CBSE Class 11 Physics Work Energy Power Solved Examples

Ex.1: A body is moving under the influence of a single constant force acting along the direction of motion.

Ex.3. The block of mass M shown in the figure initially has a velocity v0 to the right and its position is such that the spring exerts no force on it, i.e., the spring is not stretched or compressed. The block moves to the right a distance l before stopping in the dotted position shown. The spring constant is k and the coefficient of kinetic friction between block and table is μ. As the block moves the distance l,

(a) Find the work done on it by the friction force.
(b) Find the work done on it by the spring force.
(c) Are there other forces acting on the block, and if so, what work do they do ?
(d) Find the total work done on the block.
(e) Use the work-energy theorem to find the value of l in terms of M, v0, μ, g and k.
Sol.: (a) Work done by friction = –μ M g l.
(b) Work done by the spring force = – 1/2  k l ².
(c) Gravitational force and normal reaction of the table act in the direction perpendicular todisplacement, no work is done by these forces
(d) Total work done on block = –(μ M g l + 1/2  k l ²).
(e) According to work-energy theorem change in kinetic energy = Work done

Ex.4. An ideal massless spring S can be compressed 2 metre by a force of 200 N. This spring is placed at the bottom of a frictionless inclined plane which makes an angle θ = 30º with the horizontal. A 20 kg mass is released from rest at the top of the inclined plane and is brought to rest momentarily after compressing the spring by 4 metre.
(a) Through what distance does the mass slide before coming to rest ?
(b) What is the speed of the mass just before it reaches the spring ?