Kinetic Energy

Kinetic energy

Ek – the energy an object has because it is moving.

Kinetic energy

Ek – the energy an object has because it is moving.

The unit for kinetic energy is joule (J)

Kinetic energy

Ek – the energy an object has because it is moving.

The unit for kinetic energy is joule (J)

Ek = ½ mv2

Kinetic energy

Ek – the energy an object has because it is moving.

The unit for kinetic energy is joule (J)

Ek = ½ mv2 m = mass in kg

v = speed in ms-1

Kinetic energy and mass

The greater the mass of a moving object, the greater the kinetic energy of the object.

Kinetic energy and mass

The greater the mass of a moving object, the greater the kinetic energy of the object.

If the mass of an object doubles, and the speed remains the same, the kinetic energy will double too.

Question

1. Calculate the kinetic energy of the truck shown below:

Mass = Speed = 5ms-1

1000kg

Solution

Ek = ½ mv2

½ x 1000 x 52 = 12500 J

Kinetic energy and speed

The faster an object moves, the greater its kinetic energy.

Kinetic energy and speed

The faster an object moves, the greater its kinetic energy.

When the speed of an object doubles, and the mass remains the same, the kinetic energy is four times what it was.

Kinetic energy and speed

The faster an object moves, the greater its kinetic energy.

When the speed of an object doubles, and the mass remains the same, the kinetic energy is four times what it was.

This is because the amount of kinetic energy is related to the square of the speed.

Kinetic energy and speed

So, if the speed doubles, Ek increases to 4 times its original value (22 = 4)

Kinetic energy and speed

So, if the speed doubles, Ek increases to 4 times its original value (22 = 4)

If the speed increases 3 times, what’s the kinetic energy increase to?

Kinetic energy and speed

So, if the speed doubles, Ek increases to 4 times its original value (22 = 4)

If the speed increases 3 times, what’s the kinetic energy increase to?

9 times original value (32 = 9)

Question

There is a car travelling at 3 different speeds below. The car and its driver has a mass of 1000kg. Calculate the kinetic energy for each of the speeds.

1ms-1 2ms-1

3ms-1

Kinetic energy and speed

This is important in car safety – if a car doubles its speed, there is 4 times the amount of kinetic energy to lose before the car is stationary.

Kinetic energy and speed

This is important in car safety – if a car doubles its speed, there is 4 times the amount of kinetic energy to lose before the car is stationary.

= a car travelling 100km/h has four times the distance to stop than if it was travelling at 50km/h

Kinetic energy and speed

Ek

½ m v2

Kinetic energy and speed

From the triangle before, to calculate mass, use 1/2m = Ek then m =2 x ½ m

v2

Kinetic energy and speed

From the triangle before, to calculate mass, use 1/2m = Ek then m =2 x ½ m

v2

To calculate speed, use v2 = Ek then v =

½ m

Try these

Two cars travelling down the road both have a 30000J of kinetic energy.1. One car is travelling at 25ms-1 (90km/h).

Calculate its mass. 2. The other car has a mass of 1000kg.

Calculate its speed.

Answers

1. 1/2m = Ek

v2 300’000 = 480 252

then m =2 x ½ m

m = 2 x 480 = 960kg

Answers

2. v2 = Ek 300’000 = 600

½ m ½ x 1000

then v = = = 24ms-1

Questions

At an athletics meet, the winner of the men’s 100m sprint crossed the line running at a speed of 10ms-1. If his mass was 80kg calculate his kinetic energy.