Energy

Question 1

What is the equation to work out kinetic energy?

Answer 1

kinetic energy (J) = 1/2 x mass (kg) x velocity² (ms^-1)

E_k = 1/2 x m x v²

Question 2

Why does a falling object gain kinetic energy as its speed increases?

Answer 2

It gains energy from the gravitational potential energy lost.

Question 3

What is the equation to work out gravitational potential energy?

Answer 3

gravitational potential energy (J) = mass (kg) x gravitational field strength (Nkg^-1) x height (m)

E_p = m x g x h

Question 4

How could you work out the kinetic energy gained for a falling object?

Answer 4

kinetic energy gained = gravitational potential energy lost

Question 5

How could you work out the gravitational potential energy gained for an object thrown or catapulted upwards?

Answer 5

gravitational potential energy gained = kinetic energy lost

Question 6

An apple of mass 0.1kg falls from a tree of height 2m. With what speed does it hit the ground?

Answer 6

1. E_p lost = mgh = 0.1 x 9.8 x 2 = 1.96J
2. E_p lost = E_k gained
3. E_k = 1.96J = 1/2 x m x v² = 1/2 x 0.1 x v²
4. 1.96 ÷ (1/2 x 0.1) = v² = 39.2
5. v = √39.2 = 6.26ms^-1 (2d.p)

Question 7

A ball of mass 0.2kg is thrown upwards at 10ms^-1. How high does it get?

Answer 7

1. E_k lost = 1/2mv² = 1/2 x 0.2 x 10² = 10J
2. E_k lost = E_p gained = 10J
3. 10J = mgh = 0.2 x 9.8 x h
4. h = 10 ÷ (0.2 x 9.8) = 5.10m (2d.p.)

Question 8

What equation links work, force and distance?

Answer 8

work done (J) = force (N) x distance moved (m)

W = F x d

Question 9

A 10N force to the north pushes an object 15m in a north-easterly direction. What is the work done?

Answer 9

1. cosθ = adjacent ÷ hypotenuse
2. cos45 = d ÷ 15
3. 15cos45 = d = 10.6m
4. work done = force x distance = 10 x 10.6 = 106J

Question 10

What are the three possible situations of where the energy in work done goes to?

Answer 10

1. It can go entirely into the gravitational potential energy of an object.
2. It can go entirely into the kinetic energy of an object.
3. It can go into both the kinetic and gravitational potential energy of an object.

work done = increase in E_k + increase in E_p

F x d = 1/2mv² + mgh

Question 11

What equation links power, work done and time taken?

Answer 11

power (W) = work done (J) ÷ time taken (s)

P = W ÷ t

Question 12

How can one work out the power of a moving object?

Answer 12

1. power = work done ÷ time
2. work done = force x distance
3. power = (force x distance) ÷ time = force x (distance ÷ time)
4. distance ÷ time = velocity
5. power = force x velocity

Question 13

How do you work out the efficiency of a system?

Answer 13

efficiency(%) = (useful energy out ÷ total energy in) x 100