Potential and kinetic energy

Question 1

How do you calculate change in gravitational potential energy?

Answer 1

change in GPE (J) = m x g x change in vertical h (m)

Question 2

What is gravitational potential energy?

Answer 2

the energy possessed by a body due to its height above Earth

Question 3

What does gravitational energy depend on?

Answer 3

• the mass of the body
• the gravitational field strength
• the height the body is raised

Question 4

How do you calculate gravitational potential energy?

Answer 4

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

Question 5

What is the gravitational field strength on Earth?

Answer 5

10 N/kg

can also be given in m/s²

Question 6

What is kinetic energy stored in?

Answer 6

moving objects

Question 7

How is kinetic energy calculated?

Answer 7

kinetic energy (J) = 1/2 x mass (kg) x speed² (m/s)²

Question 8

What is kinetic energy directly proportional to?

Answer 8

the mass of the moving object

(doubling the mass doubles the kinetic energy)

it is also directly proportional to the square of the speed

(doubling the speed increases the KE by a factor of 4)

Question 9

A mass of 800 g is moving at 14 m/s. Calculate its kinetic energy.

Answer 9

KE = 1/2 x m x v²

KE = 1/2 x 0.8 x 14² = 78.4 J

Question 10

A motorbike of mass 80 kg is moving at 30 km/h. Calculate its kinetic energy.

Answer 10

30 km/h = 30,000 m / 3600s = 8.33m/s

KE = 1/2 x 80 kg x 8.33m/s = 2778 J

Question 11

A body of mass 4.8 kg has kinetic energy of 200 J. Calculate the speed it is moving at.

Answer 11

200 J = 1/2 x 4.8 x v²

v = √400 / 4.8 = 9.1 m/s

Question 12

a) A body of mass 73 kg is lifted through a vertical height of 26 m. Calculate how much gravitational potential energy it gains.
b) The body is now dropped from 26 m above the ground. At what speed will it hit the floor?

Answer 12

a) ∆GPE = m x g x ∆h
= 73 kg x 10 N/kg x 26 m = 18,980 J

b) loss in GPE = gain in KE
(conservation of energy)

18,980 J = 1/2 x 73 kg x v²

v² = 520 (m/s)²

v = √520 = 23 m/s

Question 13

A mass of 5 kg is raised through a vertical height of 18 m. Calculate the change in gravitational potential energy.

Answer 13

∆GPE = m x g x ∆h = 5 kg x 10 N/kg x 18 m = 900 J

Question 14

A ball of mass 3 kg falls from 34 m above the ground. Calculate its speed when it lands.

Answer 14

KE = ∆GPE

1/2x m x v² = m x g x ∆h, so v² = 2 x g x ∆h

2 x 10 N/kg x 34 m

v = √680 = 26 m/s