Work, Energy And Momentum

Question 1

What is work?

Answer 1

Work done on an object is when a force makes the object move.

Question 2

What is the relationship between work and energy?

Answer 2

Energy transferred = work done

Question 3

How do we calculate the work done?

Answer 3

Work done (J) = force applied (N) x distance moved in the direction of the force (m)

Question 4

What happens to the work done to overcome friction?

Answer 4

Work done to overcome friction is transferred as energy that heats objects that rub together and the surroundings.

Question 5

What is gravitational potential energy?

Answer 5

Energy stored in an object because of its position in the Earth’s gravitational field.

Question 6

How do we work out the change of the gravitational potential energy?

Answer 6

Change of gravitational potential energy (J) = weight (N) x change of height (m)

Question 7

What does the gravitational potential energy of an object depend on?

Answer 7

• it’s weight

* how far it is moved vertically

Question 8

What happens to the gravitational potential energy of an object when it moves up or down?

Answer 8

The gravitational potential energy of an object increases when the object goes up and decreases when the object goes down.

Question 9

How can we calculate the change of gravitational potential energy of an object when it moves up or down?

Answer 9

Change of gravitational potential energy (J) = mass (kg) x gravitational field strength (N/kg) x change of height (m)

Question 10

What is power?

Answer 10

The energy transferred per second.

Question 11

How do we calculate the power?

Answer 11

Power (W) = energy (J) / time (s)

Question 12

What does the kinetic energy of an object depend on?

Answer 12

• mass

* speed

Question 13

How can we calculate kinetic energy?

Answer 13

Kinetic energy (J) = 1/2 x mass (kg) x speed2 (m/s2)

Question 14

What do kinetic energy recovery systems (KERS) do?

Answer 14

They store energy in vehicles when the vehicle brakes and use it later.

Question 15

What so elastic potential energy?

Answer 15

The energy stored in an elastic object when work is done on it to change its shape.

Question 16

How can we calculate momentum?

Answer 16

Momentum (kg m/s) = mass (kg) x velocity (m/s)

Question 17

What happens to the total momentum of two objects when they collide?

Answer 17

Momentum is conserved whenever objects interact, provided the objects are in a closed system so that no external forces act on them.

Question 18

What is the law of conservation of momentum?

Answer 18

In a closed system, the total momentum before an event is equal to the total momentum after the event.

Question 19

Why does momentum have a direction as well as size?

Answer 19

Momentum is mass x velocity and velocity is speed in a certain direction.

Question 20

What happens when two objects push each other apart?

Answer 20

They move apart:

• with different speeds if they have unequal masses.
• with equal and opposite momentum so their total momentum is zero.

Question 21

When vehicles collide, what does the force of the impact depend on?

Answer 21

The force of the impact depends on mass, change of velocity and the duration of the impact.

Question 22

How does the impact force depend on the impact time?

Answer 22

The longer the impact time is, the more the impact force is reduced.

Question 23

What can we say about the impact forces and the total momentum when two vehicles collide?

Answer 23

• they exert equal an opposite forces on each other

* their total momentum is unchanged

Question 24

Why do seat belts and air bags reduce the force on people in car accidents?

Answer 24

Seat belts and air bags spread the force across the chest and they also increase the impact time.

Question 25

How do side impact bars and crumple zones work?

Answer 25

They give way in an impact so increasing the impact time.

Question 26

How can we work out if a car in a crash was ‘speeding’?

Answer 26

We can use the conservation of momentum to find the speed of a car before an impact.