Centre of Gravity
Centre of Gravity
- Gravity pulls on every part of an object: every arm of a crane, every page of a book.
- Physics replaces all of those small pulls with one arrow at one point. This lesson is about that point.
One point for the whole weight
- The of an object is the point where all the weight of the object may be considered to act.
- That sentence is the exam definition. The scoring words are point, all the weight and considered (taken) to act.
- For a uniform, symmetric object (a ruler, a sphere, a plank) the centre of gravity is at the middle.
- For a non-uniform object it sits closer to the heavy end, and the question must tell you where it is.
Your turn— tap to reveal the worked answer (9702/21/M/J/22 Q2(a))
State what is meant by the centre of gravity of an object. (9702/21/M/J/22 Q2(a))
Answer: the point where all the weight of the object may be considered to act.
centre of gravity- The centre of gravity does not have to be inside the object. A high jumper bends backwards over the bar, and at that moment the centre of gravity is outside the body, just below the back.
- While the jumper is in the air, that one point follows the smooth projectile path from lessons 2.8 and 2.9, whatever the arms and legs are doing.
Finding it by experiment
For a flat sheet (a lamina), a simple experiment finds the centre of gravity.
- Make small holes near the edge. Hang the sheet from a pin through one hole, so it can swing freely.
- Hang a plumb line (a string with a small weight) from the same pin.
- The sheet comes to rest with its centre of gravity directly below the pin. Draw a line on the sheet along the string.
- Repeat from a second hole. The centre of gravity is where the two lines cross.
- Why it works: a hanging object settles so that its weight acts straight below the support. So the centre of gravity must lie on the vertical line through the pin, every time.
In every force question from now on, draw the weight as one arrow, straight down, from the centre of gravity.
- How the exam uses it: a uniform plank's weight acts at its midpoint (9702/12/F/M/25 Q9). For a non-uniform bar, you find where the centre of gravity is by taking moments, which is the skill of lesson 4.04 (9702/13/M/J/25 Q12).