Download Year 11 GCSE Physics Work, Power and

Year 11
GCSE
Physics
Unit 1
W o r k , Power and E f f i c i e n c y
1.2.4
recall, understand and use t h e equation
useful output energy
Efficieny
total input energy
1.2.5
describe and explain various ways o f making b e t t e r use o f energy;
1.2.6
review primary and secondary sources relating t o t h e e f f i c i e n c y o f
domestic appliances (w - (ii)b); and
1.2.7
recall and use t h e equation
work = force
x
distance
and t h a t t h e work done equals t h e amount o f energy t r a n s f e r r e d .
1.2.8
recall and use t h e equations
energy transferred
work done
and
Power =
time taken
Power =
time taken
t o calculate power, work done, t i m e taken or energy t r a n s f e r r e d ;
1.2.9
plan and carry out experiments t o measure personal power and t h e
output power o f an electric motor, and evaluate t h e validity and
reliability o f t h e i r data (w - (i)a): and
Work
When energy is changed f r o m one f o r m t o another, work is done.
Work Done = Energy
W o r k depends on t w o things:
Transfered
rli^nn^
^woJgri
pare
-r-o
dh
Work = Force x Dis tan ce
Complete
Where: W o r k is in Joules ( J )
t h e memory
triangle
Force is in Newton's ( N )
Distance is in meters (m)
Example:
I f John l i f t s up a box o f
apples weighing 120N a
distance o f 1.25m, how
much work does he do?
91
= 110x1.25)
Year 11
pages
107-9
GCSE Physics
Unit 1
Work and energy
Extension
.Sheet
When a force moves an object, energy is transferred and work is done.
Work done = energy transferred
Work done =
(in joules)
force
(in N)
x
It is measured in joules.
distance moved
(in metres)
Example
A man lifts a parcel, mass 4 kg,
from the floor to a shelf 2 m high.
a) What is the weight of the parcel?
b) How much work is done on it?
c) Where does this energy come from?
Answer
a) Weight = 4 kg x 10 = 40 N (see page 75)
b) Work done = force x distance moved
= 40N x 2m
= 80 joules (from his food)
Questions
For each question show all your working clearly.
1. How much work is done in these situations:
a) A man pushes a van against a
friction
force of 300 N for 10 m.
b) A mother pushes a pram with a force of
30 N for a distance of 100 m.
500 N
^
2m
3cxz=> S
c) A weight-lifter lifts a weight of 500 N
through a height of 2 m.
I O O O
J
2. A worker pushes a barrow at a steady speed
of 2 m/s for 10 s, using a force of 100 N.
a) How far did he travel?
b) How much work is done? 2 o o o T
c) Where does the energy come from? chsrr->icaS c-vfcjg-j
mSLcka. i_>-x^"t_c_/
3. A boy with a mass of 60 kg climbs 10 m
vertically up a ladder.
a) What is his weight?
b) How much work is done? t y c o o n
c) What are the energy changes here? ICeS -» Ci^E
4. An archer pulls back the arrow in his bow
a distance of 0.5 m against an average force
of 200 N.
a) How much work is done? l o c t l
b) What are the energy changes here? Kj=
S^el
A car is travelling along the road with
40 000 J of kinetic energy.
The brakes are applied and it comes to rest
in 20 m.
a) Calculate the average braking force.
b) What happens to the kinetic energy? ^
_ cr^
92
20 m
GCSE Physics
Year 11
Unit 1
Power
Power is
I f you move a f o r c e o f I N (which is t h e same as a mass o f 1kg on E a r t h ) a
distance o f l m you will have done 1J o f work. I f you do t h a t work in Is, you will
have done i t with a power o f 1W.
Power =
Complete
work done
t h e memory
time taken
triangle
W h e r e : Power is in W a t t s ( W )
W o r k is in Joules ( J )
Time is in seconds (s)
Remember:
Work Done = Energy Transferee!
Examples:
1.
A crane does 5 0 0 0 J o f work in 2 seconds. W h a t is t h e output power?
p-. &**>A
2. Find t h e power o f t h e man who pushes t h e box 8m w i t h a f o r c e o f 15N in a
6seconds.
