Forces

Newton’s third law refers to two forces acting in a pair.These forces

• act in the same direction

• act on different objects.

• are different types of force

• have different magnitudes.

A brick of mass 5.0 kg falls through water with an acceleration of 0.90 m s^–2.Which of the following can be used to calculate the resistive force acting on the brick?

• 5.0 × (0.90 – 9.81)

• 5.0 × (0.90 + 9.81)

• 5.0 × 0.90

• 5.0 × 9.81

A box is pulled along the floor using two identical ropes. The tensions in the ropes act in the directions shown and have magnitudes T_P and T_Q. The box moves at a constant speed in the direction shown.The magnitude of the frictional force acting on the box is equal to

• T_P sinθ + T_Q sin2θ

• T_P cosθ + T_Q cos2θ

• T_P + T_Q

• zero

Which of the following is the unit of upthrust?

• N m⁻²

• N m⁻¹

• N m

• N

A small stone of mass m is dropped into a pond and accelerates downwards with an acceleration a. The free-body force diagram for the stone is shown.

• U + D – mg = 0

• U + D – mg = ma

• mg – U − D = 0

• mg – U − D = ma

An object of weight W is on a slope at an angle θ to the horizontal as shown. The normal contact force is R. As θ increases, R will

• decrease because R = Wcosθ.

• decrease because R = Wsinθ.

• increase because R = Wcosθ.

• increase because R = Wsinθ.

A car pulls a caravan at a slow but increasing velocity along a horizontal road.The four forces acting on the car are weight W, reaction force R of the road on the car, tension T in the tow‑bar and friction F between the car tyres and the road. Which of the following could be the free‑body force diagram for the car?

• 
• 
• 
• 

A person stands on some bathroom scales in a stationary lift. The lift begins to move upwards with a constant acceleration. The reading on the scales will

• increase but then remain constant.

• increase at a constant rate.

• decrease but then remain constant.

• decrease at a constant rate.

A child throws a ball vertically upwards and then catches it when it falls back down. When the ball is caught, the ball exerts a force on the child’s hand.According to Newton’s third law there will also be

• a downwards force of the ball on the hand.

• a downwards force of the hand on the ball

• an upwards force of the ball on the hand.

• an upwards force of the hand on the ball.

A free-body force diagram can be used to represent the forces acting on an object. Which of the following should not appear on the free-body force diagram for a block sliding down a slope?

• normal reaction of ground

• mass × gravitational field strength

• friction

• acceleration of block