Newton's laws of motionIB MYP Physics: Subtopic test

1. 1
   A bus travels at a constant velocity of 15 m/s along a straight, level road in Dubai. The driver then brakes hard and the bus slows down quickly. Passengers who are standing are thrown forwards as the bus slows.

   (a)

   Before the driver brakes, the bus moves at constant velocity. Which statement about the horizontal forces on the bus is correct?

   [1 mark]

   • AThe driving force equals the resistive forces, so the resultant force is zero
   • BThe driving force is bigger than the resistive forces, because the bus is moving
   • CThere are no horizontal forces acting on the bus at all
   • DThe resistive forces are bigger than the driving force, so the bus keeps a steady speed

   (b)

   Which property of the standing passengers explains why they keep moving forwards when the bus brakes?

   [1 mark]

   • ATheir weight
   • BTheir air resistance
   • CTheir inertia
   • DTheir upthrust

   (c)

   Explain, using Newton's first law, why the standing passengers are thrown forwards when the bus brakes.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A space agency tests a rocket. Burning fuel produces hot gas that the engine pushes out of a nozzle at the back at very high speed. Once the rocket is in space, it accelerates forwards even though there is no air or ground for it to push against.

   (a)

   The engine pushes the exhaust gas backwards. Which force is the Newton's third law pair of this force?

   [1 mark]

   • AThe weight of the rocket
   • BThe force of the exhaust gas on the rocket, pushing it forwards
   • CThe force of gravity on the exhaust gas
   • DThe force of the rocket on the exhaust gas, pushing it forwards

   (b)

   Which statement about the pair of forces between the rocket and the exhaust gas is correct?

   [1 mark]

   • AThey are equal and opposite and act on the same object, so they cancel
   • BThe force on the rocket is smaller because the rocket is much more massive
   • CThe force on the gas is larger because the gas moves faster
   • DThey are equal in size and opposite in direction, and act on different objects

   (c)

   Explain how the rocket accelerates forwards in space even though there is nothing for it to push against.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A student investigates how the acceleration of a trolley of mass 0.50 kg depends on the resultant force on it. The trolley runs along a level, low-friction runway and is pulled by a constant force measured with a force sensor. A motion sensor records the acceleration. A resultant force of 0.25 N gave an acceleration of 0.50 m/s², 0.50 N gave 1.0 m/s², and 1.00 N gave 2.0 m/s².

   (a)

   State the independent variable, the dependent variable and one control variable in this investigation.

   [3 marks]

   (b)

   Describe the pattern in the results. Use Newton's second law to calculate the acceleration of the trolley for a resultant force of 0.80 N.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A family in Mumbai drives a car of mass 1200 kg, including the passengers, along a straight, level road. The engine provides a forward driving force of 4500 N and resistive forces of 900 N act against the motion. The government is also deciding whether to keep its law that makes seat belts compulsory for all passengers.

   (a)

   The car carries 300 kg of extra luggage. The driving force and the resistive forces stay the same. Use Newton's second law to find the acceleration of the car before the luggage is added and after it is added.

   [6 marks]

   (b)

   Discuss and evaluate the law that makes seat belts compulsory, using Newton's laws of motion where appropriate.

   [6 marks]

   Total for question 4: 12 marks

End of questions