work done

A labourer on a building site lifts a heavy concrete block onto a lorry. He then lifts a light block the same distance in the same time. Which of the following is true?

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A small emergency generator supplies 432 000 000J of electrical energy in twenty-four hours. What is the average power output of the generator?

• 5000W

• 300 000W

• 18 000 000W

• 432 000 000W

A crane lifts a weight of 1000N through a vertical height of 30m. It uses 60 000J of energy. What is the efficiency of the crane?

• 20%

• 30%

• 50%

• 200%

The efficiency of an electrical generator is 65%. The energy input to the generator is 12kJ. What is the useful energy output?

• 4.2kJ

• 5.4kJ

• 7.8kJ

• 780kJ

What is efficiency?

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• 
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The diagram shows a girl lifting a box of weight 50N from a low shelf to a high shelf.

How much work is done on the box?

• 30 J

• 60 J

• 90J

• 120 J

A crane lifts a load of 6000N through a vertical distance of 15m in 30s. What is the average useful power during this operation?

• 200W

• 400W

• 3000W

• 12000W

The input power to a lamp is 6.0W. The lamp wastes 2.7J of energy in 3.0s. What is the efficiency of the lamp?

• 0.15

• 0.45

• 0.55

• 0.85

A 1000W, 240V electrical device is switched on. How much energy does it use in 5.0 minutes?

• 5000 J

• 12 000J

• 72 000J

• 300 000J

In a weightlifting contest, an athlete lifts a metal bar of mass 20kg fitted with a mass of 80 kg on each end. The lift from ground to a height of 2.5m takes 0.50 s.

The gravitational field strength g is 10 N/ kg. What average power does the weightlifter exert in providing the gravitational potential energy during this lift?

• 0.90 kW

• 5.0 kW

• 8.0 kW

• 9.0 kW