- How do you calculate torque?
- Torque is τ = r F sin θ, where r is the distance from the pivot to where the force acts, F is the force and θ is the angle between r and F. The unit is newton metres (N m). A force at right angles to a 0.8 m lever with 20 N gives 16 N m.
- Why is a door easier to open at the handle?
- For the same force, a longer lever arm gives a bigger torque. Pushing near the hinge makes r small, so the torque is small. Pushing at right angles to the door (sin θ = 1) gives the most turning effect.
- What does Στ = Iα mean?
- The net torque on a body equals its moment of inertia times its angular acceleration. It is the rotational form of Newton’s second law. The bigger I is, the more torque is needed for the same angular acceleration.
- What is moment of inertia?
- Moment of inertia I measures how hard a body is to spin: I = Σmr². A solid disc has I = ½MR², a ring or hoop has I = MR², and a rod spun about its centre has I = ⅓ML². Mass further from the axis gives a larger I.
- When is a beam balanced?
- A beam is balanced when the net torque about its pivot is zero, Στ = 0. The clockwise and anticlockwise torques cancel. For a beam with its own mass, this happens when the centre of mass is over the pivot.
- What is angular momentum?
- Angular momentum is L = Iω. With no net external torque, angular momentum is conserved, which is why a skater spins faster with arms pulled in.