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Physics: High School18 chapters | 212 lessons

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Lesson Transcript

Instructor:
*David Wood*

David has taught Honors Physics, AP Physics, IB Physics and general science courses. He has a Masters in Education, and a Bachelors in Physics.

After watching this video, you will be able to solve calculus problems involving work and explain how that relates to the area under a force-displacement graph. A short quiz will follow.

**Work** is an important quantity in physics. It's defined as the energy transferred from one type to another whenever a force is applied causing an object to move a distance. In equation form, work is a force multiplied by the displacement moved in the direction of that force. Or in other words, *W* = *Fx* cos (theta).

The cos (theta) part is a factor that just makes sure that the force you're looking at is in the right direction. It's the angle between the force and displacement vectors. So if you're pushing a block forwards, applying your force in the same direction as the displacement, theta is zero, and cos (0) = 1. So then work is just *Fx*. Or, if you're pushing down at an angle by putting in the angle between the direction, you're pushing and the direction you're moving, and you will get the component of your force in the direction it's moving multiplied by the displacement.

So far, none of this should be especially new. Work was introduced in another lesson in quite a bit of detail. But now, it's time to add a bit of calculus.

In a basic sense, work is just force multiplied by displacement. Or, if you have a force-displacement graph, like the one shown, it would be the area under the graph. If the force is 20 newtons and the eventual displacement is 10 meters, then the work is 20 multiplied by 10, which is 200 Joules. The area of a rectangle is the product of each side, so it's the same as the area under the graph. But what if your force isn't a nice, constant 20 newtons? What if it's varying gradually? Or what if it's a curve?

If the force is constantly changing, finding the area under the graph becomes more complex. And that's when you need a more fundamental equation for work. Work isn't just force multiplied by displacement. It's the integral of the force OVER the displacement. Or, in other words, if you have a force as a function of displacement and integrate it with respect to that displacement, you will get an expression for the work. Using the integral form shown, no matter how complex the shape of the force-distance graph, it will be possible to solve the problem.

Let's start with a simple diagonal graph:

In this graph, the force is equal to *kx* - a constant times the displacement. That constant is just the slope of the graph. It's basically just a *y* = *mx* graph.

To find the work, we have to integrate this expression with respect to the force. So work equals the integral of *kx dx*, integrated from the initial displacement, 0, to the final displacement, *x f*. The integral of

Time for some practice problems. Problem 1:

*The force applied to an object is given by the equation F = 3x + 2. If the total displacement is 5 meters, how much work is done in this process?*

To solve this problem, we first need to find an equation for the work done. The work done is the integral of the force function with respect to the displacement. So work equals 3*x* + 2*dx*. Integrating this, we get 3/2 *x*^2 + 2*x*. And putting in the limits, these *x* values become *x f* - the final or total displacement. Plug in our value of 5 meters for the total displacement, and we find that 47.5 Joules of work was done.

Problem 2:

*The force applied to an object is given by the equation F = x^2 + 2. If the total displacement is 10 meters, how much work is done in this process?*

Again, we need to put our expression into the work integral and integrate it. Work is equal to the integral of *x*^2 + 2, with respect to *x*. The integral of *x*^2 + 2 equals 1/3*x*^3 + 2*x*. And those *x* values once again become the total displacement. Plug in our value of 10 meters, and we find that the work done is 353.3 Joules. And that's our answer.

**Work** is an important quantity in physics. It's defined as the energy transferred from one type to another whenever a force is applied causing an object to move a distance. In equation form, work is a force multiplied by the displacement moved in the direction of that force. Or in other words, *W* = *Fx* cos (theta). The cos (theta) part is a factor that just makes sure that the force you're looking at is in the right direction.

This value of *Fx* is just the area under a perfectly flat force-displacement graph. But what if your graph isn't flat? What if it's diagonal? Or curved?

In that situation, you need calculus. Work isn't just a force multiplied by displacement; it's the integral of the force OVER the displacement. Or in other words, if you have a force as a function of displacement and integrate it with respect to that displacement, you will get an expression for the work. Using this integral form, you can solve any problem that could be thrown your way.

After this lesson, you'll have the ability to:

- Define work in terms of physics
- Identify the equation for work
- Explain how to use the integral form to solve work problems in calculus

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Physics: High School18 chapters | 212 lessons

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