.
p.
p .
^
iSxS.
93
GCSE Physics
Year 11
WORK AND
Unit 1
POWER
1. A cyclist moves along a flat road against a resistive force of 100K. If the cyclist travels
1000m calculate the work done by the cyclist.
ICO.
QQqTJ.
Z. Ponna lifts a parcel of weight 100N onto a shelf that is 2m above the ground.
a) Calculate the work done in lifting the parcel onto the shelf.
b) What type of energy does the parcel gain?
. QP£........
3. fit ear is driven up a mountain pass. Ft gains a vertical height of 300w. The weight of the car and
its passengers is 10,000N.
a) Calculate the work done by the car against gravity.
3 CCCs. OCG.ZF..
b) What is the gain in potential energy of the car?
3 o c o .coo.
.d
Matt cycles a distance of 2000m against a resistance force of 150N. He travels this distance in 400s.
a) Calculate the work done by Matt.
(5r%
3co..cxx>.J. ..
b) What is Matt's power output?
. ."^tSauJ
5. The diagram shows a pumped storage system used to store water in a dam.
a) Calculate the work done in pumping 10,000N of water
from the lower to the upper reservoir.
o . c o..cm."J.
100m
b) If it takes 10s to move 10000N of water from the top to the bottom calculate the power output of the pump.
lOO. ...CCO. LO.
6. The output power of a crane Is 1.6kW. Calculate how long tt will take to lift a load of 5000N
through a distance of 8m.
b.
s
•
f...2S:
94
GCSE
Year 11
page
118
Physics
Unitl
Power
Extension
Sheet
work done (in joules)
Power
=
time taken (in seconds)
(in watts)
or
Power =
energy transferred (in J)
time taken (in s)
1 watt = 1 joule per second
Example
A force of 100 N moves a
distance of 5 m in 2 seconds.
a) What is the work done?
b) What is the power?
Answer
a) Work done f force x distance moved (see p. 107)
- 100N x 5m
= 500 J
,„
work done
500 J
b) Power =
=
= 250 W
time taken
2s
Questions
For each question show all your working clearly.
1. A boy does 500 J of work in 10 seconds, ^q^^
What is his power output?
2. A mother pushes a pram with a force of 30 N
for a distance of 100 m in 50 s.
toco
What is her power output?
An electric lamp is marked 60 W.
How much energy does it transfer
a) in 1 second?
b) in 100 seconds?
i&oooZ
What are the energy transfers here? eveeWicc^ -=> W«j*v
4. An athlete runs a 100 m race in 10 s against a
friction force (drag) of 100 N.
What is his power output?
5. A weightlifter lifts an object of mass 30 kg
through a height of 2 m in 3 seconds.
a) What is the weight of the object? 3 c o « o
(Hint: see p. 75.)
b) What is the work done on the object? t o o t l
c) What is his power output? '2oo<—>
6. A boy weighing 600 N runs up the stairs,
a) in 3 seconds, and then
Soo«-~>
b) in 4 seconds.
boo L-O
The vertical height of the stairs is 4 m.
What is his power output in each case?
7. A lift containing 6 people is raised through a
height of 20 m in 10 s. The total weight of
the lift and passengers is 6000 N.
What is the power of the lift motor,
a) in watts?
'^ooci—>
b) in kilowatts?
12jc.ua
95
60 w
GCSE Physics
Year 11
Unit 1
Personal Power
Plan and carry out an experiment t o measure t h e amount o f power you can
develop when running up a f l i g h t o f stairs.
Method:
Results:
H e ^ *
c u r b e d
(uS)
W h a t a r e t h e sources o f e r r o r in t h i s experiment and how can you make t h e
measurement more reliable?
r\n>
96
Year 11
GCSE Physics
Unit 1
Power o f an Electric M o t o r
Plan an experiment t o determine t h e output power o f an electric motor by
plotting a graph o f work done against t i m e taken.
Method:
Results:
we.
Graph:
97
Year 11
GCSE Physics
Unit 1
Efficiency
A machine changes energy f r o m one type t o another, but not all t h e energy
produced is useful - usually some is lost as heat.
