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Tuesday, 12 November 2013

F.Sc Notes: Physics XI: Chapter 04 Work and Energy Exercise Short Questions:

F.Sc Notes: Physics XI: Chapter 04 Work and Energy Exercise Short Questions:


Question 4.1 A person holds a bag of groceries while standing still, talking to a friend. A car is stationary with its engine running. From the stand point of work, how are these two situations similar?
Answer 4.1
Since Work = Force* displacement. In both cases displacement is zero so work done in both cases is zero.


Question 4.2 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.
Answer 4.2
In this case work is stored in the body in the form of potential energy
So Work = P.E = mgh = 10*9.8*10 = 980 J = 0.98 KJ.


Question 4.3 A force F acts through a distance L. The force is then increased to 3F, and then acts through a further distance of 2L. Draw the work diagram to scale.
Answer 4.3
  A force F acts through a distance L, then work done =FL.
If force is increased to 3F and distance is increased to 2L.
Then work done = 3F.2L = 6FL. Total work done = 6FL+FL=7FL.


Question 4.4 In which case is more work done? When a 50 kg bag of books is lifted through 50 cm, when a 50 kg crate is pushed through 2m across the floor with a force of 50N?
Answer 4.4
(a) m= 50kg h= 50cm = 0.5m.
So Work Done = W1= mgh= 50*9.8*.5.
Work done = W1 = 245J.     

(b) m= 50kg  d= 2m F=50N.
So Work Done =  W2= F.d= 50*2.
Work done = W2 = 100J.


Question 4.5 An object has 1J of potential energy. Explain what does it mean?
Answer 4.5
An object has potential energy when it is in a force field such as gravitational field or it is in a constrained state such as stretched spring.
  1. When an object is raised by doing 1J work. P.E = mgh = W.h =1J (1N*1m). When 1N force is applied on a body to lift it through 1m. The work appears as gravitational P.E.
  2. When a spring is stretched or extended by doing 1J of work. The work is conserved in it as elastic P.E.


Question 4.6 A ball of mass m is held at a height h1 above a table. The table top 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?
Answer 4.6
Both statements are correct. It is a matter of relative position for specifying the P.E of the ball. The P.E relative to the table top is mgh1 and P.E relative to ground is mg (h1+h2).


Question 4.7 When a rocket re-enters the atmosphere, its nose cone becomes very hot. Where this heat does comes from?
Answer 4.7
When a rocket re-enters the atmosphere, its nose cone becomes very hot due to air friction and dust particles.


Question 4.8 What sort of energy is in the following:
  1. Compressed spring
  2. Water in a high dam
  3. A moving car.
Answer 4.8 Energy stored in stated above is given below:
  1. Elastic P.E.
  2. Gravitational P.E.
  3. Kinetic Energy.


Question 4.9 A girl drops a cup from a certain height, which breaks into pieces. What energy changes are involved?
Answer 4.9
The cup has gravitational P.E due to its height. When it is dropped its P.E changes to K.E just before hitting the ground the P.E completely changes to K.E. On hitting the ground K.E changes to heat, sound, and work done in breaking the cup.


Question 4.10 A boy uses a catapult to throw a stone which accidentally smashes a green house window. List the possible energy changes?
Answer 4.10
The elastic P.E of the catapult is given to stone as its K.E. When the stone smashes the green house window the K.E of the stone is changed into sound, heat and work done in breaking the glass.

Written By: Asad Hussain

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