# Calculating work done by a force?

• chrismeng
In summary, the conversation discusses finding the work done on an object by an applied force, given its initial and final elevations and velocities. However, since the mass of the object is unknown, a numerical value for the work cannot be determined. It is possible that the question is simply asking for a formula with the mass as a parameter.
chrismeng

## Homework Statement

An object initially at an elevation of 5 m relative to the Earth's surface with a velocity of 50 m/s is acted on by an applied force R and moves along a path. Its final elevation is 20 m and its velocity is 100 m/s. The acceleration of gravity is 9.81 m/s^2. Determine the work done on the object by the applied force, in kJ.

## Homework Equations

Work of all non-gravity forces = Change in KE + Change in PE

## The Attempt at a Solution

I can get the work done by R in terms of the mass using the equation
Work of all non-gravity forces = Change in KE + Change in PE
but the problem doesn't tell us the mass and I don't know what other equation to use to set up a system of equations to eliminate the mass.

Welcome to PF!

You are right. Without the mass, you cannot find a numerical value for the work done by the applied force.

TSny said:
Welcome to PF!

You are right. Without the mass, you cannot find a numerical value for the work done by the applied force.
I thought so! thanks for the help

I get the impression that the conclusion is that the question is invalid. Not necessarily so however. Since the result is dependent on the unknown mass, it likely should be presumed that the question is simply asking for a formula as an answer, with the mass as a parameter in it. One would also have to specify the units of the mass in such an answer since the question is so specific in asking that the work deduced is explicitly in kj.

## 1. What is work done by a force?

Work done by a force is a measure of the energy transferred to an object when the force acts on it. It is calculated by multiplying the force applied by the displacement of the object in the direction of the force.

## 2. How do you calculate work done by a force?

To calculate work done by a force, you need to multiply the magnitude of the force by the distance the object moves in the direction of the force.

## 3. What is the unit of work done by a force?

The unit of work done by a force is joule (J) in the International System of Units (SI). It can also be expressed in other units such as foot-pounds (ft-lb) or kilogram-force meter (kgf-m).

## 4. Can work done by a force be negative?

Yes, work done by a force can be negative. This happens when the force and the displacement are in opposite directions. Negative work indicates that the force is acting against the direction of motion, resulting in a decrease in the object's energy.

## 5. What is the difference between work and power?

Work is a measure of the energy transferred to an object when a force acts on it, while power is the rate at which work is done. In other words, power measures how quickly work is done, while work measures the total amount of energy transferred.

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