VISUAL PHYSICS ONLINE

2 DYNAMICS

P20 001.m

(A)

Two 6.0 kg objects are attached to a light spring balance calibrated in newtons by light inextensible strings passing over smooth pulleys as shown in the diagram. What is the string tension and the reading on the spring balance? What is the net force acting on the spring balance?

(B)

A sphere and a block are connected by a light, inextensible string placed over a smooth pulley. In Diagram 1, the two objects remain at rest at the same level. Then, block is pulled down and held while in its new position as shown in Diagram 2. Describe the motion of the block and sphere after the block is released from the hand. Justify your answer in terms of physical principles.

View solution below only after you have completed the answering the question.

Solution

(A)

Mass of each object

Weight of each object

Consider the left-hand object as holding the spring balance in place. The right-hand object pulls on the spring balance through the tension of the string. The pull is the weight of the object. So, the string tension is 60 N which would be reading on the spring balance.

Many students give 0 N as the reading on the spring balance. It is the net force acting on the spring balance that is zero.

(B)

Identify the two Systems the Block and the Sphere.

Draw free body diagrams for the two Systems.

Apply Newton’s Second Law to both Systems.

For Diagram 2 the situation in terms of forces is identical to Diagram 1. The block and sphere will remain stationary since the forces acting on the block and sphere do not depend upon the position of the block or position of the sphere. The block and sphere are both in equilibrium.