Ans. In both cases work is zero, as there is no displacement; W = F d cos θ = F x 0 x cos θ = 0
Q.2 Calculate the work done in kilo joules in lifting a mass of 10 kg (at a steady velocity) through a vertical height of 10 m.
Ans. W = F d cos0o = F d = m a d = 10x9.8x10 = 980 J = 0.98 KJ
Q.3 A force F acts through a distance L. the force is then increased to 3 F, and then acts through a further distance of 2 L. Draw the work diagram to scale.
Ans. COMING SOON....
Q.4 In which case is more work done? When a 50 kg bag of books is lifted through 50 cm, or when a 50 kg crate is pushed through 2 m across the floor with a force of 50 N?
Ans. For books: W = F d cos θ = mgh cos 0o = mgh = 50 x 9.8 x 0.5 = 245 J (more work) For crate: W = F d cos θ = Fdcos 0o = Fd = 50 x 2 = 100 J More work is done in lifting bag of books.
Q.5 An object has 1 J of potential energy. Explain what it means?
Ans. PE
g = mgh = (1/9.8)x9.8x1 = 1 (N) x 1(m)= 1 N-m = 1 J ⇒ Force of one N is applied to a body to raise through 1 m. PEe = ½ kx2 = ½ k (√2/k)2 = 1 J ⇒ A spring is copressed or extended through a distance (√2/k), if k, elastic constant = 1 N, then the work of 1 J is conserved by compressing (or extending) of √2 m of spring.
Q.6 A ball of mass m is held at a height h1 above a table. The tabletop is at a height h2 above the floor. One student says that the ball has potential energy mgh1 but another says that it is mg(h1 + h2). Who is correct?
Ans. Both are correct if they give right reference. PEwrt table = mgh1 PEwrt floor = mg (h1 + h2)Q.7 When a rocket re-enters the atmosphere, its nose cone becomes very hot. Where does this heat energy come from?
Ans. Due to air friction, the nose cone of the rocket becomes very hot.
Q.8 What sort of energy is in the following:
0a) Compressed spring
b) Water in a high dam
c) A moving car
Ans. a) Elastic PE in compressed spring. b) Gravitational PE in water in a high dam. c) Kinetic energy in a moving car.
Q.9 A girl drops a cup from a certain height, which breaks into pieces. What energy
changes are involved?
Ans. PE → gain in KE → sound energy + heat energy
(holding the cup) (dropping the cup) (breaking into pieces)
Q.10 A body uses a catapult to throw a stone, which accidentally smashes a green house window. List the possible energy changes.
Ans. PEelastic → gain in KE → sound + heat energy + W. done in breaking
(stone in catapult) (throwing the stone) (smashing of window)
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