Dynamics

A man pulls a sledge of mass 25 kg across level ground with a horizontal force of 60 N. A constant force of friction of 20 N acts on the sledge. What is the acceleration of the sledge?

• 0.63 m/s²

• 1.6 m/s²

• 2.4 m/s²

• 3.2 m/s²

Answer explanation

A metal ball of mass 0.30 kg and weight 3.0 N is held so that it is below the surface of oil.

It experiences an upwards force of 0.30 N.

When the ball is released, what is its initial acceleration?

• 1.0 m/s²

• 9.0 m/s²

• 10 m/s²

• 11 m/s²

Answer explanation

An apple of mass 0.15 kg and weight 1.5 N falls from a tree. At one point during its fall, the air resistance on the apple is 0.60 N upwards. What is the acceleration of the apple at this point?

• 4.0 m/s²

• 6.0 m/s²

• 10 m/s²

• 14 m/s²

Which object has the largest resultant force?

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• Object A
• Object B
• object C

• Object D

In which example does friction act in the direction of forward motion of the object on which it acts?

• on a suitcase accelerating on a conveyor belt

• on sandpaper rubbing a wooden block

• on the skis of a skier accelerating downhill on smooth snow

• on the tyres of a braking car

In which situation is the resultant force on the body equal to zero?

• a car turning a corner at constant speed

• a rock falling freely on the Moon

• a train going up a straight hill at constant speed in a straight line

• an aircraft accelerating along a runway in a straight line

A body slides down a frictionless slope as shown. As the body presses on the surface, the surface pushes back on the body. The force of the surface on the body is sometimes called the reaction force. In which direction does the reaction force act?

• Direction A

• Direction B

• Direction C

• Direction D

Answer explanation

Normal reaction force is perpendicular to the inclined slope.

A car of mass 1500 kg travels along a horizontal road. It accelerates steadily from 10 m/s to 25 m/s in 5.0 s. What is the resultant force needed to produce this acceleration?

• 300 N

• 500 N

• 4500 N

• 7500 N

Answer explanation

Balanced forces are acting on a moving body. What happens to the direction of movement and to the speed of the body?

• A

• B

• C

• D

Answer explanation

A skier of mass 80 kg accelerates down a slope at 2 m/s². The gravitational field strength g is 10 N/kg.

What is the weight of the skier and the resultant force on the skier?

• A

• B

• C

• D