Yes, mechanical energy comprises of both potential energy and kinetic energy. Zero momentum means that velocity is zero. Hence, there it no kinetic energy but the object may possess potential energy.
Q.2. The potential energy of a body is 39600J. How high is the body if its mass is 20kg?
The potential energy of a body = mgh
h = PE/mg = 39600j/20kg x9.8m/s2=198m
Q.3. A force of 20 N displaces a body through a distance of 1 m at an angle of 60° from its own direction. Calculate the amount of work done.
Here, force F = 20 N, displacement, s = 1 m. Angle between force and displacement 60°.
Work done,W =Fscos0 =20 X 1 X cos60°=20X 1 X 1/2 = 10J.
A man of 50 kg jumps up to a height of 1.2 m. What is his potential energy at the highest point?
The potential energy of man = mgh = = 50 + 10 X 1.2 J = 600 J
Q.4. How much work is done by a force of 10 N in moving an object through a distance of 4 m in the direction of the force.
Work done Force x Displacement =F.s = (10 N) x (4 rn) = 40 joule or 40J.
Q.5. A light and a heavy object have the same momentum find out the ratio of their kinetic energies.
Which one has a larger kinetic energy?
Linear momentum of 1st object = p1=m1v1
Linear momentum of 2nd object = p2=m2v2
Given, p1 > p2 ---------------------------------(i)
Þ m1v1 > m2v2
But, m1<m2 (A light and a heavy object) Þ v1 > v2 ------------(ii)
Ke = ½ mv2 = ½ m vx v =1/2 pv
From (i)and (ii) p1v1 > p2v2 Þ ½ p1v1 > ½ p2v2 Þ KE1> KE2
Q.6. What is power? How do you differentiate kilowatt from kilowatt hour?
Power is the rate of doing work. Kilowatt is the unit of power and kilowatt hour is the unit of energy.
Q.7. 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?
Initial KE/Final KE = ( ½ mu2) /( ½ mv2) = ( ½ mu2) /{ ½ m(3v2)} =1:9
Q.8. Calculate the work done in lifting 200 kg of water through a vertical height of 6 m.
Work done in lifting a body = Weight of body X vertical distance
The work done in lifting = W = mgh = 200 kg x 10m/s2 x6 m = 1200J
Q.9. Give one example each of potential energy (i) due to position (ii) due to shape.
(i) Potential energy due to position: Water stored in dam has potential energy.
(ii) Potential energy due to shape: In a toy car, the wound spring possesses potential energy, and as the spring is released, its potential energy changes into kinetic energy due to which the car moves.
Q.10. What kind of energy transformation takes place when a body is dropped from a certain height?
When a body falls, its potential energy gradually gets converted into kinetic energy. On reaching the ground, the whole of the potential energy of the body gets converted into kinetic energy
IX Physics Chapter: Work Power and Energy
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IX Physics Chapter: Work Power and Energy
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9th Work, Energy and Power CBSE Test Paper
9th Work, Energy and Power CBSE Guess Paper
IX Work, Energy and Power Study material Download Files
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