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