Friction and dragIB MYP Physics: Subtopic test

1. 1
   Cyclists in Rotterdam fit rubber brake pads that press against the metal rim of a wheel to slow the bicycle down. A cyclist notices that her brakes work much less well on a rainy day, and that her bicycle chain needs a little oil to run smoothly.

   (a)

   What causes friction between two surfaces in contact?

   [1 mark]

   • AGravity pulling the surfaces apart
   • BTiny bumps on the two surfaces catching on each other
   • CMagnetic attraction between the surfaces
   • DAir trapped between the surfaces

   (b)

   Why do the brakes work less well in the rain?

   [1 mark]

   • AWater makes the bicycle heavier so there is less friction
   • BWater makes the rubber pads harder so they grip more
   • CRain removes all contact between the pad and the rim
   • DWater acts as a lubricant between the pad and the rim, reducing friction

   (c)

   Explain how putting oil on the bicycle chain reduces the energy wasted as it moves.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A student in Cairo tries to push a heavy wooden box across a concrete floor. She pushes with a gradually increasing force. At first the box does not move at all, then it suddenly starts to slide, and she finds it easier to keep it sliding than it was to get it moving.

   (a)

   The student pushes with 40 N and the box does not move. What is the friction force on the box?

   [1 mark]

   • A40 N, acting in the opposite direction to her push
   • B0 N, because the box is not moving
   • C40 N, acting in the same direction as her push
   • DMore than 40 N, acting in the opposite direction to her push

   (b)

   Which statement explains why the box is easier to keep sliding than to start moving?

   [1 mark]

   • ASliding friction is greater than static friction
   • BThe box becomes lighter once it moves
   • CSliding friction is smaller than the maximum static friction
   • DFriction disappears once the box is moving

   (c)

   Suggest one change the student could make so the box is easier to slide, and explain why it works.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A skydiver in Queenstown, New Zealand, jumps from an aeroplane and falls with her parachute closed. After about 12 seconds her speed stops increasing and stays at a steady 55 m/s. Later she opens her parachute and, a few seconds after that, she is falling at a steady 5 m/s until she lands.

   (a)

   Explain why the skydiver stops accelerating after about 12 seconds, even though she is still falling.

   [3 marks]

   (b)

   Explain, in terms of forces, how opening the parachute reduces her speed from 55 m/s to a steady 5 m/s.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A freight company in Rotterdam runs lorries that drive long distances at motorway speeds. Air resistance acts on a lorry and it increases with speed and with how blunt the front of the lorry is. To explore how shape affects drag, a student helping the company drops modelling-clay models of different shapes, all with the same mass, through a tall cylinder of liquid soap.

   (a)

   The student's hypothesis is that a more streamlined shape falls through the liquid faster. Describe a safe investigation to test this hypothesis, including the variables you would control.

   [6 marks]

   (b)

   The company is thinking of fitting streamlined air deflectors to the front of all its lorries. Discuss and evaluate whether it should do this.

   [6 marks]

   Total for question 4: 12 marks

End of questions