# Work done concepts

Here are the questions for identifying the statement whether they are true or not which I am confused with

A man has less total energy after he performed work on another object

When some amount of fuel is used, the universe has less energy

The KE of an object must be changed if its momentum is changed

A car must develop a larger power to move up an inclined road

There is a larger work done against friction when moving up an inclined road

And a question: A truck is pushed towards a spring which is fixed on a wall.

(1) The truck compresses the spring when the truch compresses it.

(2) It exists an instant such that all the KE of the truck has transformed to the elastic potential energy of the spring

(3) When the truck leaves the spring, the momentum of the truck has changed while the KE of the truck remains unchange.

### 1 個解答

• 天同
Lv 7
1 十年前
最愛解答

Q: A man has less total energy after he performed work on another object.

False. Metabolism and respiration would replensih the energy used by the man.

Q: When some amount of fuel is used, the universe has less energy

False. This is only a transformation of energy. By the Law of Conservation of Energy, energy can neither be created nor destroyed.

Q: The KE of an object must be changed if its momentum is changed

False. KE is a scalar quantity, whereas momentum is a vector quantity. There may happen in a change in direction of a moving object, but with the speed be kept constant. This results in a change in momentum, but not a change in kinetic energy.

Q: A car must develop a larger power to move up an inclined road

True. It is because there is an increase in potential energy of the car.

Q: There is a larger work done against friction when moving up an inclined road

False. The friction is reduced because of a decrease in normal reaction on an inclined road.

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Q: When the truck leaves the spring, the momentum of the truck has changed while the KE of the truck remains unchange.

Yes. KE is a scalar quantity, whereas momentum is a vector quantity. The direction of the truck has changed, but the speed remains unchanged. This results in a change in momentum, but not a change in kinetic energy.