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Class 9 Physics (India)

Course: Class 9 Physics (India)>Unit 4

Lesson 3: Work energy theorem

Work-energy theorem

Let's explore the work-energy theorem.  Created by Mahesh Shenoy.

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• while moving the cupboard we applied force and that cupboard displaces in the same direction as that of force right. As there is displacement and force how the workdone is zero?
• Let's take the force applied on the cupboard by you as "F" and the displacement as "s". Then, you will be doing a work of "F ⋅ s". At the same time, friction will be exerting a force in the opposite direction. Its force will be "-F". The work done by friction on the cupboard will be "-F ⋅ s". Therefore, the total work done would be "F ⋅ s - F ⋅ s". This means that the total work done on the object is zero.
• at , the author says if we raise a puppy, we do 0 work. However, when you lift objects, you do work because you cause a displacement and apply force. Additionally, PE changes of that puppy... so Work= change in PE... am I incorrect?
• If you listen carefully up till , you will understand that while pushing the puppy upwards, gravity pushes the puppy downwards. Hence, there is no change in K.E. So, zero work is done. Also, one more way to look at it is is that whilst you are pushing the puppy upwards, the force applied is perpendicular to the puppy. So, even in that case, work done will be zero.
(1 vote)
• but at you increase the potential energy of that puppy....
• True, but we don’t care
(1 vote)
• meaning of theta

cos theta

you know what mean
right??
• These are trigonometric functions. They are basically just scenarios in a right-angle triangle. Theta (Θ) is an angle in a right-angle triangle.
Example: sin = opposite side ÷ hypotenuse
∴ sin Θ = opposite side of Θ ÷ hypotenuse
• Hi,

I must say that this video should be removed or modified, because there is a fundamental error.

Work is not (only) the difference in kinetic energy.

If that was true, after you have finished pushing on the cupboard, the speed is 0, so no work would have been done.
This is wrong : you had to spend energy to move that cupboard. Some work was added, and the amount of this work is F.s

For the puppy, you increase its potential gravitational energy = you add energy to it = you add some work to it. (work is energy, & both are in J)
There again, W = F.s
and F = the force pushing up - gravity.