10 minutes maximum! Can you do it in 5?

1. Which one of the following best describes a 'centripetal' force?

• A. A force acting against motion that produces circular motion.
• B. A force acting at right angles to the velocity of a body, that produces circular motion.
• C. A force caused by circular motion, acting at right angles to the velocity and pushing a body away from the centre of the circle.
• D. A force caused by circular motion, making the speed of a body change.

2. Which of the following best describes the changes to speed and velocity (if any) as an object moves in orbit, such as a moon around a planet?

3. A child is riding on a roundabout ride. After 2 seconds, the child has completed half a rotation.

What is the angular velocity of this roundabout?

• A. 0 rad s^-1
• B. ¼π rad s^-1
• C. ½π rad s^-1
• D. π rad s^-1

4. What force keeps an electron in orbit around a nucleus?

• A. An electrical force.
• B. A gravitational force.
• C. A centrifugal force.
• D. Frictional forces.

5-7. The diagram below shows a 4 kg mass being whirled round in a horizontal orbit at the end of a 2m long rope. The velocity of the mass is constant at 10 ms^-1.

5. What is the value of the angular velocity?

• A. 10 ms^-1.
• B. 20 ms^-1
• C. 10 rad s^-1
• D. 5 rad s^-1

6. What is the value of the centripetal acceleration?

• A. 20 ms^-2
• B. 10 ms^-2
• C. 5 ms^-2
• D. 50 ms^-2

7. What is the tension in the rope keeping the mass moving in a circle?

• A. 200 N
• B. 20 N
• C. 100 N
• D. 50 N

8-10. A fan used to cool a room rotates at a frequency of 10 Hz.

The length of each blade is 0.5 m from the centre of rotation.

8. What is the period of the fan?

• A. 2π s
• B. π s
• C. 10 s
• D. 0.1 s

9. What distance has been travelled, in meters, by the tip of the fan blade in this time period?

• A. 2
• B. 0.5
• C. 2π
• D. π

10. The fan slows down to a frequency of 1 Hz. What is the centripetal acceleration on the tips of the blades at this frequency?

• A. 2π² ms^-2
• B. 2π ms^-2
• C. 4π ms^-2
• D. 4π² ms^-2