Resultant ForcesOxford AQA IGCSE Physics: Subtopic test

1. 1
   Two warehouse workers test a heavy crate resting on a factory floor by each pushing on opposite sides at the same time, to see whether it will slide.

   (a)

   Two warehouse workers push a heavy crate from opposite sides in a tug-of-war style test: one pushes with 150 N to the left, the other pushes with 150 N to the right, and the crate does not move. What is the resultant force acting on the crate?

   [1 mark]

   • A300 N
   • B150 N
   • C0 N
   • D75 N

   (b)

   One worker then pushes with a larger force of 200 N to the left, while the other still pushes with 150 N to the right. Which statement correctly describes the resultant force and its likely effect on the crate?

   [1 mark]

   • AThe resultant force is 50 N to the left, so the crate accelerates to the left
   • BThe resultant force is 350 N to the left, so the crate accelerates strongly to the left
   • CThe resultant force is 50 N to the right, so the crate accelerates to the right
   • DThe resultant force is 0 N, so the crate stays still

   (c)

   The crate has a mass of 25 kg. Using the resultant force found when one worker pushes with 200 N and the other with 150 N in the opposite direction, calculate the acceleration of the crate, and state Newton's law that relates resultant force, mass and acceleration.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   During the launch of a rocket from a coastal launch site, the engines burn fuel and expel hot exhaust gas downwards at very high speed, while the rocket itself gradually rises from the launch pad.

   (a)

   During the launch of a rocket, the engines push hot gas downwards out of the rocket. Which law explains why this causes an upward force to act on the rocket itself?

   [1 mark]

   • ANewton's First Law
   • BNewton's Second Law
   • CThe law of conservation of energy
   • DNewton's Third Law

   (b)

   Just before lift-off, the rocket's engines produce a total upward thrust force exactly equal to the rocket's weight. What can be said about the rocket at this instant?

   [1 mark]

   • AIt accelerates rapidly upwards
   • BIt remains at rest, since the resultant force is zero
   • CIt accelerates downwards
   • DIt moves upwards at constant increasing speed

   (c)

   The rocket's engines produce a constant thrust of 6.6 million N. At lift-off the rocket's mass is 500,000 kg and its weight is 4.9 million N. Calculate the resultant force on the rocket at lift-off and use it to find the rocket's acceleration.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A cyclist rides along a straight, flat cycle path. Their velocity is recorded every second: they increase steadily from 0 m/s to 8 m/s over the first 4 seconds, then travel at a constant 8 m/s for the next 6 seconds, before braking steadily to a stop over the final 2 seconds.

   (a)

   Describe what the gradient of a velocity–time graph represents, and calculate the cyclist's acceleration during the first 4 seconds.

   [3 marks]

   (b)

   During the final 2 seconds, the cyclist brakes steadily, decelerating from 8 m/s to 0 m/s. Calculate the total distance travelled by the cyclist over the whole 12-second journey (the first 4 seconds accelerating, the next 6 seconds at constant speed, and the final 2 seconds braking), explaining how you use the velocity–time graph to find this.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A car safety research team studies a test collision in which a car travelling at a steady speed on a straight test track crashes into a fixed barrier. High-speed cameras record the velocity of the car and of a crash-test dummy inside it before, during and immediately after the impact.

   (a)

   Explain, using Newton's First and Second Laws, what happens to the car and to a crash-test dummy inside it during the collision, and why the dummy experiences a large force even though no external force pushed it forwards before the impact.

   [6 marks]

   (b)

   The research team repeats the test with an identical car but a heavier dummy on board, keeping the impact speed and stopping time the same. Using Newton's Second Law, explain how the force experienced by the heavier dummy compares with the force experienced by the original dummy, and explain how the resultant force acting on the car itself changes as it decelerates from its test speed to rest.

   [6 marks]

   Total for question 4: 12 marks

End of questions