Question

Discuss whether any work is being done by each of the following agents and, if so, whether the work is positive or negative. (a) a chicken scratching the ground (b) a person studying (c) a crane lifting a bucket of concrete (d) the gravitational force on the bucket in part (c) (e) the leg muscles of a person in the act of sitting down.

Short answer

(a) The work done by the chicken on the ground is ‘positive’.

(b)There is no work done when a person is studying.

(c)The work done by a crane lifting a bucket of concrete is ‘positive’.

(d) The work done on the bucket of concrete is ‘negative’.

(e) The work done by the leg muscles of a person in the act of sitting down is ‘negative’.

Step-by-step solution

Work due to external force

When an external force of certain amount is applied on a body then the body moves to a distance and the work is done on the body.

The work done on a body or by a body relies directly upon the applied external force and the distance moved by the body.

Work done in this case

(a)

It is given that the chicken is scratching the ground, so there is certain amount of work done by the chicken on scratching the ground. Since, the work is done by the chicken on the ground, its value would be positive.

Hence, the work done by the chicken on the ground is ‘positive’.

Work done in this case

(b)

It is given that a person is studying, so the person does not do any work on anything while studying. There are some chemical energy exchanges inside person’s body but these changes are very small and cannot be calculated.

Hence, there is no work done when a person is studying.

Work done in this case

(c)

It is given that, a crane is lifting a bucket of concrete, so there is a certain amount of work being done by the crane in lifting the bucket. Since, the work is done by the crane on the bucket, the value of the work done is positive.

Hence, the work done by a crane lifting a bucket of concrete is ‘positive’.

Work done in this case

(d)

It is given that, in part (c), a crane is lifting a bucket of concrete, then the ‘gravitational force’ acts on the bucket. The gravitational force on the bucket is due to its weight acting downwards. So, the work is being done on the bucket by the gravitational force, then its value would be negative.

Hence, the work done on the bucket of concrete is ‘negative’.

Work done in this case

(e)

It is given that, a person is sitting down, so the work is done by the leg muscles of the person. The leg muscles of the person compresses to a certain amount while sitting down. So, the work done by the muscles is considered as the negative work done.

Hence, the work done by the leg muscles of a person in the act of sitting down is ‘negative’.