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Chapter Four – Work and Energy

This chapter deals is about work done in several situations, energy, types of energy and also relation between work and energy.
Dot Product of force and displacement is called work done by a constant force. If the force does not remains constant, then work done is said to be work done by a variable force.
Work done In the gravitational field does not depend on path followed, so gravitational field is called a conservative field.
Time rate of doing work is called power.
Ability to do work is known as energy. There are two types of energy; Kinetic energy and Potential energy. Energy due to motion of body is kinetic energy, whereas energy due to its position is potential energy.

Q 4.1: A man pushes a lawn mower with a 40N force directed at a angle of 20 degree downward from the horizontal. Find the work done by the man as he cuts a strip of grass 20m long.

Q 4.2: A rain drop m = 3.35 * 10(-5)kg falls vertically at a constant speed under the influence of the forces of gravity ad friction. In falling through 100m, how much work is done by: (a) Gravity: (b) Friction:

Q 4.3: Ten bricks each 6cm thick and mass 1.5kg, lie flat on a table. How much work is required to stack them one on the top of another.

Q 4.4: A car of mass 800kg traveling at 54km/h is brought to rest in 60 meters. Find the average retarding force on the car. What has happened to the original K.E?

Q 4.5: A 1000kg automobile at the top of an inclined plane 10m high and 100m long is released and rolls down the hill. What is its speed at the bottom of inclined if the average retarding force due to friction is 480N?

Q 4.6: If 100m(3) of water is pumped from a reservoir into a tank, 10 m higher than the reservoir, in 20 minutes. If density of water is 1000kg/m(3), find (a) The increase in P.E (b) The power delivered by pump.

Q 4.7: A force(thrust) of 400N is required to overcome road friction and air resistance in propelling an automobile at 80km/h. What power kW must the engine develop?

Q 4.8: How large a force is required to accelerate an electron (m 9.1 * 10 (-31)kg) from rest to a speed of 2 * 10(7)m/s) through a distance of 5cm.

Q 4.9: A diver weighing 750N dive from a board 10m above the surface of a pool of water. Use the conservation of mechanical energy to find the speed at a point 5m above the water surface, neglecting air friction.

Q 4.10: A child starts from the rest at the top of a slide of height 4m. (a) What is his speed at the bottom if the slide is friction less? (b) If he reaches the bottom, with a speed of 6 m/s. What percentage of his total energy at the top of the slide …