When building a machine engineers want i t t o produce as much useful energy as
possible - t h e y want i t t o be e f f i c i e n t .
The e f f i c i e n c y can be calculated f r o m e i t h e r o f these equations:
Efficieny =
useful output energy
—
•
total input energy
gr
useful output
Efficieny
power
=
total input
power
Why will t h e efficiency o f a machine always be less than 100%?
W h a t does an e f f i c i e n c y o f 0.8 mean?
ill L
-
Example:
Find t h e e f f i c i e n c y o f t h e ramp shown.
-• 1 X 0 0 3
cx-><-
nod
Weight = SOON
Physics f o r CCEA Questions 17 - 24, Page 51
Physics f o r CCEA Questions 3c, 4 - 6c + 8, Pages 5 6
Physics f o r You Questions 18b, 19b, 20b, 23 - 26c 2 7 - 2 9 , Pages 150 + 151
98
Year 11
GCSE Physics
pages
112, 122
Energy transfer and efficiency
An Energy Transfer Diagram (Sankey diagram)
shows what happens during an energy transfer:
Efficiency =
useful energy transferred
total energy input
Unit 1
for a torch
chemical
energy
stored in
* e battery
X 100%
Questions
For each question show all your working clearly.
K light energy
r (useful energy)
energy heating u|
torch + room
(wasted energy)
10O3
1. A lot of energy is wasted in a car. For every
100 J of chemical energy in the petrol, only
25 J are transferred to useful kinetic energy.
The rest just heats up the engine and the air.
a) Draw an Energy Transfer Diagram for this,
to scale.
b) Calculate die efficiency. 167.
energy heating
up the room
600 J
2. The diagram shows the energy transfers for a
Bunsen burner heating a beaker of water.
What is its efficiency as a water heater? L\(fl
i
Useful energy
> heating water
Y 400 J
0
In a solar cell, for every 80 J of solar energy
shining on it, only 4 J is transferred to useful
energy (as electricity).
b Wr
a) What happens to the oflier 76 J? U3qab}jJ QS koctVb) What is its efficiency? S°/
c) Draw a Sankey diagram of this, to scale.
p £ > ^ T eWcW>cc\
3
0
4. A pulley system lifts a load and gives it
6000 J of potential energy. The person
pulling on the rope gives it 8000 J of energy. "^-tSTo
What is the efficiency?
5. An electric kettle has a power rating of 2 kW
and is switched on for 100 seconds.
While heating up, it loses 60 000 J to the
surroundings.
a) How much energy is supplied to the kettle?
cccrX
(1 kW = 1000 W = 1000 joules per second)
b) How much is given to the water? ' ^ o 0 0 0 3
c) What is the efficiency of heating water?
6. An electric motor on a building site has a
power rating of 400 W and lifts a load of
bricks weighing 600 N through a height of
10 m in 20 seconds.
a) How much energy is needed to lift the bricks?
(See page 107 or page 116.)
°
b) How much energy is supplied to the motor
in 20 seconds?
S^cotJ
c) What is the efficiency of the motor in
doing this job? "}SY
too
3
0
99
motor
Year 11
GCSE Physics
Unit 1
Making Good Use o f Energy
List as many ways as you can t h a t we unnecessarily use energy in our homes and
some ways in which we could r e c t i f y t h i s waste:
e.g.
leaving TV on standby - t u r n o f f completely
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T h e E f f i c i e n c y o f Domestic Appliances
All electrical appliance in t h e UK must display information regarding i t s
e f f i c i e n c y . There are various ways o f displaying t h i s information:
Task: Outline how each o f these systems work and list t h e info t h e y contain.
EU Label
-
Energy
€3
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/
£ - lensr e f f
3'
info
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SEDBUK
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SEDBUK Rating
90.0%
91.3%
86.0%
90.0%
82.0%
96.0%
78.0%
82.0%
74.0%
7BJO%
70.0%
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710%
70%
I O O ^ V J J J S . or-«
100
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^podb)
GCSE Physics
Year 11
Unit 1
Kinetic and Gravitational Potential Energy
1.2.10 recall and use t h e equations
kinetic
energy
= j mass x velocity
=
potential
energy
2
imv*
= mass * acceleration
due to gravity
x height
= mgh
Kinetic Energy
KE is t h e energy t h a t a body possesses because i t is moving.
