7) Energy, Work and Power

Question 1

Define work done.

Answer 1

Work done is the energy transferred to or from an object through the application of force along a distance in a straight line.

Question 2

Formula for work done

Answer 2

W = F x d

W: Work done (J)

F: force applied (N)

d: distance moved in the straight line along the direction of the force applied (m)

Question 3

What is the SI unit for work done?

Answer 3

joule (J)

Question 4

Define energy.

Answer 4

Energy is the ability to do work.

Question 5

What is kinetic energy?

Answer 5

Energy associated with the motion of a body.

Question 6

What is gravitational potential energy?

Answer 6

Energy associated with the position of a body in a gravitational field.

Question 7

What is the law of conservation of energy?

Answer 7

The law of conservation of energy states that energy can neither be created nor destroyed - only converted from one form of energy to another.

Question 8

Define power.

Answer 8

Power is the rate of doing work.

(Power is how fast energy is being converted from one form to another)

Question 9

Formula for power

Answer 9

P = E/t

P: power (W)

E: energy change / work done (J)

t: time taken (t)

Question 10

Formula for kinetic energy.

Answer 10

KE = 0.5mv²

KE: kinetic energy (J)

m: mass (kg)
v: velocity (m/s)

Question 11

Formula for gravitational potential energy.

Answer 11

GPE = mgh

m: mass (kg)
g: gravitational field strength (N/kg) - 10 N/kg on Earth
h: height above the ground (m)

Question 12

What is the formula linking the energy as the pendulum bob travels from the center to the right?

(Use the law of conservation of energy)

Answer 12

Loss in KE = Gain in GPE

0. 5mv² = mgh
   m: mass (kg)
   g: gravitational field strength (N/kg)
   h: height of object from lowest point (m)
   v: velocity (m/s)

Question 13

What is the formula linking the energy as the pendulum bob travels from the left to the center?

(Use the law of conservation of energy)

Answer 13

Loss in GPE = Gain in KE

mgh = 0.5mv²

m: mass (kg)
g: gravitational field strength (N/kg)
h: height of object from lowest point (m)
v: velocity (m/s)

Question 14

What is the formula linking the energy between when the car is moving and when it brakes to a stop?

(Use the law of conservation of energy)

Answer 14

Loss in KE = work done against friction

0. 5mv² = F_fd
   m: mass (kg)
   v: velocity (m/s)

F_f: frictional force (N)

d: distance moved when frictional force is acting (m)