Moments and balance — final check

1. describe / calculate / state

A beam is pivoted at its middle; its own weight acts through the pivot. (a) Describe what is meant by the moment of a force. (b) A force of 25 N acts at right angles to a door, 0.40 m from the hinge. Calculate its moment about the hinge. (c) An 8.0 N weight hangs 0.25 m to the left of the beam’s pivot. Calculate how far to the right of the pivot a 5.0 N weight must hang to balance it. (d) State the two conditions for an object to be in equilibrium. (e) Supplement. With the beam balanced as in (c), a 2.0 N weight is added 0.50 m to the left of the pivot. Calculate where a 4.0 N weight must hang on the right, with the 5.0 N weight still in place, to balance the beam again. (f) Supplement. Describe an experiment to show that there is no resultant moment on an object in equilibrium.

(a) Moment of a force

(b) Moment about the hinge

(c) Distance of the 5.0 N weight

(d) Conditions for equilibrium

(e) Supplement: distance of the 4.0 N weight

(f) Supplement: the experiment

2. state / calculate / describe / explain

(a) State what is meant by centre of gravity. (b) A uniform metre rule balances on a pivot at its 35 cm mark when a 0.90 N weight hangs at its 5 cm mark. Calculate the weight of the rule. (c) Describe an experiment to find the centre of gravity of an irregularly shaped plane lamina. (d) A tall, narrow vase and a short, wide vase have the same weight. Explain which one is more stable.

(a) Centre of gravity

(b) Weight of the rule

(c) The experiment

(d) The vases