Important Diagrams: Work and Energy | Science Class 9 PDF Download

Q1: Answer the following questions based on the diagram given below:

(i) What is work according to the experiment?
Ans: Work is done when a constant force is applied on an object in the direction of its motion and causes a displacement.

(ii) What is the SI unit of work?
Ans: The SI unit of work is the joule (J), where 1 joule is equal to 1 newton-meter (1 J = 1 N·m).

(iii) From the figure, what is the amount of work done?
Ans:  It must be
Work = F × S
= 7×8
= 56 N

(iv) In what conditions is work done said to be zero based on the experiment?
Ans: Work done is said to be zero when the force applied on the object is zero or when the object does not move (displacement is zero), even if a force is applied.

(v) How is work related to the displacement of the object in this experiment?
Ans: In this experiment, work is directly proportional to the displacement of the object in the direction of the applied force. The greater the displacement, the more work is done if the force remains constant.

Q2: Answer the following questions based on the diagram given below:

(i) Fill in the blank
(a) When a ball is thrown upwards, ______ energy is transformed into ________  energy.
(b) The sum of the potential and kinetic energies of a body is called ________ energy.
(c) Work is measured as a product of ______ and ______.
(d) The electricity meter installed in our home measures electric energy in the units of ______.
(e) The work done on a body moving in a circular path is ______.
Ans: 
(a) When a ball is thrown upwards, kinetic energy is transformed into potential energy.
(b) The sum of the potential and kinetic energies of a body is called mechanical energy.
(c) Work is measured as a product of force and distance.
(d) The electricity meter installed in our home measures electric energy in the units of kWh.
(e) The work done on a body moving in a circular path is zero.

(ii) When do we say that work is done?
Ans: Work is done when a force acts on an object and displacement occurs in the direction of force.

(iii) Define 1 J of work.
Ans: 1 J is that work which is done when a force of 1 N is applied on an object and it moves a distance of 1 m in the direction of force.

Q3: Answer the following questions based on the diagram given below:

(i) What is kinetic energy?
Ans: Kinetic energy is the energy an object possesses due to its motion. It is the energy stored in an object when work is done to change its velocity from rest or a lower value to a higher value.

(ii) How is kinetic energy defined and calculated?
Ans: Kinetic energy (KE) is defined as half the product of the mass (m) of an object and the square of its velocity (v). The formula for kinetic energy is KE = (1/2)mv².

(iii) What is the work-energy theorem?
Ans: The work-energy theorem states that the net work done on a moving body is equal to the change in its kinetic energy. Mathematically, it is represented as: W_net = KE_final - KE_initial.

(iv) Give examples of objects possessing kinetic energy.
Ans: Some examples include a rolling ball, a fired bullet, a moving torpedo, flowing water, strong wind, a moving hammer, and a falling body.

(v) What factors affect kinetic energy?
Ans: Kinetic energy depends on two main factors: mass and velocity. The greater the mass of an object and the higher its velocity, the more kinetic energy it possesses. Momentum is also related to kinetic energy, as it is the product of mass and velocity (momentum = mv).