KE = i m v
2
Complete
t h e memory
W h e r e KE = k l , ^ g h
(
r
triangle
^
Rearranging KE= £ mv
To g e t m: - 1 l £
X/
To g e t v:
1
W o r k done and Kinetic energy
A car is moves along a road a t 60mph what has t o be done t o bring i t t o a stop?
Explain t h e connection between t h e KE o f t h e moving car and t h e work done t o
Stop i t .
1
leg
o£>-
=
(>-3<^
V=x^
lp^o\Co_S,
-V
UgiQr-
losk
>K-> W - Q V I ^ S
Example:
A bullet o f mass 10g travels a t a speed o f 200m/s. Calculate i t s kinetic energy.
=
2. v O.OV
Sj
xloo
V
ZooT
101
Year 11
GCSE Physics
KINETIC
L
ENERGY
a) What is kinetic energy?
Unit 1
#
Energy
, g r ^ ^ . . . \ c ^ o 3 j j ^ . . . . v ^ . . . Q f € ooo^^c^
b) A truck of mass ZOOOkg and a car of mass 1000kg are travelling down a motorway at the same speed.
(i) Which one has the greatest kinetic energy?
(ii) Explain your answer. 1 p/Qjgdr:.. rv^aso
c) Two cars of the same mass are travelling down a road. Explain how one car could have more
kinetic energy than the other.
one.
brc9s^k\n.c\.... £Q,s*c-ey ... fa\o.o.... .-^WrA,... .c^Us-a-
1. For the following pairs of objects state which has the most kinetic energy,
al A car of mass 1000kg or a lorry of mass SZOOkg, both moving at lOm/s.
b) A car of mass 1000kg moving at lOm/s or a car of mass 1000kg moving at ZOm/s.
3. A car of mass 1000kg moves along a road at a constant speed of ZOm/s. Calculate it's kinetic energy.
2QO..
.000.3.
4. A -truck of mass 3Z,000kg moves along a road with a speed of lOm/s. Calculate the kinetic energy of the truek
lbcx> ODD. T.
5. A skier of mass 90kg is skiing down a hill at a speed of 15m/s. What is the kinetic energy of the skier?
6. The kinetic energy of a cyclist moving along a road is 5000J. If the mass of the cyclist is 100kg
calculate the speed of the cyclist.
7. A motorcyclist and motorcycle have a combined mass of 900kg. If they have 140,000J of kinetic
energy calculate their speed.
•
102
GCSE Physics
Year 11
Unit 1
Gravitational Potential Energy
GPE is t h e energy t h a t an o b j e c t has gained because o f a change in i t s vertical
position.
GPE = m g h
Complete
t h e memory
\ h h ^ a \ gnO^(T)
'
W h e r e GPE = Cm^\nY\nrA
m=
h -
r/Yl^S
t r i Q n 9
e
(V-r^
IA^IOWV
( rv\)
Potential Energy and W o r k bone
When you l i f t your school bag onto t h e table i t gains PE. W h e r e did t h i s energy
come f r o m ?
JQj '
rJ^pnrMml
( rrv^ri)
Explain t h e connection between t h e work you did l i f t i n g your bag and t h e energy
i t gained.
, v i -to
hpl-
»
f.P£
ggnrxpd
Conservation o f Energy
A ball is dropped f r o m a height, h.
?
PE = mgh KE = 0
PE a t t h e Top = KE a t t h e b o t t o m
A t t h e t o t a l amount o f energy
must remain t h e same.
PE = KE
o
When t h e ball h i t s t h e ground t h e
KE will be c o n v e r t e d into h e a t and
sound energy.
