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Work and Energy
Physics IX : Complete NCERT Exercise Solutions
Access free NCERT Solutions for Class 9 Science Chapter 11: Work and Energy. Understand the scientific conception of work, kinetic & potential energy, law of conservation of energy, power, and commercial unit of energy with solved problems.
Question 11.1
A force of 7 N acts on an object. The displacement is 8 m in the direction of the force. Calculate the work done on the object.
Answer & Explanation:
Given:
Force (F) = 7 N
Displacement (s) = 8 m
Work done is given by:
W = F × s
W = 7 × 8 = 56 J
Therefore, the work done on the object is 56 joules.
Force (F) = 7 N
Displacement (s) = 8 m
Work done is given by:
W = F × s
W = 7 × 8 = 56 J
Therefore, the work done on the object is 56 joules.
Question 11.2
When do we say that work is done?
Answer & Explanation:
Work is said to be done when:
• A force acts on an object, and
• The object undergoes displacement in the direction of the force.
If there is no displacement, no work is done.
• A force acts on an object, and
• The object undergoes displacement in the direction of the force.
If there is no displacement, no work is done.
Question 11.3
What is the SI unit of work? Define it.
Answer & Explanation:
The SI unit of work is joule (J).
One joule of work is said to be done when a force of 1 newton displaces an object through a distance of 1 metre in the direction of the force.
One joule of work is said to be done when a force of 1 newton displaces an object through a distance of 1 metre in the direction of the force.
Question 11.4
Is work done when a person holds a heavy suitcase without moving it? Give reason.
Answer & Explanation:
No work is done because although a force is applied, there is no displacement of the suitcase.
Since displacement is zero, work done is also zero.
Since displacement is zero, work done is also zero.
Question 11.5
Define energy.
Answer & Explanation:
Energy is defined as the capacity to do work.
An object possessing energy can perform work.
An object possessing energy can perform work.
Question 11.6
What is kinetic energy? Write its formula.
Answer & Explanation:
Kinetic energy is the energy possessed by a body due to its motion.
Formula:
KE = ½ mv2
Formula:
KE = ½ mv2
Question 11.7
What is potential energy? Give an example.
Answer & Explanation:
Potential energy is the energy possessed by an object due to its position or configuration.
Example: Water stored in a dam has potential energy.
Example: Water stored in a dam has potential energy.
Question 11.8
Write the expression for gravitational potential energy.
Answer & Explanation:
The expression for gravitational potential energy is:
PE = mgh
where m = mass, g = acceleration due to gravity, h = height.
PE = mgh
where m = mass, g = acceleration due to gravity, h = height.
Question 11.9
What is the law of conservation of energy?
Answer & Explanation:
The law of conservation of energy states that energy can neither be created nor destroyed; it can only be transformed from one form to another.
The total energy of a system remains constant.
The total energy of a system remains constant.
Question 11.10
What is power? Write its SI unit.
Answer & Explanation:
Power is defined as the rate at which work is done.
Formula:
P = W / t
SI unit of power is watt (W).
Formula:
P = W / t
SI unit of power is watt (W).
📘 Exam Preparation Tip:
Remember the scientific definition of work (W = F*s*cosθ). Practice numericals on calculating kinetic and potential energy. Understand the law of conservation of energy with examples. Learn the conversion between Joules and kilowatt-hour.