Download Acceleration P3:Higher Physics of Transport

Written by
M J Bradley
P3:Higher Physics of Transport
400
300
Thinking distance is the distance you go while reacting. It is affected by
alcohol, drugs and speed
Braking distance is the distance travelled once the foot is on the brake
and is affected by how hard the brakes are pressed (the harder the pedal
is pressed the more the friction between the brake pad and wheel),
speed, weather, road and tyre conditions. Anything which reduces
friction will increase braking distance.
Distance-Time graph for two cars
500
distance
(m)
stopping = thinking + braking
distance distance distance
Stopping Distance
Speed(or velocity) = distance (m) Example: What is
the distance travelled
(m/s)
time (s)
in 30 seconds by a
car travelling with a
velocity of 30m/s?
d
answer: d = s x t
= 30 x 30
t
s
= 900m
Typical Questions
1) What is the distance
travelled by car A? 500m
stationary
constant
car A speed
2) What is the speed of car
A?
s =d/t = 500/20 = 25m/s
200
stationary
100
3) What is the distance
travelled by car B? 200m
constant
speed
car B
10
20
4) What is the speed of car
B?
s = d/t = 200/25 = 8m/s
30
time in seconds
Acceleration
This is the change in speed per second. The
units of acceleration are m/s/s
acceleration = change in speed (m/s)
time (s)
Example: What is the acceleration of a rocket if it
accelerates from 20m/s to 100m/s in 2 seconds?
Answer a = change in speed
time
= 100-20 = 80/2 = 40m/s/s
2
chs
a
t
Calculating distance
from a Speed-time
graph
Speed - Time graph for a car
30
speed
(m/s)
constant speed
constant acceleration
This is done by calculating the
area under the graph.
The distance travelled by the
car while it is accelerating is
the area of a triangle.
area of triangle = 1/2 b x h
=1/2 x 10 x 30
= 150m
Typical Questions
20
1) What is the maximum speed
of the car? 30m/s
2) What is the car's acceleration?
acceleration = change in speed
time
=30/10 = 3m/s/s
10
5
10
The Link between Force,
Mass and Acceleration
acceleration = force(N)
(m/s2)
mass(Kg)
F
a
m
a=F
m
A big force on a small
mass gives a very big
acceleration
Example: What force is needed to make
a 100000 Kg boat accelerate at 0.5m/s2?
answer
F = ma = 100000 x 0.5
= 50000N
Terminal Speed
time in seconds
Objects fall to the ground
because of the pull of gravity.
When an object falls on Earth
it first gets faster (accelerates)
then it reaches a steady speed
just like when a cyclist
reaches top speed on a bike.
This maximum speed is called
the terminal speed and it
happens when the forces on
an object become balanced
The forces become balanced
because the air drag increases.
Strength of Earth's Gravity
a = f/m so g = W/m
So weight, W = mg
g is the strength of
Earth's gravity. It is
about 10N/kg. On the
moon it is 1.6N/kg
Example How heavy is a 100kg man on Earth?
answer W = mg = 100 x 10 = 1000N
terminal velocity (or speed)
velocity m/s
air drag
weight
The weight and air
drag forces are
balanced at the
terminal speed
time (s)
Work
Work is done when a force moves an object through a distance.
Work is a transfer of energy. E.g. lifting a book chemical potential
energy in muscles is changed to gravitational potential energy
Work Done = force x distance
(J)
(N)
(m)
Example The work done by a car
pulling a trailer with a force of 200N
through a distance of 1000m is
WD = F x d = 200 x 1000 = 200000J
Power = work done (J)
(W)
time (s)
WD
F
Power
Power is how quickly work is done. If a machine does a lot of
work in a short space of time it has a high power. It is measured
in joules per second which scientists call watts (W)
d
Example: The power of a machine
that does 2000J of work in 2s is
P = WD = 2000 = 500W
t
2
WD
P
t
Gravitational Potential Energy Potential Energy = mass x g x height
This is the energy something has due to it height
(J)
(kg)
(m)
above the ground
g is the acceleration due to gravity = 10 m/s2
PE
Example What is the potential energy of a steel girder of mass 1000kg when a crane lifts it to a
height of 100m.?
