Turning Effects of Forces：练习题

Which of the following is the S.I. unit for the moment of a force?

A force of \(15\text{ N}\) acts perpendicularly on a door at a distance of \(0.80\text{ m}\) from its hinges. What is the moment of this force about the hinges?

A uniform wooden plank \(AB\) of length \(3.0\text{ m}\) and weight \(60\text{ N}\) rests horizontally on two supports located at its ends \(A\) and \(B\). A heavy load of weight \(140\text{ N}\) is placed at a point \(0.90\text{ m}\) from end \(A\). What is the upward force exerted by support \(B\) on the plank?

A force is applied as shown in the diagram. What is the moment of this force about the pivot \(P\)?

A uniform wooden plank of length \(2.0\text{ m}\) and weight \(40\text{ N}\) is pivoted at a point \(0.60\text{ m}\) from its left end. A downward force \(F\) is applied at the extreme left end to keep the plank horizontal and in equilibrium. What is the magnitude of the force \(F\)?

Explain what is meant by the centre of gravity of an object.<\/p>

A uniform beam is balanced horizontally on a pivot as shown in the diagram. A weight of \(450 \text{ N}\) is placed at a distance of \(1.2 \text{ m}\) from the pivot. Calculate the weight \(W\) that must be placed at a distance of \(1.8 \text{ m}\) from the pivot on the opposite side to keep the beam in equilibrium.<\/p>

The diagram shows a uniform beam balanced horizontally on a pivot at its center point. A weight of \(12\text{ N}\) is placed at a distance of \(0.50\text{ m}\) to the left of the pivot. A second weight of \(4.0\text{ N}\) is placed at a distance \(d\) to the right of the pivot to maintain equilibrium.
(a) Calculate the anticlockwise moment acting on the beam about the pivot.
(b) State the Principle of Moments for a body in equilibrium.
(c) Calculate the distance \(d\) required to keep the beam balanced.
(d) If the \(12\text{ N}\) weight is moved further left to a new distance of \(0.60\text{ m}\) from the pivot, calculate the new weight that must be placed at distance \(d\) to restore balance.