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**Q.1 Â Â A rocket is moving up with a velocity v. If the velocity of this rocket is suddenly tripled, what will be the ratio of two kinetic energies?
Sol. Â Â Â **Â Since K.E , if velocity is tripled, the K.E will become 9 time. Ratio will be 1:9.

**Q.2 Â Â Avinash can run with a speed of 8 against the frictional force of 10 N, and Kapil can move with a speed of 3 m against the frictional force of 25 N. Who is more powerful and why?
**

Â Â Â Â Â Â Â Power of Kapil = 10 Ã— 8 = 80 W.

Â Â Â Â Â Â Â Power of Avinash = 25 Ã— 3 = 75 W.

Â Â Â Â Â Â Â Â Kapil is more powerful.

**Q.3 Â Â Can any object have mechanical energy even if its momentum is zero? Explain.
**

**Q.4 Â Â Can any object have momentum even if its mechanical energy is zero? Explain.
**

**Q.5 Â Â The power of a motor pump is 2 KW. How much water per minute the pump can raise to a height of 10 m?(Given g = 10 m)
**

Â Â Â Â Â Â Â Mass of water =

**Q.6 Â Â The weight of a person on a planet A is about half that on the earth. He can jump up to 0.4 m height on the surface of the earth. How high he can jump on theÂ planet A?
**

Â Â Â Â Â Â Â Since, his energy is same; on the planet it will beÂ

Â Â Â Â Â Â Â Therefore, mg Ã— 0.4 =

Â Â Â Â Â Â Â Therefore, h= 0.8 m on the planet.

**Q.7 Â Â The velocity of a body moving in a straight line is increased by applying a constant force F, for some distance in the direction of the motion.
**

Â Â Â Â Â Â Â Than work done = F Ã— d = mad...........(i)

Â Â Â Â Â Â Â Now by Newton's 3rd eq. of motion.

Â Â Â Â Â Â Â

Â Â Â Â Â Â Â ..................(ii)

Â Â Â Â Â Â Â Substituting the value of ad from (ii) in (i), we get ,

Â Â Â Â Â Â Â Work done =

Â Â Â Â Â Â Â = Final K.E - Initial K.E.

Â Â Â Â Â Â Â Thus work done = change in kinetic energy

**Q.8 Â Â Is it possible that an object is in the state of accelerated motion due to external force acting on it, but no work is being done by the force?** **Explain it with an example.
**

hence work do by the gravitational force is zero. Thus work done can be zero for an accelerated body.

**Q.9 Â Â A ball is dropped from a height of 10 in. If the energy of the ball reduces by 40% after striking the ground, how much high can the ball bounce back?** **(g = )
**

Â Â Â Â Â Â Â = mg 10.

Â Â Â Â Â Â Â After striking the ground, energy left Ã— mg 10. = 6 mg.

Â Â Â Â Â Â Â If this energy is converted to P.E = mgh

Â Â Â Â Â Â Â mgh = 6 mg

Â Â Â Â Â Â Â h = 6 m

**Q.10 Â Â If an electric iron of 1200 W is used for 30 minutes everyday, find electric energy consumed in the month of April.
**

Â Â Â Â Â Â Â Â = 1200 Ã— Ã— 30 days.

Â Â Â Â Â Â Â Â = 1800 Wh = 18 KWh = 18 Units