PE = m x g x h = 1000 x 10 x 100 = 1000000J
mxgxh
kinetic energy = 1/2 x mass x velocity2
(J)
(kg) (m/s)
Anything which moves has kinetic energy. If
Kinetic Energy
an object has a big mass and a high speed it
has a very big kinetic energy
Examples: (1) What is the Kinetic energy of a 100 000kg train moving with a velocity of 30m/s?
answer: KE = 1/2 x m x v2 = 1/2 x 100 000 x 302 = 1/2 x 100 000 x 900 = 45000000J
Transfer of Potential and Kinetic Energy
B
A
C
Imagine
you are
towed up to
the top of a
high slope
and the
cable is
released
from the
carriage
Crashes
In any collision kinetic energy needs to be absorbed and
The law of energy conservation is that energy is transferred to other forms of energy. Car brakes transfer
never destroyed but can be transferred
kinetic energy to heat.
A - The ball has kinetic and potential energy
Crumple zones, car bumpers, air bags absorb energy
B- The ball has maximum potential and minislowly and reduce the impact force
mum kinetic
C - On the point of impact the ball has maximum Seat belts keep the occupants in their seat. In a crash
kinetic energy and zero potential energy
they are best replaced because their is a danger of them
being permanently stretched and weakened - dangerous in
a further crash
All gravitational
potential energy, no
kinetic
The kinetic energy is
transferred back to
potential energy
WEDNESFIELD
HIGH SCHOOL
EXTREME RIDES
If the brakes are never
applied will this ride
last forever?
What will happen?
kinetic energy is
maximum and the
potential energy is
zero
UNIT P3 FORCES AND MOTION
FOLD
1) distance/time
2) d = sxt = 20 x 20 = 400m
3) acceleration = speed change/time
4) a = chs/t = 20/4 = 5 m/s2
5) unbalanced (resultant)
6) bigger
bigger
small
big
7) a = f/m
8) a = f/m = 8000/1000 = 8m/s2?
9) f = ma = 1000 x 9 = 9000N
10) balanced
speed
1) speed = ?
2) What is the distance travelled by a car in 20s if travelling at 20m/s?
3) acceleration = ?
4) What is the acceleration of a car if it accelerates from 10m/s to 30m/s in
4s?
5) To make something accelerate there must be an ______ force acting
upon it
6) The bigger the resultant force acting on an object the _______ the
acceleration. The smaller the mass the ______ the acceleration. For a
racing car the best combination for high acceleration is ______ mass and
_____ thrust from the engine.
7) What equation links force, mass and acceleration?
8) What is the acceleration of a car if the force from the engine is 8000N
and the mass is 1000 Kg?
9) What is the force required to make a 1000 kg car accelerate at 9m/s2?
10) When a falling object reaches its terminal speed the forces on it are
________
11) Sketch a graph of speed against time for a falling object starting from
rest (zero speed).
time
speed (m/s)
12) seat belts, air bags, crumple zones
13) increase the time for the collision,
12) Three safety features of cars are.......
13) An air bag reduces the force on a driver during a crash because reducing the deceleration and reducing
the force because f = ma
................[3]
14) thinking distance + braking distance
14) Stopping distance = ________ + _________
15) distance travelled with foot on brake
15) What is meant by braking distance?
16) wet weather, poor brake condition,
16) What things increase braking distance?
17) distance travelled while reacting
17) What is meant by thinking distance?
18) alcohol, drugs, tiredness
18) What things increase thinking distance?
19) What is the thinking distance of a driver if the reaction time is 0.5s and 19) d = sxt = 40 x 0.5 = 20m
20) sd = td + bd = 15 + 75 = 90m
the speed is 40m/s?
20) What is the stopping distance of a car if the thinking distance is 15m
and the braking distance is 75m?
21) Use the graph to describe the motion of a bus in terms of speed and 21) accelerates then steady speed then
acceleration
rapid deceleration
20
4
16
18
time (s)
distance (m)
22) a = chs/t = 20/4 = 5 m/s2
22) What is the acceleration of the bus?
