Forces in Equilibrium

Question 1

Define centre of mass.

Answer 1

The centre of mass of a body is the point through which a single force on the body has no turning effect.

Question 2

How could you calculate the mass of a metre rule?

Answer 2

Locate the centre of mass of the ruler by balancing it on a knife edge. Note this point. The rule is uniform if it’s centre of mass is exactly at the middle of the rule. Now balance the rule off centre on a knife edge using a known weight. Using moments and the distance from the knife edge one can work out the mass of the rule.

Question 3

Define a couple.

Answer 3

A couple is a pair of equal and opposite forces acting on a body, but not along the same line.

Question 4

What is another term for the moment of a couple?

Answer 4

Torque.

Question 5

What is the difference between stable and unstable equilibrium?

Answer 5

A body is in stable equilibrium if when displaced and released, it returns to equilibrium. If a body in unstable equilibrium is displaced and released it moves even further away from equilibrium.

Question 6

What are the criteria for a body to be in equilibrium?

Answer 6

• The resultant force must be zero. If there are only three forces they must form a closed triangle.
• The principle of moments must apply.

Question 7

What is the principle of moments?

Answer 7

The sum of the clockwise moments = the sum of the anticlockwise moments.