PE = 0
KE = £mv
2
Physics f o r CCEA Questions 25 - 3 3 , Pages 53 + 54
Physics f o r CCEA Questions 3b, 6 d • 7, Pages 5 5 - 5 7
103
GCSE Physics
Year 11
page
117
o r
Potential energy kinetic energy
Change in
gravitational PE
= weight x
change in
gravitational PE
—
= „„„„
mass
(in kg)
(in J)
Kinetic energy
(inJ)
Unit 1
=
v
change in
height
since weight = mass x g (see p. 75)
x „g ~x change
. •» in
height
(N)
6
(N/kg)
(in m)
VJ x mass x speed squared
(kg)
(kg)
(N/kg)
g = 10 here on Earth
(m/s)
J
See the examples on page 117.
Questions
For each question show all your working clearly.
1.
A diver, of mass 40 kg, climbs up to a diving
platform 1.25 m high,
a) What is his weight, in N? te-o
b) What is his change in P.E.? SooJ
c) Where does this energy come from? <^Ke»v>.c
d) He walks off the platform and falls down. f
What is his K.E. as he hits the water? S o o t J
e) What is his speed as he hits the water?
o o d
2. The same diver now climbs to the 5 m
platform, four times as high.
a) What is his change in P.E. now? ^ o c o j
b) What is his speed as he hits the water? t o i l s
c) What do you notice about this answer?
5 m
3. Another diver, of mass 80 kg, climbs to the
5 m platform.
a) What is her speed as she hits the water? /cvnte
b) What do you notice about your answer? $ ,wa as ^ojcg dnje/
(sps=d readied *\ (yee. fed I
4. A stone is dropped from a window 5 m high.
Of. m a s s )
At what speed does it hit the ground?
iorr»\s
a
5. A tennis player hits a ball vertically with a
speed of 10 m/s.
How high does it go?
10 m/s
6. A car of mass 600 kg is travelling at 10 m/s.
When the brakes are applied, it comes to rest
in 10 m.
What is the average force exerted by the brakes?
3CCCM
7. A car of mass 800 kg is at rest
The engine exerts a resultant force of 2000 N
for a distance of 5 m.
t o d - fd
a) What is then its K.E.? ooooT
.
S
b) What is then its speed?
. iooco3
104
fo^eookg^
10 m
3c»oco = f * '
0
Year 11
GCSE Physics
Unit 1
Kinetic Energy and Potential Energy
1.
A tourist's Fiat is driving along a mountain road. The combined mass
of the car and luggage is 2920kg. The car is powering uphill at 23m/s.
a) How much kinetic energy does the car have? '^'wlSuO'J
At the top of the road, the car has gained a total height of 1200m.
b) Calculate the potential energy the car has gained. 3 5 ^ 0 0 0 3 ~ 3
As the car rounds a bend at the top of the mountain, a suitcase falls
from the roof into the valley below. The suitcase has a mass of 20kg.
c) Work out the potential energy the suitcase lost when it had fallen a distance of 60m. 12 OCOj
d) If all of this potential energy of the suitcase is converted into kinetic energy, how fast will
it be travelling when it has fallen 60m?
3U.lo rV>ls
e) Explain why it will not actually be travelling as fast as this. • f ^ ' ,
c r l £ r (
Some workmen are using a rope to lower a bucket full of bricks from a window.
They tie off the rope when the bucket is just above the ground. As they are making
their way downstairs to unload the bucket, a strong wind sets the bucket swinging.
Draw a diagram of the path of the swinging bucket. On your diagram:
mark with the letter A — where the potential energy is greatest.
mark with 'he letter B — where the kinetic energy is greatest.
mark with the letter C — where the bucket is travelling fastest.
mark with the letter D — where the bucket's velocity is zero.
A bouncy ball has a mass of 0.3kg. It is dropped from a height of 3.0m.
a) How much potential energy has the ball lost when it hits the ground? ^ 3
Ignoring air resistance, how fast will the ball be travelling?.-)
The ball rebounds vertically at a speed of 7.0m/s.
What kinetic energy does it now have? " ^ . j S J
What height will it reach on the rebound?
