Newton's laws of motionAQA A-Level Physics: Subtopic test

1. 1
   A car of mass 1200 kg is accelerating along a straight, level road. At one instant the driving force from the road on the car is 3000 N and the total resistive force on the car is 800 N.

   (a)

   What is the resultant force on the car at this instant?

   [1 mark]

   • A3800 N
   • B3000 N
   • C800 N
   • D2200 N

   (b)

   What is the acceleration of the car at this instant?

   [1 mark]

   • A0.67 m s⁻²
   • B1.8 m s⁻²
   • C2.5 m s⁻²
   • D3.2 m s⁻²

   (c)

   The driver keeps the driving force at 3000 N. The resistive force increases as the car speeds up and the car eventually reaches a constant speed. Explain, using Newton's laws, why the car stops accelerating.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A person of mass 70 kg stands on bathroom scales on the floor of a lift. The scales read the normal contact force exerted on the person by the floor of the lift. Take g = 9.81 N kg⁻¹.

   (a)

   Which force forms the Newton's third law pair with the weight of the person (the gravitational force of the Earth on the person)?

   [1 mark]

   • AThe gravitational force of the person on the Earth
   • BThe contact force of the floor on the person
   • CThe force of the person on the floor
   • DThe tension in the lift cable

   (b)

   The lift accelerates upwards at 1.5 m s⁻². What do the scales read?

   [1 mark]

   • A580 N
   • B690 N
   • C790 N
   • D105 N

   (c)

   The lift now moves upwards at a constant speed of 2.0 m s⁻¹. Determine the reading on the scales and explain your answer using Newton's first law.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A crate of mass 8.0 kg slides down a rough slope that is inclined at 25° to the horizontal. A constant frictional force of 12 N acts on the crate. Take g = 9.81 N kg⁻¹.

   (a)

   State the three forces acting on the crate and give the direction of each.

   [3 marks]

   (b)

   Calculate the acceleration of the crate down the slope.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A truck of mass 3500 kg tows a trailer of mass 1500 kg along a straight, level road using a rigid coupling. The driving force from the road on the truck is 11 000 N. The resistive force on the truck is 1000 N and the resistive force on the trailer is 500 N. A student says: 'By Newton's third law the truck and trailer pull on each other with equal and opposite forces, so these forces cancel and the vehicles can never accelerate.'

   (a)

   Calculate the acceleration of the truck and trailer, and the tension in the coupling.

   [6 marks]

   (b)

   Evaluate the student's statement about Newton's third law.

   [6 marks]

   Total for question 4: 12 marks

End of questions