6. Plotting graph - data
Speed km/h Speed m/s Reaction
distance/ m
Braking
distance /m
Stopping
distance /m
50 21 21
60 25 31
70 29 42
80 33 55
90 37 70
100 42 85
110 46 104
Plot a graph of reaction distance against speed and braking
distance against speed
7. Analysing the graphs
1. Use the gradient of the first graph and v=s/t to calculate the
reaction time used. Draw a conclusion.
2. What do you notice about the speed against braking distance
graph? Plot a new graph to investigate.
3. Use the equations of motion and the table below to plot a graph
of braking time against speed. What do you notice?
Initial speed (u)
m/s
u2 s a t
8. LI…
Use ideas of work done, momentum and
kinetic energy to explain vehicle motion
Use W=Fd (or E=Fd), p=mv and Ek= ½ mv2
9. Braking – what happens
A moving object has
kinetic energy
A stationary object has
none
Brakes apply a force on
the wheels
The brakes use
frictional forces
Brake discs and shoes
heat up
KE
Force
Braking distance
Work done = force x distance
Work done is energy and
measured in Joules (J)
10. Work done and kinetic energy
Brakes do work (apply
a force over a distance)
to transfer the kinetic
energy of the vehicle.
1. Explain your speed
braking distance
graphs using these
ideas
2. A goods train has
mass of 2400 tonnes
and travels at
100km/h. Calculate
it’s kinetic energy. It
takes 1 ½ km to
stop. Calculate the
force of it’s brakes.
Work done = kinetic energy
by brakes of vehicle
Fd = ½ mv2
For a braking vehicle
F - the brake force
d – braking distance
m- mass of the vehicle
V – is the speed of the vehicle
11. Momentum
Momentum of an object
depends upon it’s mass and
velocity.
As a vehicle brakes it’s
velocity and so it’s
momentum is reduced over
time
So when a car brakes the
loss of momentum is the
braking force applied over
time.
Force x time = mass x acceleration x time
momentum = mass x velocity p=mv
F x t= m x a x t
Ft = m x v-u x t
t
Ft = mv - mu
Ft=Δp
a=v-u/t
Force x time = change in momentum