,
2.^5^
Explain what has happened to the energy that the ball has lost. (Yichcry
Three students carry out an experiment to compare their own personal power. They measure their
mass, then time how long it takes them to run up a flight of stairs 12m high. Their results are shown
in the table below. Copy and complete the table.
Name
Weigh! (N) Time (s)
Alex
520
Billie
450
16
Jack
600
15
Potential Energy
Power (W)
Gained (J)
14
oL'OO
105
Year 11
Unit 1
Assessed Homework
Karl takes Katie on a date t o a theme park. They decide t o have a go on t h e
roller coaster. The roller coaster car weights 9 0 0 0 N and is brought t o a height
of 45m above i t s s t a r t i n g point in 30 seconds.
a) W h a t is t h e work done in raising t h e car t o t h i s height (point B)?
[4]
b) W h a t is t h e power of t h e motor used t o pull t h e car up t h e t r a c k ?
[4]
c) W h a t is t h e gravitational potential energy gained by t h e car a t point B?
[4]
d) I f t h e roller coaster is let run down f r o m point B t o a point 15m above i t s
initial s t a r t i n g position (point C), what kinetic energy will i t have gained
assuming t h a t t h e roller coaster is 100% e f f i c i e n t ?
e) W h a t is t h e velocity o f t h e roller coaster car a t point C?
[4]
[5]
f ) I f t h e roller coaster is actually only 8 2 % e f f i c i e n t what velocity will t h e car
have a t point C?
[4]
Total [ 2 5 ]
106
.Assessed
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GCSE Physics
Year 11
Unit 1
Momentum
By t h e end o f t h i s section you should be able t o :
1.1.18 recall t h a t
Momentum = mass x velocity
1.1.19 recall and understand t h a t
Change in momentum - force x time
and apply t h i s t o t h e solution o f mathematical problems;
1.1.20 apply t h e principles o f momentum, f o r c e s and time t o an analysis o f
s a f e t y f e a t u r e s o f modern cars, t o include car air bags, car seat belts,
car crumple zones and crash b a r r i e r s ;
1.1.21 investigate, using data loggers or computer simulations, one-dimensional
inelastic collisions and, through mathematical modelling, use t h e data
obtained t o show t h a t t h e momentum is conserved in such collisions; and
1.1.22
recall and use t h e principle o f conservation o f momentum t o solve simple
problems involving one-dimensional inelastic collisions.
Momentum
Momentum is a useful quantity t o consider when o b j e c t s collide.
Think about a Year 8 pupil colliding w i t h a Year 14 pupil. Who comes o f f t h e
worst? W h a t f a c t o r s does t h i s depend on?
Momentum is product o f t h e mass o f a body and i t s velocity.
P=m v
Complete
t h e memory
triangle
m -
Example:
An o b j e c t has a momentum o f 15.90kgm/s and a velocity o f 9.04m/s. W h a t is
t h e mass o f t h e o b j e c t ?
IIS.^
'
Y~Y^
107
K<=\
,OU
Year 11
GCSE Physics
Unit 1
Impulse
Consider a f o r c e F acting on a mass m f o r a time t j so t h a t i t accelerates f r o m
initial velocity u t o final velocity v.
The f o r c e x t i m e is called t h e impulse.
As
a =
Then
v
—
u
F = m
Impulse = F t
And F = ma
(v-u^
v
t
Force
mv - mu
I . e . Force
change
time taken
J
x time = change
in momentum
in momentum
=
= Impulse
Questions:
1.
While playing basketball in PE class, Logan lost his balance a f t e r making a
lay-up and colliding w i t h t h e padded wall behind t h e basket. His 74kg body
decelerated f r o m 7.6m/s t o Om/s in 0.16 seconds.
a) Determine t h e f o r c e acting upon Logan's body.
b) I f Logan had h i t t h e concrete wall moving a t t h e same speed, his
momentum would have been reduced t o zero in 0.008 seconds. Determine
what t h e f o r c e on his body would have been f o r such an abrupt collision.
