Round and round, pulled inward

Science · Forces & Motion · ages 17-18

Name ______________________ Date ____________

A disc spins with angular speed 3 radians per second. A point sits 2 m from the centre. What is its linear speed in metres per second?
Answer: ______________
What is centripetal force?
- A new outward force
- The inward job done by a real force
- Friction inside every machine
A real outward centrifugal force pulls on objects moving in circles.
Circle one: True False
A 0.5 kg ball on a 1 m string snaps the string at 50 N. What is the greatest steady speed in metres per second?
Answer: ______________
A ball circles with period 0.25 seconds. What is its frequency in hertz?
Answer: ______________
Which force keeps a car on a roundabout?
- Tyre friction pointing inward
- Engine thrust pointing forward
- An outward centrifugal push
Nina doubles the radius at fixed omega and says rim speed is unchanged. What is her error?
- It doubles
- It halves
- It stays level
Speed doubles on the same bend with the same mass. What happens to inward demand?
- It doubles
- It stays level
- It quadruples

Answer key

For grown-ups. Fold this page away before handing over the rest.

6 · Rim speed is omega times r: 3 times 2 is 6 metres per second.
The inward job done by a real force · It is a job title for tension, friction, or gravity pointing inward, not a new force.
False · Only inertia heads straight while the real force hauls inward. No outward force acts.
10 · Set 0.5 times v squared over 1 equal to 50: v squared is 100, so v is 10.
4 · Frequency is one over period: 1 over 0.25 is 4 hertz.
Tyre friction pointing inward · Tyre friction does the inward job here, hauling the car round the bend.
It doubles · Rim speed is omega times r, so doubling r at fixed omega doubles v.
It quadruples · Demand runs as v squared, so doubling v multiplies demand by four.