9.1 Types of Forces

Which of the following descriptions applies to Newton's Third law?

   A   Forces are of equal size and act in the same direction

   B   Forces are of equal size and act in opposing directions

   C   Forces are unequal in size and act in the same direction

   D   Forces are unequal in size and act in opposing directions

Higher Tier Only

In a physics class, the students are asked to give an example of a force pair in accordance with Newton's third law of motion.

One student gives the following example

'The weight of Sam is opposed by the normal contact force of Sam's chair.'

State whether you agree that this is force pair. Explain your answer.

Higher Tier Only

While Sam is sitting still on their chair, Sam is in equilibrium.

Explain what is meant by the term 'equilibrium'.

Use ideas about resultant forces in your answer.

Higher Tier Only

A toy cart with a mass of 35 g rolls down an incline as shown in Figure 1.

Figure 1

The cart travels at a constant speed.

Explain why the cart does not accelerate.

Higher Tier Only

The angle of the incline is increased. The cart now accelerates.

Explain why the cart now accelerates.

The resultant force on the cart is 0.65 N.

Calculate the acceleration of the cart.

The acceleration of the cart can be increased by reducing the frictional force.

Suggest a way to reduce the force of friction acting on the cart.

Higher Tier Only

Figure 1 shows a man pulling a child on a sledge. 

Figure 1

The mass of the child and the sledge is 42 kg. 

The unbalanced force acting on the sledge is 46 N.

What type of force opposes the motion of the sledge?

   A   Non-contact, friction

   B   Contact, friction

   C   Non-contact, normal reaction

   D   Contact, normal reaction

The sledge is then placed at the top of a hill and accelerates down the hill at 0.75 m/s² 

Calculate the resultant force acting on the sledge.

Give the unit.

Higher Tier Only

Assuming that the force opposing the motion of the sledge is the same, calculate the total force acting down the hill.

You may include a diagram in your answer.

Higher Tier Only

A student throws a tennis ball vertically upwards and catches it as it returns to her.

Figure 1 shows the motion of the ball whilst it is in the air. 

Figure 1

Explain the motion of the ball as it moves from position A to position B.

You should include ideas about Newton's second law of motion in your answer.

Higher Only

The magnitude and direction of a force can be represented by a vector.
Figure 22 shows the forces acting on four identical trolleys. The arrows show the magnitude and direction of the forces.

Which diagram shows a pair of forces that will produce zero acceleration?

Figure 22

Higher Only

Figure 23 shows two small boats pulling a much larger ship.

Figure 23

The ship is connected to the boats with ropes.
The tension, T, in each of the ropes has a magnitude of 20 kN.
The ropes are at right angles to each other.

Draw a vector diagram and use it to determine the resultant force that the boats exert on the ship.

magnitude of resultant force on the ship = .............................. kN

Higher Only

Figure 24 shows a wooden block connected to a weight by a string.
The string goes over a pulley.

The surfaces of the table and the wooden block are both rough.

Figure 24

The wooden block moves across the table at a constant horizontal velocity.
Several vertical and horizontal forces act on the wooden block as it moves.

Explain how the forces keep the wooden block moving across the table at a constant horizontal velocity.
Your answer should refer to all forces acting on the wooden block.
You may add to the diagram to help with your answer.