Force in the Direction of Displacement
The work done by a constant force is proportional to the force applied times the displacement of the object.
Learning Objectives
Contrast displacement and distance in constant force situations
Key Takeaways
Key Points
- Understanding work is quintessential to understanding systems in terms of their energy, which is necessary for higher level physics.
- Work is equivalent to the change in kinetic energy of a system.
- Distance is not the same as displacement. If a box is moved 3 meters forward and then 4 meters to the left, the total displacement is 5 meters, not 7 meters.
Key Terms
- work: A measure of energy expended in moving an object; most commonly, force times displacement. No work is done if the object does not move.
Work Done by a Constant Force
When a force acts on an object over a distance, it is said to have done work on the object. Physically, the work done on an object is the change in kinetic energy that that object experiences. We will rigorously prove both of these claims.
The term work was introduced in 1826 by the French mathematician Gaspard-Gustave Coriolis as “weight lifted through a height,” which is based on the use of early steam engines to lift buckets of water out of flooded ore mines. The SI unit of work is the newton-meter or joule (J).
Units
One way to validate if an expression is correct is to perform dimensional analysis. We have claimed that work is the change in kinetic energy of an object and that it is also equal to the force times the distance. The units of these two should agree. Kinetic energy – and all forms of energy – have units of joules (J). Likewise, force has units of newtons (N) and distance has units of meters (m). If the two statements are equivalent they should be equivalent to one another.
[latex]\text{N} \cdot \text{m} = \text{kg} \frac{\text{m}}{\text{s}^2} \cdot \text{m} = \text{kg} \frac{\text{m}^2}{\text{s}^2} = \text{J}[/latex]
Displacement versus Distance
Often times we will be asked to calculate the work done by a force on an object. As we have shown, this is proportional to the force and the distance which the object is displaced, not moved. We will investigate two examples of a box being moved to illustrate this.
Example Problems
Here are a few example problems:
(1.a) Consider a constant force of two newtons (F = 2 N) acting on a box of mass three kilograms (M = 3 kg). Calculate the work done on the box if the box is displaced 5 meters.
(1.b) Since the box is displaced 5 meters and the force is 2 N, we multiply the two quantities together. The object’s mass will dictate how fast it is accelerating under the force, and thus the time it takes to move the object from point a to point b. Regardless of how long it takes, the object will have the same displacement and thus the same work done on it.
(2.a) Consider the same box (M = 3 kg) being pushed by a constant force of four newtons (F = 4 N). It begins at rest and is pushed for five meters (d = 5m). Assuming a frictionless surface, calculate the velocity of the box at 5 meters.
(2.b) We now understand that the work is proportional to the change in kinetic energy, from this we can calculate the final velocity. What do we know so far? We know that the block begins at rest, so the initial kinetic energy must be zero. From this we algebraically isolate and solve for the final velocity.
[latex]\text{Fd} = \Delta \text{KE} = \text{KE}_\text{f} - 0 = \frac{1}{2} \text{m} \text{v}_\text{f} ^2 \\ \text{v}_\text{f} = \sqrt{2 \frac{\text{Fd}}{\text{m}}} = \sqrt{2 \frac{4\text{N} \cdot 5\text{m}}{2 \text{kg}}} = \sqrt{10} \text{m}/\text{s}[/latex]We see that the final velocity of the block is approximately 3.15 m/s.
