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Energy formulas- guess and match
Heat energy Kinetic Energy (motion) Gravitational Potential
Energy Photon Energy (EM
radiation)
Transport Engineering 1
Physics on the Road
Lesson 12
LI… Investigate stopping distances Use graphs to analyse stopping distances
Stopping distances - revision
Stopping distances – made up of 2 parts
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
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
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
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 distanceWork done is energy and measured in Joules (J)
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 energyby brakes of vehicle
Fd = ½ mv2
For a braking vehicle
F - the brake forced – braking distancem- mass of the vehicleV – is the speed of the vehicle
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 tFt = m x v-u x t
tFt = mv - mu
Ft=Δp
a=v-u/t
Force x time = change in momentum