State the principle of moments.

A student uses the principle of moments to determine the weight of a heavy key. This is the student's method:

• He balances a uniform metre rule at its mid-point (50 cm) on a pivot.
• He hangs a small paper cup from the 30 cm mark on the rule.
• He places the key inside the cup.
• He hangs a 0.4 N metal hook weight from the rule on the other side of the pivot.
• He adjusts the position of the hook weight until the rule is perfectly balanced when the hook weight is suspended from the 85 cm mark.

Draw on this

Describe how the student could use an electronic balance to check that the weight of the metal hook weight is 0.4 N.

Suggest how the student could improve the precision of one of the distance measurements.

State the equation linking moment, force, and perpendicular distance from the pivot.

Use the principle of moments to calculate the force acting on the metre rule at the 30 cm mark.

Suggest a reason why the weight of the key will be different from the force calculated in part (iv).