5.5 Newton's Laws of Motion

Sam is a veterinarian at a zoo. They are performing routine health inspections on the turtles and the rhinos today.

The animals must be sedate for these inspections and are pulled out of their enclosures on wheeled trolleys, as shown in Figure 1.

Figure 1

Using the concept of inertia, explain why Sam can move the turtle on their own but three zookeepers are needed to move the rhino.

Sam exerts a force  on the trolley. Following Newton's third law, the trolley exerts an equal force in the opposite direction. 

Explain how Sam causes the trolley to move forwards despite this, referring to the forces on Sam and the forces on the cart. 

After the inspection, when Sam isn't looking the turtle rides the trolley down a hill (see Figure 2). A frictional force acts uphill.

Figure 2

Explain why the turtle accelerates down the incline of the hill.

Higher Only

A student places a trolley on an incline, as in Figure 1. Three masses are attached to the trolley via a string which passes over a pulley.

Figure 1

Ignore the effects of friction. 

Describe the origins of forces A and B.

Force B can be described as , where  is the weight of the trolley.

is the total mass of the three weights on the end of the string.

The system accelerates to the right with acceleration .

Using Newton's second law, derive the following expression for the system:

,

where  is the acceleration due to gravity.

Each mass on the end of the string is 100 g and the weight of the trolley is 5.0 N.

Calculate the acceleration of the system.

Give your answer to 2 significant figures.

A student draws Newton’s third law pair of force on a book, as shown in Figure 1.

Figure 1

F_g is the gravitational force and F_N is the normal reaction force.

State whether the student has drawn a Newton’s third law pair of forces correctly. 

Explain your answer.

A child balances on a pogo stick as shown in Figure 2. The child and the pogo stick are stationary.

Figure 2

Draw a Newton’s Third law force pair on Figure 2

Using Newton’s Third law, explain how the forces on a person’s foot enables them to walk on the ground.

Using Newton’s First law, explain why a comet moves in a straight line at constant speed whilst travelling in space.

Figure 1 shows the forces on a car that is travelling to the right.

Figure 1

Calculate the resultant force on the car. State an appropriate unit for your answer.

The car initially accelerates at 0.25 m/s².

Calculate the mass of the car.

The car eventually reaches a top speed which it cannot go any faster.

State the acceleration of the car at this point. Explain your answer.

Higher Only

All objects have inertia.

Which of Newton’s Laws of motion is also called ‘the law of inertia’. Explain your reasoning.

Higher Only

A trolley accelerates at 3.8 m/s² when a force of 5.0 N is applied to it.

Calculate the inertial mass of the trolley.

A group of students apply a constant force of 0.5 N to a trolley with a mass of 200 g. They then measure the trolley's acceleration. 

They want to investigate how changing the mass of the trolley affects the acceleration of the trolley. 

Name a control variable in this experiment.

The students increase the mass added to the trolley by 200 g each time and record the acceleration using light gates. 

They draw a predicted curve on Figure 2 using Newton's second law and neglecting the mass of the trolley.

Their actual data were recorded with crosses on the graph in Figure 2.

Use data from Figure 2 to show that mass and the actual values of acceleration are inversely proportional.

Suggest why the actual values of acceleration are lower than those predicted. 

The difference between the curves could be due to an external force acting. 

Calculate the magnitude of this external force on the trolley.