2. A boy kicks a stone o f mass 1kg, accelerating i t f r o m
r e s t t o lOm/s. The stone is rigid so t h e f o r c e acts f o r
only 1/100 o f a second. He t h e n kicks a football o f t h e
same mass t o give i t t h e same final speed. The football
is s o f t in comparison t o t h e stone so t h e f o r c e acts f o r
1/10 o f a second t h i s t i m e . Which kick h u r t s less?
Physics f o r CCEA Question 11, Page 20
108
GCSE Physics
Year 11
Unit 1
Conservation o f Momentum
Udr»'
The law o f conservation o f momentum states t h a t : ^ m o m = n h
-tdrol
Cr?lll,1\Qp
CC
Vyzfrr-e.
o
^-yfdriSlno
There are no exceptions t o t h i s law.
When t w o o b j e c t s collide t h e i r changes in momenta will be equal in size but
opposite in direction. The momentum gained by one is equal t o t h e momentum
lost by t h e other.
Tackling Momentum Problems:
1.
Choose a positive direction.
2. Draw b e f o r e and a f t e r sketches o f t h e o b j e c t s involved.
3. Calculate every momentum you can
4.
Apply t h e law o f conservation o f momentum f o r collisions involving two
bodieS.
Pbefore = Pafter
m i u i + m2U2 = m i V i + m2V2
Questions:
1.
A railway wagon o f mass 800kg moves a t a steady speed o f 2.5m/s. I t
collides w i t h another wagon o f mass 1000 kg. The two wagons couple
t o g e t h e r a f t e r t h e collision. Calculate t h e final speed and t h e loss in KE i f
t h e second wagon was stationary initially.
2. Rex (86kg) and Tex (92kg) board t h e bumper cars a t t h e local carnival. Rex
is moving a t a full speed o f 2.05m/s when he rear-ends Tex who is a t r e s t in
his path. Tex and his 125kg car lunge f o r w a r d a t 1.40m/s. Determine t h e
post-collision speed o f Rex and his 125kg car.
109
Year 11
GCSE Physics
Unit 1
3. A candy-filled pinata is hung f r o m a t r e e f o r Matthew's birthday. During an
unsuccessful a t t e m p t t o break t h e 4.4kg pinata, Hayden cracks i t with a
0.54kg stick moving a t 4.8 m/s. The stick stops and t h e pinata undergoes a
gentle swinging motion. Determine t h e swing speed o f t h e pinata immediately
a f t e r being cracked by t h e stick i f i t initially was a t r e s t .
4.
During an in-class demonstration o f momentum change and impulse, M r . H
asks Jerome (102kg) and Michael (98kg) t o s i t on a large 14kg skate c a r t .
M r . H asks Suzie (44kg) t o s i t on a second 14kg skate c a r t . The two carts
are placed on low f r i c t i o n boards in t h e hallway.
Both carts are initially a t
r e s t and Jerome pushes o f f o f Suzie's c a r t . Measurements are made t o
determine t h a t Suzie's c a r t acquired a post-impulse speed of 9.6m/s.
Determine t h e expected recoil speed of Jerome and Michael's c a r t .
5. Jaclyn plays singles f o r South's varsity tennis team. During t h e match
against N o r t h , Jaclyn won t h e sudden death tiebreaker point w i t h a crosscourt passing shot. The 57.5g ball h i t her racket with a velocity of 26.7m/s.
Upon impact with her 331g racket, t h e ball rebounded in t h e exact opposite
direction (and along t h e same general t r a j e c t o r y ) with a speed o f 29.5m/s.
a. Determine t h e pre-collision momentum of t h e ball.
b. Determine t h e post-collision momentum o f t h e ball.
c. Determine t h e momentum change of t h e ball.
d. Determine t h e velocity change of t h e racket.
6. To M r . H's disgust, a 450g black crow is raiding t h e recently-filled bird
f e e d e r . As M r . H runs out t h e back door with his broom in an e f f o r t t o
scare t h e crow away, i t pushes o f f t h e 670gram f e e d e r with a t a k e o f f speed
of 1.5m/s. Determine t h e speed a t which t h e f e e d e r initially recoils
backwards assuming t h e crow and f e e d e r were stationary initally.
