Download Document

GCSE Forces and
Motion
W Richards Worthing High School
Edited by J. Catchpole, Hockerill AEC
Distance, Speed
and Time
D
Speed = distance (in metres)
time (in seconds)
S
T
1) Dave walks 200 metres in 40 seconds. What is his speed?
2) Laura covers 2km in 1,000 seconds. What is her speed?
3) How long would it take to run 100 metres if you run at 10m/s?
4) Steve travels at 50m/s for 20s. How far does he go?
5) Susan drives her car at 85mph (about 40m/s). How long does it
take her to drive 20km?
Distance-time graphs
2) Horizontal line =
40
4) Diagonal line
downwards =
30
Distance
(metres)
20
10
0
Time/s
20
1) Diagonal line =
40
60
80
100
3) Steeper diagonal line =
40
Distance
(metres)
30
20
10
0
Time/s
20
40
60
80
100
1) What is the speed during the first 20 seconds?
2) How far is the object from the start after 60 seconds?
3) What is the speed during the last 40 seconds?
4) When was the object travelling the fastest?
Speed vs. Velocity
Speed is simply how fast you are travelling…
This car is travelling at a
speed of 20m/s
Velocity is “speed in a given direction”…
This car is travelling at a
velocity of 20m/s east
Acceleration
V-U
Acceleration = change in velocity (in m/s)
(in m/s2)
time taken (in s)
A
T
1) A cyclist accelerates from 0 to 10m/s in 5 seconds. What is her
acceleration?
2) A ball is dropped and accelerates downwards at a rate of 10m/s2
for 12 seconds. How much will the ball’s velocity increase by?
3) A car accelerates from 10 to 20m/s with an acceleration of 2m/s2.
How long did this take?
4) A rocket accelerates from 1,000m/s to 5,000m/s in 2 seconds.
What is its acceleration?
Velocity-time graphs
1) Upwards line =
80
Velocity
m/s
4) Downward line =
60
40
20
0
10
2) Horizontal line =
20
30
40
50
3) Steeper line =
T/s
80
60
Velocity
m/s
40
20
0
T/s
10
20
30
40
1) How fast was the object going after 10 seconds?
2) What is the acceleration from 20 to 30 seconds?
3) What was the deceleration from 30 to 50s?
4) How far did the object travel altogether?
50
Balanced and unbalanced forces
Consider a camel standing on a road.
What forces are acting on it?
Reaction
These two forces would be equal –
we say that they are BALANCED.
The camel doesn’t move anywhere.
Weight
Balanced and unbalanced forces
Reaction
What would happen if we took the
road away?
Weight
Balanced and unbalanced forces
What would happen if we took the
road away?
The camel’s weight is no longer
balanced by anything, so the camel
falls downwards…
Weight
Balanced and unbalanced forces
What would happen if we took the
road away?
The camel’s weight is no longer
balanced by anything, so the camel
falls downwards…
Balanced and unbalanced forces
1) This animal is either
________ or moving
with _____ _____…
3) This animal is getting
_______….
2) This animal is getting
_________…
4) This animal is…
Balanced and unbalanced forces
What is happening
to these cars?
Force and acceleration
If the forces acting on an object
are unbalanced then the object will
accelerate, like these wrestlers:
Force (in N) = Mass (in kg) x Acceleration (in m/s2)
F
M
A
Force, mass and acceleration
1) A force of 1000N is applied to push
a mass of 500kg. How quickly does
it accelerate?
F
2) A force of 3000N acts on a car to
make it accelerate by 1.5m/s2. How
heavy is the car?
3) A car accelerates at a rate of
5m/s2. If it weighs 500kg how
much driving force is the engine
applying?
4) A force of 10N is applied by a boy
while lifting a 20kg mass. How
much does it accelerate by?
M
A
Terminal Velocity
Consider a skydiver:
1) At the start of his jump the air
resistance is _______ so he
_______ downwards.
2) As his speed increases his air
resistance will _______
3) Eventually the air resistance will be
big enough to _______ the
skydiver’s weight. At this point
the forces are balanced so his
speed becomes ________ - this is
called TERMINAL VELOCITY
Terminal Velocity
Now he opens his parachute:
4) When he opens his parachute the
air resistance suddenly ________,
causing him to start _____ ____.
5) Because he is slowing down his air
resistance will _______ again until
it balances his _________. The
skydiver has now reached a new,
lower ________ _______.
Velocity-time graph for terminal velocity…
Parachute opens –
diver slows down
Velocity
Speed
increases…
Terminal
velocity
reached…
Time
New, lower terminal
velocity reached
Diver hits the ground
Newton’s Laws of Motion
These are my three laws of
motion (summarised):
1) If an unbalanced force acts on an
object that object will either accelerate
or change direction:
F
2) That force is given by F=ma
3) When a force acts on an object
there is an equal force acting in
the opposite direction (“Action and
reaction are equal and opposite”)
M
A
Newton’s 2nd Law
Instead of F=ma Newton actually said that the force acting on
an object is that object’s rate of change of momentum. In
other words…
mv
Force = Change in momentum (in kgm/s)
(in N)
Time (in s)
F
T
For example, David Beckham takes a free kick by kicking a stationary
football with a force of 40N. If the ball has a mass of 0.5kg and his
foot is in contact with the ball for 0.1s calculate:
1) The change in momentum of the ball,
2) The speed the ball moves away with
Example questions
1) A golfer strikes a golf ball with a force of 80N. If the ball
has a mass of 200g and the club is in contact with it for
0.2s calculate a) the change in momentum of the golf ball,
b) its speed.
2) A tennis player strikes a serve with a force of 30N. If the
ball has a mass of 250g and the racket is in contact with it
for 0.15s calculate the ball’s change in momentum and its
speed.
3) A footballer takes a goal kick by kicking a 0.4kg football
away at 10m/s. If his foot was in contact with the ball for
0.1 seconds calculate the force he applied to the ball.
4) A golfer strikes a 200g golf ball away at 50m/s. If he
applied a force of 50N calculate how long his club was in
contact with the ball for.
Friction
1) What is friction?
2) Give 3 examples where it is annoying:
3) Give 3 examples where it is useful:
4) What effect does friction have on the surfaces?
Stopping a car…
Tiredness
Too many
drugs
Thinking
distance
Poor
visibility
Wet roads
Icy roads
Tyres/brakes
worn out
Too much
alcohol
Stopping
distance
Driving too
fast