4.3 Turning Effect of Force 2020 JnN

A uniform plank rests on a pivot at its centre.

Two children P and Q sit on the plank in the positions shown.

The mass of child P is 25 kg. The plank is balanced.

What is the mass of child Q?

• 20 kg

• 25 kg

• 31 kg

• 45 kg

An object is in equilibrium on the Earth. Which statement is correct?

• All the forces acting on the object are in the same direction.

• All the forces acting on the object have the same value.

• The object is weightless.

• The resultant force acting on the object is zero.

A force of 4.0 N acts on a beam as shown in the diagram.

The line of action of the force is a distance x from a pivot.

The moment of this force about this pivot is 8.0 N cm.

What is distance x?

• 0.50 cm

• 2.0 cm

• 12 cm

• 32 cm

The diagram shows a stand.

The stand holds a heavy mass above the bench.

Which two changes would definitely make the stand more stable?

• Lower the mass and make the base narrower.

• Lower the mass and make the base wider.

• Raise the mass and make the base narrower.

• Raise the mass and make the base wider.

Students X and Y are sitting on a seesaw.

Student X has a weight of 400 N and student Y has a weight of 600 N. The seesaw is in equilibrium.

Which statement correctly describes why the seesaw is in equilibrium?

• The resultant force is zero and the resultant moment is zero.

• The resultant force is 200 N and the resultant moment is zero.

• The sum of the downward forces is zero and the resultant moment is zero.

• The total downward force is 1000 N and the resultant moment is 200 N·m.

The diagram shows a beam lying on the ground.

End Q is lifted from the ground by the force F.

End P of the beam remains on the ground.

The length of the beam is 3.0 m and its weight is 600 N.

The centre of mass of the beam at G is 1.0 m from end P.

What is the size of the force F when it just raises end Q from the ground?

• 200 N

• 300 N

• 400 N

• 600 N

The diagram shows a trolley used to transport a load of 400 N.

A force F vertically downwards is needed to balance the trolley as shown.

The centre of mass of the trolley is vertically above the pivot.

What is the value of F?

• 133 N

• 150 N

• 300 N

• 400 N

A force F is applied to a spanner, as shown.

Which action increases the moment of F about the centre of the nut?

• apply the force F to the end of the spanner handle

• apply the force F parallel to the spanner handle

• spray oil on the nut

• use a shorter spanner

An athlete with mass 70kg trains by performing press-ups with a load on his back.

The diagram shows the perpendicular distances involved.

The centre of mass of the athlete is CM and the centre of mass of the load he is carrying is CL.

The mass of the load is 6.0kg.

What is the upward force exerted by his two arms?

• 54 N

• 76 N

• 540 N

• 760 N

Forces are applied to four identical objects.

The lengths of the arrows indicate the magnitude of each force. Which object is in equilibrium?