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# Work example problems

David goes through some example problems on the concept of work. By reviewing these, you'll have a better knowledge of how to calculate work done by individual forces on an object in motion. You'll also understand the formula definition of work and how forces like tension, friction, and gravity play a role, and be able to apply the work-energy principle to determine an object's final speed. Created by David SantoPietro.

## Want to join the conversation?

• At he says no net work due to no change in kinetic energy. Is there not a change in potential energy that would account for work being done?
• If the trashcan's velocity is constant, that means it has no net acceleration. If it has no acceleration, there is no net force on the object, since F=ma=4kg(0)=0. If there is no net force, then no net work is being done on the object, since W=fd=0(2m). However, you are accelerating the trashcan, applying a force to it, and doing work on it when you lift it, it is just that all of your work is being negated by gravity in this example of constant velocity. Hope this helps!
• At , I don't understand why the force lifting the trash can up is 39.2N. If it's 39.2N up and 39.2N down, would the forces balance on both sides and the trash can would not be lifted up? Doesn't the upward force have to be greater in some way for it to move in that direction?
• Yes, you need slightly more than 39.2N to trigger the initial acceleration.
• He uses kinetic friction in the video but how would it be effected if he also had Static Friction?
• static friction applies when there is no movement. Hence, no displacement = no work done.
• How is there no change in KE in the example, at the very end of the video (). Initially velocity was zero and then after a 2 mt displacement, it had some velocity, so there has to be a change in KE?
• Yeah, this is a bad example. He needed to make one of the following conditions:
1) the can starts from rest and ends at rest. In that case you put in some extra work to accelerate the can but then you get back the same amount when you decelerate it, so the total work is just F*d
or
2) the can somehow already had upward velocity and all we did was maintain that upward velocity to get it to the new height. That's sort of what he implied when he said the can had constant velocity, but it doesn't make practical sense because the can started on the ground.

A better example might to put the can in an elevator. The elevator starts at the first floor and stops at the 4th floor. How much work was done on it between floors 2 and 3 when it moved at constant velocity? That's f*d.
• But wait. I know that the net force needs to be zero for avoiding any acceleration, but if there was no budging force in the beginning, why would the trashcan even go in the air with a constant velocity? Both forces are equal, so it'd rather stay on the ground.

I do understand that Fn = 0 makes sense when it's already in a velocity, but how does it make sense when it was at rest and went into a constant motion just because a force equal to gravity began pushing it from below?
• Well we assume the body is already in motion.So we ignore the fact that an initial force was applied which was larger than gravitational force and whose acceleration was used to attain the constant velocity.
• will a very less force just greater than zero can lift the block because normal from the ground will be balancing mg and it could move up with a little force
• Good question. The answer is no. It might seem that the laws of physics say yes, because if normal is equal and opposite to mg, then adding a small upward force will result in a net upward force. So what happens? Well, the normal force is a funny kind of force, which you might call a "reactive" force -- it only pushes exactly hard enough to keep the object from pushing through the surface, but no harder. So if a 100N block is sitting on a horizontal table, the normal force will be 100N, up. If you pull upwards with a force of 2N, the normal force instantaneously (well, almost) drops to 98N, so that the net force is still zero, and the block does not move. If you increase your upward pull to 99N, the normal force will drop to 1N. This should agree with common sense and your experience -- if you want to lift a 100N object, YOU have to pull upward with 100N of force! (note: you mention the time in your question, but this video is only in total length!)
• I feel like we just jumped through a lot of stuff
• I wonder if they might have changed this lesson over the years because they talk as if this is a review when this is all completely new info following along the HS physics course.
• At why is the angle listed at 90 degrees and not 270 degrees? I'm watching all of the Khan Academy videos on physics, and in an earlier physics video on vectors, they said that vector angles are measured in a counter clockwise direction starting from the east.
• It does not matter. There certainly is no law of physics that says angles must be measured that way.