4.2 Centre of mass

Question 1

The centre of mass of an object…

Answer 1

…is a point through which any externally applied force produces straight-line motion but no rotation.

Question 2

What is the centre of gravity?

Answer 2

through which the object’s weight acts, coincides with its centre of mass.

Question 3

How do you find the centre of mass of a piece of card?

Answer 3

Make small holes along the edges of the object made from card. Insert a pin through one of the holes and hold the pin firmly in a clamp. Allow the object to swing freely. It will come to rest with its centre of gravity vertically below the pin. Hang a plumb-line from the pin and draw a line along the vertical string of the plumb-line. Repeat this process for other holes. The centre of gravity will be the point of intersection of the lines.

Question 4

Describe how you could determine the centre of gravity of an irregular shaped piece of card using the edge of the ruler instead of the plumb-line method above?

Answer 4

Make small holes along the edges of the object made from card. Insert a pin through one of the holes and hold the pin firmly in a clamp. Allow the object to swing freely. It will come to rest with its centre of gravity vertically below the pin. Hang a plumb-line from the pin and draw a line along the vertical string of the plumb-line. Repeat this process for other holes. The centre of gravity will be the point of intersection of the lines.

Question 5

Explain why the centre of gravity for a table-tennis ball is in the empty space inside the ball, rather than in the plastic of the ball itself.

Answer 5

The weight is due to the plastic wall.
The ball is symmetrical so the centre of mass will be in the centre.