DSE Physics: Force and Motion — Practice Questions & Answers

The output power of a machine is the useful

• work done by the machine per unit time
• total energy stored in the machine
• force exerted by the machine
• time taken by the machine

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An object of mass 0.5 kg is raised vertically 2.5 m at uniform speed in 1.5 s by a motor. Neglecting air resistance and using g = 9.81 m s^-2, the output power of the motor is about

• 8.2 W
• 5.5 W
• 11.0 W
• 16.4 W

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A motor of output power 300 W raises a load at a steady speed of 0.50 m s^-1. Using g = 10 m s^-2, the mass of the load is (P = mgv)

• 60 kg
• 6 kg
• 600 kg
• 150 kg

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Power equals work done divided by the taken.

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Answer: time

A motor raises a 6.0 kg mass 3.0 m in 2.0 s at constant speed (g = 10 m s^-2). Its output power is W.

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Answer: 90 / 90.0

A crane with output power 4.0 kW lifts a load at a steady 0.80 m s^-1. The lifting force is N.

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Answer: 5000

A block resting on a 30 degree incline is given a slight push and then slides down with uniform speed. There is

• no net force acting on the block
• a net force down the incline
• a net force up the incline
• a net force equal to its weight

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A block of mass m slides down a 30 degree incline at uniform speed. The frictional force acting on the block is

• 0.5 mg
• 0.87 mg
• mg
• 2 mg

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A block slides down a rough 30 degree incline at uniform speed. If it is instead given a greater initial speed down the same incline (kinetic friction unchanged), it will

• continue to slide down at uniform speed
• slide down with acceleration
• decelerate to rest
• slide back up the incline

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A body moving with uniform velocity experiences zero force.

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Answer: net / resultant