REVISION CONCEPTS
Work Done: Work done on an object is defined as the magnitude of the force multiplied by the distance moved by the object in the direction of the applied force.
Work done = force × displacement = F × s
1 Joule = 1 Newton × 1 metre. The S.I. unit of work is joule.
• Work done by a force acting obliquely is equal to W = F cos θ × s
• Work done on a body moving in circular path is zero.
Energy : The energy of a body is its capacity of doing work. The S. I. unit of energy is Joule (1 KJ = 1000 J).
Form of energy : (P.E & K.E.), heat energy, light energy, sound energy etc.
Kinetic energy : Kinetic energy is the energy possessed by a moving body by virtue of its motion.
Potential energy : Potential energy is the energy possessed by the body due to its position or shape.
Law of conservation of energy states that whenever energy gets transformed from one form to another, the total energy remains unchanged. i.e., energy can neither be created nor destroyed.
Total Energy (E) +
Power(P) Power is defined as the rate of doing work. S.I. unit of power is Watt. (W).
Commercial unit of energy is kWh (kilowatt –hour). 1 Unit = 1 kWh= 3.6 ×106Jule
NCERT SOLUTIONS (By Concept Academy)
QUESTIONS FROM NCERT TEXTBOOK
Q1. A force of 7N as on an object. The displacement is, say 8 m, in the direction of the force. Let us take it that the force acts on the object through the displacement. Wtm is the work done in this case?
Ans. F=7N s = 8 m W = F × s = 7 N × 8 m W = 56J ∴ Work done is 56 J.
NCERT TEXTBOOK PAGE 149
Q1. When do we say that work is done?
Ans. Work is said to be done when force applied on an object shows the displacement in that object W = F x s.
Q2. Write an expression for the work done when a force is acting on an object in the direction of its displacement.
Ans. Work done = W Force = F Displacement = s
Work done is equal to the product of the force and displacement W = F × s
Q3. Define 1 J of work.
Ans. 1 Joule of work is said to be done when force of 1 Newton acts on a by and shows displacement of 1 metre.
Q4. A pair of bullocks exerts a force of 140 N on a plough. The field being ploughed is 15 m long. How much work is done in ploughing the length of the field?
Ans. Force = 140 N s = 15m W = ?
W = F × s = 140 N x 15 m = 2100 Nm W = 2100 J
∴ Work done in ploughing the length of the field is 2100 J.
NCERT TEXTBOOK PAGE 162
Q1. What is the kinetic energy of an object?
Ans. The energy possessed by an object due to its motion is called kinetic energy of that object
Where m is mass of the object and v is velocity of the object.
Q2. Write an expression for the kinetic energy of an object.
Q3. The kinetic energy of an object of mass, m moving with a velocity of 5 ms 1 is 25 What will be its kinetic energy when its velocity is doubled? What will be its kine energy when its velocity is increased three times.