0825 2.

asked • 01/04/22

Can we represent energy negative?

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Grigoriy S. answered • 01/04/22

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If it is potential energy, then the answer is yes. Kinetic energy of the body is always positive or zero. But if we talk about the potential energy, you need to recall that the zero's level of potential energy selected arbitrary. Say you can set the zero's level of potential energy of the book, that seats on the table, on the floor. From another hand, if you want, you can set it to zero on the table's level. What matters - is the difference of potential energies (it will be equal to the work with negative sign). BTW, total energy of the body in the Earth's gravitational field is also negative!

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0825 2.

Yes, but I think KE can also be zero when deceleration takes place, where u would be greater than v..
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01/04/22

Grigoriy S.

tutor
I wrote that kinetic energy could be 0. In case of the decelerated motion the change of kinetic energy is negative, because the final velocity is less. Keep in mind that we consider any motion with respect to the reference frame. If with respect to the reference frame the velocity of the body is zero, then its kinetic energy is also zero.
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01/04/22

0825 2.

oh right! I meant to say negative instead of 0
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01/05/22

Grigoriy S.

tutor
By definition kinetic energy is one half of the product of mass of the body and square of its velocity. Mass is always positive, square of the number is not negative (zero or positive), hence kinetic energy is not negative. In case of potential energy the picture is different. I told you about the zero's level of potential energy. We choose it as we want. If it is level of the table, then with respect to the floor the potential energy of the body is negative. But kinetic energy cannot be negative; it is against definition of this type of energy. BTW, in case of linear momentum it can be negative , if we select the positive direction of motion, say to the right, and the body moves to the left.
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01/05/22

0825 2.

But change in KE can be negative(deceleration), so isn't that too KE?
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01/05/22

Grigoriy S.

tutor
No, energy and change of energy - different things. Still if you calculate energy in case 2, it will be positive: you are taking positive mass, multiply by square of velocity (also positive number) and divide by positive 2. Result will be positive.
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01/05/22

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