110
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GCSE Physics
Year 11
Unit 1
Elastic and I n e l a s t i c Collisions
I n some collisions kinetic energy as well as momentum is conserved.
The t o t a l kinetic energy o f t h e bodies b e f o r e t h e collision is equal t o t h e t o t a l
kinetic energy o f t h e bodies a f t e r t h e collision.
No energy will be lost in t h e collision as heat or sound o r in t h e permanent
deformation o f t h e colliding bodies.
T
2
TuUi +
i
1TI2U2
2
2
= £ mivi + |
m2V22
I f t h e collision is elastic t h e r e is no loss in kinetic energy.
Collisions in which t h e kinetic energy is not t h e same b e f o r e and a f t e r are called
inelastic collisions.
Although kinetic energy may or may not be conserved in a collision, momentum is
always conserved and so is t o t a l energy.
Total momentum b e f o r e is equal t o t o t a l momentum a f t e r , providing no external
f o r c e is applied.
Types o f collision;
•
Elastic collision
No loss o f kinetic energy on impact
•
Inelastic collision
Some kinetic energy lost on impact
•
Completely
O b j e c t s stick t o g e t h e r on impact
inelastic collision
NB: Remember t h a t momentum is always a vector quantity and so magnitude and
direction are important.
Hence i f t w o o b j e c t s are travelling towards each o t h e r t h e t o t a l momentum is
found by subtracting t h e individual momentum o f t h e objects.
Physics f o r You Questions 1 - 7 , Page 147
111
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Year 11
GCSE Physics
Car S a f e t y
Make a poster presentation detailing how one o f t h e following car s a f e t y
f e a t u r e s keeps passengers safer during a collision.
Crumple zones
Seat be Its
I
I I I
«
Revision Questions
Phys cs f o r CCEA Questions 1 - 4 , Pages 11-12
Phys cs f o r CCEA Questions 12 - 1 6 , Page 22
Phys cs f o r You Questions 12 - 2 2 , Page 121
Phys cs f o r You Questions 1 - 6, Page 128
Phys cs f o r You Questions 8 - 1 7 , Page 141
Phys cs f o r You Question 3 9 , Page 153
113
Unit 1
Car S a f e t y
All of t h e s a f e t y f e a t u r e s have one aim in mind, t o increase t h e time t o stop and t o
increase
•
t h e distance over which t h e passengers stop.
By increasing t h e time t o stop, you are decreasing t h e r a t e o f change o f velocity,
i.e. decreasing t h e acceleration, so decreasing t h e f o r c e experienced.
mv - mu
F = m
V
•
By increasing t h e distance over which t h e f o r c e is acting, t h e average f o r c e is less.
Other information
Crumple Zones
Crumple zones are part of a car designed t o collapse during a collision - usually t h e
f r o n t end. The f r o n t o f t h e car crumples and stops, but t h e passengers continue t o
move a crucial half m e t r e or so.
S e a t belts
I f you were not wearing a seat belt and t h e car came t o a sudden stop, you would
continue t o move due your inertia. Your body would most likely be stopped as a result
of t h e f o r c e o f t h e windscreen or other rigid part of t h e car.
Airbags
Airbags need t o be used in conjunction with seat belts. They p e r f o r m t h e same
function as a seat belt and should be fully inflated b e f o r e you h i t them. I f you were not
wearing a seat belt then t h e r e would not be enough time f o r t h e airbag t o i n f l a t e
b e f o r e you h i t i t . Airbags are designed t o inflate in 0.05s, and t o d e f l a t e within 0.3s.
The quarter o f a second between these two times is s u f f i c i e n t t o slow you down.
A f l e x i b l e nylon bag is f o l d e d into t h e steering wheel or
dashboard.
When t h e f r o n t end o f t h e spring is suddenly stopped, t h e
mass continues f o r w a r d s t o make contact w i t h t h e switch,
s t a r t i n g a chemical reaction. This occurs when t h e
acceleration is around -lOg.
An inflation system in which a spark ignites a violent chemical
reaction between sodium n i t r i d e (NalSb) and potassium n i t r a t e
(KNO3) producing nitrogen gas t o inflate t h e airbag.