22b) area under graph =0.5bh =
22b) What is the distance travelled while accelerating
0.5 x 4 x 20 = 40m
23) For how long was it at steady speed?
23) 12s
24) What was the deceleration?
24) a = chs/t = 20/2 = 10m/s2
25) Use the graph below to describe the motion of a cyclist in terms of 25) steady speed for 5s then stationary
speed
30
5
time (s)
26) What was the cyclists' speed in the first 5 seconds?
26) s = d/t = 30/5 = 6m/s
27) Work is a measure of _______ transfer
27) energy
28) Work = _______ x _______ or wd = _____
28) force x distance , wd= fd
29) How much work is done by a 1000N man climbing stairs 10m
high?
29) wd = fd = 1000x10 = 10000J
30) Power is the rate at which ________ is done
30) work
31) If a machine does a lot of work in a small amount of time it has a
________ power. If it does little work in a long time (like some of
you) it has a ______ power. One joule per second is one _______
31) high , low, watt
32) Power = work done /time, p = wd/t
32) Power = ______ or p =
33) What is the power of a machine if it does 10000J of work in 10s?
34) Gravitational potential energy is the__________ energy of an
object which is ________ up and is equal to the amount of ______
done to get it there
35) Potential energy = mass x _____ x ______
or PE = ________
36) What is the potential energy of a 10Kg mass 20m above the
ground?
33) p = wd/t = 10000/10 = 1000w
34) stored, high, work
35) = mass x gravity strength x height
PE = mgh
36) PE=mgh = 10x10x20 = 2000J
( take g = 10m/s2)
37) Moving objects have ________ energy
37) Kinetic
38) Kinetic energy = 0.5 x mass x _________ or KE = ________
38) 0.5 x mass x velocity2
39) What is the kinetic energy of a 1000Kg car travelling at 20m/s?
39) KE = 0.5mv2 = 0.5x1000x 202
= 0.5x1000x400 = 200000J
40) Energy cannot be _________ but can be __________ to other
forms
40 ) created or destroyed, transferred
41) A playground slide converts _________ energy to ________
energy
41) potential, kinetic
42) The brakes on a car do work to stop it. _______ energy is
transferred to ________ energy.
42) kinetic, heat
43) A ball has 50J of potential energy. If it is dropped how much
kinetic energy does it have the moment before it hits the ground?
44) Work done to stop = ke of car
43) 50 J
44) If a car has 200000J of kinetic energy what distance will it take to
stop if the braking force is 4000N?
fd = KE so d = KE/f = 200000/4000
45) At the top of the ride below Mr Bradley's _______ energy is at a
______ and his kinetic energy is _______. As he falls potential
energy is transferred to _______. His kinetic energy is at a maximum
at the ______ of the ride. This kinetic energy is then transferred back
t o _____
_ energy.
45) potential, zero, kinetic , bottom,
potential
WEDNESFIELD
HIGH SCHOOL
EXTREME RIDES
= 50m
P3 TEST: Transport Physics
Name ___________________________ Form:___________
1(a) Nick thinks cars travel to fast near his school. He calculates the speeds of cars as they move
past the school. Write about the experiment Nick did to calculate the cars speed
In your answer include
(i) what equipment he used
(ii) what measurements he took
__________________________________________________________________________________________
__________________________________________________________________________________________
__________________________________________________________________________________________
__________________________________________________________________________________________
__________________________________________________________________________________________
__________________________________________________________________________________________
__________________________________________________________________________________________
________________________________________________________________________________________[4]
2)
New cars are tested in crashes. 15_ car A mass 1000kg
Scientists study the results. Two cars crash
speed in m/s
into a wall. Look at these graphs
10_
(a) Car A takes 0.2 s to stop during the
crash.
Calculate the acceleration of car A during 5__
the crash.
15_ car B mass 1500kg
__________________________________ 0
__________________________________
__________________________________
__________________________________
Answer______________m/s2
[3]
0
time in seconds
0.2 s
speed in m/s
10_
5__
time in seconds
0.1 s
(b) Calculate the distance car A travels during the crash.
