P2.2 The Kinetic Energy of Objects

Question 1

What is meant by ‘work done’ in physics?

Answer 1

When a forces moves an object through a distance, energy is transferred and work is done

Question 2

Can you explain ‘work done’ in more detail? (4)

Answer 2

• If something moves, something else is providing some sort of ‘effort’ to move it
• The thing putting in the effort needs a supply of energy (e.g. food, fuel)
• The thing does ‘work’ by moving the object - and transfers the energy it receives as fuel into other forms
• The energy transferred can be useful or wasted, but it is still work done

Question 3

What is ‘work done’ measured in?

Answer 3

It is equal to energy transferred and is measured in Joules

Question 4

What is the formula to work out ‘work done’?

Answer 4

W = F x d
W = work done (J)
F = force (N)
d = distance (m)

Question 5

What is gravitational potential energy?

Answer 5

The energy an object has due to it’s vertical position in a gravitational field

Question 6

What happens to the GPE when an object is lifted up from the ground?

Answer 6

Work is done against the form of gravity and the object gains GPE

Question 7

What is formula to work out GPE?

Answer 7

GPE (J) = mass (kg) x gravitational field strength (N/kg) x height (m)

Question 8

Anything that is moving has kinetic energy, what is the formula to find kinetic energy?

Answer 8

KE = 1/2 x mass (kg) x speed² (m/s)

Question 9

What does kinetic energy depend on?

Answer 9

The mass and speed of an object

Question 10

What happens when the brakes are pressed in a car?

Answer 10

When a car brakes its kinetic energy is transferred into heat energy by friction between the brake pads and the wheels, and work is done

Question 11

Work done by brakes = energy transferred, what does this mean?

Answer 11

1/2 x mass x speed² = braking force x braking distance (or F x d)

Question 12

What is an objects GPE converted to when it falls?

Answer 12

Kinetic energy

Question 13

What is the formula to show the transfer of energy when an object is falling? (GPE = KE)

Answer 13

Mass x gravitational field strength x height = 1/2 x mass x max speed²

Question 14

What other forms of energy are transferred from a falling object?

Answer 14

Sound and heat - heat is transferred due to the friction between the object and the particles in the air

Question 15

What is power?

Answer 15

The rate of doing work - how much per second

Question 16

What is meant powerful machine?

Answer 16

One which transfers a lot of energy in a short period of time

Question 17

What is the formula for power?

Answer 17

Power (watts) = work done or energy transferred (J) ÷ time taken (seconds)

Question 18

What is one watt equal to?

Answer 18

1 Joule of energy transferred

Question 19

How can you work out the power output of a person?

Answer 19

You may have to use other equations to find the energy transferred, but it is possible

Question 20

What is momentum?

Answer 20

A property of moving objects

Question 21

What is the difference between momentum and speed?

Answer 21

Momentum has a size and direction (unlike speed)

Question 22

What is the equation for momentum?

Answer 22

Momentum (kg m/s [direction]) = mass (kg) x velocity (m/s)

Question 23

What is conservation of momentum?

Answer 23

The total momentum before an event (like a collision or explosion) is the same as after the event

Question 24

What different types of collisions are there?

Answer 24

Objects bumping into each other and sticking or objects bouncing off of each other

Question 25

What happens when a force acts on an object?

Answer 25

There is a change in momentum

Question 26

How do traditional brakes work?

Answer 26

The brakes transfer kinetic energy (of the car) into heat and sound energy

Question 27

How do regenerative brakes work?

Answer 27

In some electric and hybrid cars, instead of coverting the KE into heat energy the brakes put the vehicle’s motor in reverse - which slows the wheels

Question 28

What is a benefit of regenerative brakes?

Answer 28

The motor converts the kinetic energy into electrical energy that is stored as chemical energy in the motor - thus storing the energy of braking rather than wasting it

Question 29

What happens when a car crashes?

Answer 29

It slows down very quickly - this means that a lot of KE is converted into other forms of energy in a short amount of time which could be fatal for the people inside

Question 30

Where are crumple zones on a car found?

Answer 30

At the front and back of the car

Question 31

How do crumple zones work?

Answer 31

They crumple on impact - crumple zones increase the impact time, decreasing the force produced by the change in momentum

Question 32

What are side impact bars?

Answer 32

strong metal tubes fitted into car door panels

Question 33

How do side impact bars help to reduce the force on the passengers?

Answer 33

They direct the KE of the crash towards other areas of the car away from the passengers and towards areas like crumple zones

Question 34

How do seatbelts help reduce the force on the passengers?

Answer 34

Not only do they stop passengers from hitting hard surfaces in the car or being thrown out the windscreen - but they also stretch slightly increasing the time taken for the wearer to stop, which reduces the force acting on the chest

Question 35

How do airbags help to reduce the force on the passengers?

Answer 35

They slow you down more gradually and prevent you from hitting hard surfaces in the car