Energy 1

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1. Which one of the following must remain constant if balanced forces act on an object?
A. mechanical energy
B. Ek
C. Ep
D. Ep and Ek
Answer
Ek
If forces are balanced, then an object can only move at constant velocity. Hence Ek constant.
Note that balanced forces can be used to lift an object, and Ep would hence increase.
2. A pupil lifts a block from the floor and places it on a table. Which one of the following physical quantities does not affect the magnitude of the work done by the pupil?
A. the mass of the book
B. the weight of the book
C. the time taken to lift the book
D. the height through which the book was lifted
Answer
C.
The work done is given by Ep = mgh
Time is not part of this equation.
The time taken affects the POWER.
3. A stone is projected vertically upwards from the surface of the Earth at a speed u. If the effect of air friction is taken into account, then the stone will arrive back at the Earth's surface with a speed :
A. equal to zero
B. less than u
C. equal to u
D. greater than u
Answer
B.
The stone would continue to lose kinetic energy upwards and downwards.
4. If a stone of mass x falls freely through a vertical distance h, it's kinetic energy gained will be :
A. xh
B. gh
C. ½xh2
D. xgh
Answer
D.
Loss in Ep equals gain in Ek
5. A stone falls freely from rest from a height h. At which point would it's Ep and Ek be equal?
A. ½h
B. h
C. at ground level
D. could never be equal
Answer
A. ½h
6. What is meant by “a closed system”? Answer
No external forces act in the system, such as friction, forces from an engine, etc.
7. What is meant by “an elastic collision”? Answer
There is no loss of kinetic energy in the collision. The total kinetic energy before the collision is equal to the total kinetic energy energy after the collision.
8. For an object thrown vertially upwards, which statement is true when it reaches its highest point?
A. Ep = Ek
B. acc = 0
C. p = 0
D. Em = 0
Answer
C.
The velocity would be zero, hence p would be zero.

9. A constant resultant force acts on a body that is free to move. Which one of the following quantities of the body would not change in magnitude while this force acts?
A. velocity
B. acceleration
C. momentum
D. kinetic energy
Answer
B.
From Newton's Second Law, acc is proportional to FR
10. The unit kilowatt-hour measures:
A. power
B. energy
C. time
D. impulse
Answer
B.
kilowatt-hour is really Pt.
And W = Pt
Hence energy.

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