____________________________________________________________________________________
____________________________________________________________________________________
____________________________________________________________________________________
____________________________________________________________________________________
Answer___________________m
[3]
(c) After the crash car B is more damaged. Car B must have had much greater forces on it during the
crash. Explain why.
_____________________________________________________________________________________
_____________________________________________________________________________________
_____________________________________________________________________________________
_____________________________________________________________________________________
_______________________________________________________________________________[3]
(d) Elaine wants to buy a car. She has a choice of
three engines. All the cars have the same mass. Each
engine produces a different maximum force.
Engine
capacity
Maximum
Look at the information in the table.
force
(i) The GSi model has the highest acceleration. Explain Mass
S model
GS model
GSi model
1.3 litres
1.6 litres
2.0 litres
2500N
3000N
4000N
1000Kg
1000Kg
1000Kg
why.
____________________________________________________________________________________
_________________________________________________________________________________[2]
(ii) The GSi model now pulls a caravan. Its maximum acceleration is now 2.5m/s2.
Calculate the mass of the caravan. _______________________________________________________
____________________________________________________________________________________
____________________________________________________________________________________
____________________________________________________________________________________
_________________________________________________________________________________[4]
____________________________________________________________________________________
____________________________________________________________________________________
_____________________________________________________________________________
(3) Look at the diagram on stopping distances for cars.
car moving
at 30 km/h
thinking distance
braking distance
car has
stopped
Describe the factors that will increase stopping distance___________________________________
_________________________________________________________________________________
_________________________________________________________________________________
_________________________________________________________________________________
________________________________________________________________________________[5]
4) Five cars travel along the motorway. They go through two tunnels. Both tunnels are the same
length.
They go through tunnel A first. Look at the results.
Car
Astra
Focus
Golf
Mini
Saxo
Time to pass through tunnel A Time to pass through tunnel B
in seconds
in seconds
6
8
6
3
7
3
5
5
10
11
(a) Which cars increase their speeds?
__________________________________________________________________________________[1]
(b) Which car has the highest acceleration?
__________________________________________________________________________________[1]
(e) Tunnel A is 200m long. Calculate the speed of the Mini through tunnel A
____________________________________________________________________________________
____________________________________________________________________________________
__________________________________________________________________________________[2]
5) Barry must lift some heavy
weights onto a lorry. Each
weight is 600N and he must
lift them 1.1m onto the lorry.
(a) How much work is done
lifting one weight onto the
lorry?
1.1m
________________________
_______________________[2]
____________________________________________________________________________________
__________________________________________________________________________________[4]
b) How much work is done lifting all 3 weights onto the lorry? _________________________________
_________________________________________________________________________________[1]
c) If it takes Barry 100 seconds to load the lorry with all three weights what is his average power?
____________________________________________________________________________________
____________________________________________________________________________________
________________________________________________________________________________[4]
6) Look at the picture of the ball. It is stationary on the edge of a table
i) What is the gravitational potential energy of the ball? (take g as
10m/s2)
mass = 0.1Kg
__________________________________________________________
________________________________________________________[4]
1.5m
(ii) If the ball is stationary what is its kinetic energy?
_______________________________________________________[1]
(iii) Describe what happens to the potential and kinetic energy of the ball as it falls to the ground
_____________________________________________________________________________________
___________________________________________________________________________________[2]
(iv) What is the kinetic energy of the ball just as it hits the ground?
___________________________________________________________________________________[1]
7a (i) A car has a mass of 1000kg and a speed of 20m/s. What is its kinetic energy?
_____________________________________________________________________________________
___________________________________________________________________________________[4]
(ii) The brakes provide the force to stop the car. As the car stops what type of energy is the kinetic
energy converted to? __________________________________________________________ [1]
(iii) If the brakes provide a force of 5000N how far does the car travel while stopping? [4]
___________________________________________________________________________________
___________________________________________________________________________________
(b) Look at this data of the braking distance of a
car
Describe in detail how braking distance changes
with speed. Use the kinetic energy equation to
explain your description
____________________________________________
____________________________________________
____________________________________________
__________________________________________[3]
speed
braking distance
0
0
1
2
2
8
3
18
4
32
5
50