Physics 2019 JAMB Past Questions - Question 14: A uniform metre rule QR is balanced on a knife edge which is 55cm from R. when a mass of 10g is hung at P as shown below the mass of the metre rule is

Correct Answer

A

Explanation

Taking moment about P, 10 x(90 - 55) = m x (55 - 50) m = 70g
To find the mass of the meter rule, we can use the principle of moments. The total anticlockwise moment about the knife edge must be equal to the total clockwise moment.

Let's denote the mass of the meter rule as M, and the distance from the knife edge to the center of mass of the meter rule as x. The mass of 10g at P is 0.01kg.

The anticlockwise moment due to the 10g mass at P is 0.01kg * 55cm, and the clockwise moment due to the meter rule is M * x. Since the meter rule is balanced, these two moments are equal.

So, 0.01kg * 55cm = M * x

To find x, we can use the fact that the center of mass of a uniform meter rule is at its midpoint, which is 50cm from either end.

So, x = 50cm

Now we can solve for M:

0.01kg * 55cm = M * 50cm

M = (0.01kg * 55cm) / 50cm
M = 0.011kg

Therefore, the mass of the meter rule is 0.011kg.