power, work done and efficiency (topic 1).

Question 1

explain why the work done on the box is greater than the increase in GPE store of the block (2)

Answer 1

• the block needs to overcome friction (or air resistance)
• energy is transferred to the surroundings (by mechanical work)

Question 2

describe the energy transfers that take place as the brakes slow the bicycle from 15 m/s to 6m/s (2)

Answer 2

• kinetic energy store of bicycle decreases
• thermal store of brakes/surroundings increases

Question 3

Answer 3

J

Question 4

Answer 4

2,848 J

Question 5

an electric motor has an input of 250 W, the efficiency of the motor is 49%, calculate the useful output power of the motor

Answer 5

Question 6

a student is using an electric motor to lift a mass off the ground, the motor operates at 12 V with a current of 20 mA, calculate the power of the motor

Answer 6

Question 7

calculate how long it takes for the motor to lift the 200 g mass to a height of 80 cm, give your answer to 1 decimal place (gravitational potential energy of the mass when it has been lifted through a distance of 80 cm = 1.57 J, efficiency = 0.14 J/s)

Answer 7

Question 8

the elastic potential energy stored in the bow before the archer released was 560 J, calculate the efficiency of conversion from elastic potential energy to kinetic energy, give your answer to 2 significant figures (kinetic energy of the arrow = 480J)

Answer 8