Moments, centre of gravity and springs — GCSE Physics practice questions

Question 1

A toddler's high-chair sometimes tips over. A designer changes the chair to make it more stable.

Which row shows two changes that both make the chair more stable?

Accepted Answer

D: wider · lower. An object topples when it is tilted so far that the vertical line through its centre of gravity passes outside its base. A wider base and a lower centre of gravity both mean it must be tilted further before this happens, so the chair is more stable.

Question 2

A light bar rests on a pivot. Three forces X, Y and Z act on the bar, as shown. Forces X and Z act upwards and force Y acts downwards.
The bar is in equilibrium.

Which equation is correct?

Accepted Answer

C: 0.30 X + 0.50 Y = 1.1 Z. X (up, left of the pivot) turns the bar clockwise, and Y (down, right of the pivot) also turns it clockwise. Z (up, right of the pivot) turns it anticlockwise. In equilibrium, the sum of the clockwise moments equals the anticlockwise moment: 0.30 X + 0.50 Y = 1.1 Z.

Question 3

A shop sign hangs from the end of a light horizontal bar. The bar is hinged to a wall. A cable joins a point 0.80 m along the bar from the hinge to the wall above the hinge.
The bar is in equilibrium.

Which row describes the force of the hinge on the bar?

Accepted Answer

A: away from the wall · downwards. The cable pulls the bar towards the wall, so the hinge must push it away from the wall to give no resultant horizontal force. Taking moments about the cable's attachment point, the sign (0.40 m to the left of it) turns the bar anticlockwise, so the hinge force (0.80 m to the right of it) must turn the bar clockwise: it must act downwards. (With numbers: the cable's vertical pull is 54 N, more than the 36 N sign, so the hinge pulls down with 18 N.)

	width of base	height of centre of gravity
A	narrower	higher
B	narrower	lower
C	wider	higher
D	wider	lower

	horizontal part of the force	vertical part of the force
A	away from the wall	downwards
B	away from the wall	upwards
C	towards the wall	downwards
D	towards the wall	upwards