Turning Effect of A Force (3N)

• A

• B

• C

• D

Answer explanation

Since Moment = F x d

the smallest force would be

at the largest perpendicular distance from the pivot!

The handle on a door is pulled by a force, F, in the direction shown. To open the door, F produces a moment of 0.30 N m about the hinge. What is the value of F?

hint: which distance is PERPENDICULAR?

• 0.15 N

• 0.18 N

• 0.50 N

• 0.60 N

Answer explanation

Since M = F x d, substitute the values to find the unknown:

0.30 N m = F x d

Which d should we use? Remember that the moment ONLY uses perpendicular distance. Which distance is perpendicular to the force F?

A uniform beam is pivoted at its centre. In which diagram is the beam in equilibrium?

• A

• B

• C

• D

Answer explanation

For this question, we are using the principle of moments!

For a balanced beam about the same pivot,

Total clockwise moments = total anticlockwise moments

A uniform bar is pivoted at its centre. What force F is needed to balance the bar?

• 2.0 N

• 4.5 N

• 6.0 N

• 8.0 N

Answer explanation

Using principle of moments for this question:

For a balanced beam about the pivot,

Total clockwise moments = total anticlockwise moments

Two equal forces F act on each of the four planks. Which plank turns?

• A

• B

• C

• D

Answer explanation

If you have difficulty visualizing the question, you can use a ruler to make a mini version of it!

A uniform plank, XY, measures 2.0 m in length. The plank weighs 100 N. It rests on a pivot 0.50 m from

Y. It is balanced by an upward force of 20 N at X and a downward force F at Y. What is force F?

• 5 N

• 20 N

• 40 N

• 80 N

Answer explanation

Here we use the principle of moments.

For the beam to balance about the pivot;

Total clockwise moments = total anticlockwise moments

Which forces generate moments for clockwise and anticlockwise moments?

Don't forget to use the perpendicular distance between the force and the pivot too!

A simple lever is used to lift a bucket of water from a river. A force of 40.0 N is applied. Ignore the mass of the lever and the bucket.

What is the maximum weight of water lifted?

• 10.9 N

• 15.0 N

• 24.0 N

• 107 N

Answer explanation

We are using principle of moments.

For a balanced beam about the pivot,

total clockwise moments = total anticlockwise moments

The diagrams show a uniform rod with its midpoint on a pivot. Two equal forces F are applied to the rod. Which diagram shows the rod in equilibrium?

• A

• B

• C

• D

Answer explanation

The beam in equilibrium should mean that the moments are all balanced!

Two blocks X and Y are placed on a uniform beam. The beam balances on a pivot at its centre. What does this show about X and Y?

• They have the same mass and the same density.

• The have the same mass and the same weight.

• They have the same volume and the same density.

• They have the same volume and the same weight.

Answer explanation

Since X and Y are at the exact same location away from the pivot and the beam is balanced, this shows that the clockwise moment and anticlockwise moment is the same.

Hence, we can conclude that X and Y have the same weight!

The beam in the diagram is not balanced. What change balances the beam?

• moving the 2 N weight 10 cm to the right.

• moving the 5 N weight 10 cm to the right.

• replacing the 2 N weight with a 7.5 N weight.

• replacing the 5 N weight with a 3 N weight.

Answer explanation

To make the beam balanced,

the moments on both sides of the pivot must be the same!