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Force and Laws of Motion
Complete NCERT Solutions for Force and Laws of Motion.
Get answers for Newton's laws problems for Class 9 physics.
Question 9.1
An object experiences a net zero external unbalanced force. Is it possible for the object to be travelling with a non-zero velocity? If yes, state the condition.
Answer & Explanation:
Yes, an object can travel with a non-zero velocity even when the net external force acting on it is zero.
This happens when the object is moving with a constant velocity, i.e., neither its speed nor its direction is changing, according to Newton’s First Law of Motion.
This happens when the object is moving with a constant velocity, i.e., neither its speed nor its direction is changing, according to Newton’s First Law of Motion.
Question 9.2
When a carpet is beaten with a stick, dust comes out of it. Explain.
Answer & Explanation:
When the carpet is beaten, it comes into motion suddenly, but the dust particles tend to remain at rest due to inertia of rest.
As a result, the dust particles get separated from the carpet and fall off.
As a result, the dust particles get separated from the carpet and fall off.
Question 9.3
Why is it advised to tie any luggage kept on the roof of a bus with a rope?
Answer & Explanation:
The luggage has inertia of motion. During sudden braking or a sharp turn, it may continue moving and fall off the bus.
Tying the luggage with a rope prevents it from sliding or falling, ensuring safety.
Tying the luggage with a rope prevents it from sliding or falling, ensuring safety.
Question 9.4
A batsman hits a cricket ball which then rolls on a level ground. After covering a short distance, the ball comes to rest. Why does the ball slow down?
Answer & Explanation:
The ball slows down due to the force of friction between the ball and the ground, and also due to air resistance.
These forces oppose the motion of the ball and eventually bring it to rest.
These forces oppose the motion of the ball and eventually bring it to rest.
Question 9.5
Explain Newton’s First Law of Motion.
Answer & Explanation:
Newton’s First Law of Motion states that an object remains at rest or continues to move with uniform velocity in a straight line unless acted upon by an external unbalanced force.
This law is also known as the law of inertia.
This law is also known as the law of inertia.
Question 9.6
Define inertia. Give an example.
Answer & Explanation:
Inertia is the property of a body by which it resists any change in its state of rest or motion.
Example: Passengers fall backward when a bus starts suddenly due to inertia of rest.
Example: Passengers fall backward when a bus starts suddenly due to inertia of rest.
Question 9.7
Define momentum. Write its SI unit.
Answer & Explanation:
Momentum is defined as the product of the mass and velocity of a body.
Mathematically: p = mv
SI unit of momentum is kg m s−1.
Mathematically: p = mv
SI unit of momentum is kg m s−1.
Question 9.8
Explain Newton’s Second Law of Motion.
Answer & Explanation:
Newton’s Second Law states that the rate of change of momentum of an object is directly proportional to the applied force and takes place in the direction of the force.
Mathematically: F = ma
Mathematically: F = ma
Question 9.9
Define one newton of force.
Answer & Explanation:
One newton is the force that produces an acceleration of 1 m s−2 in a body of mass 1 kg.
Question 9.10
Explain Newton’s Third Law of Motion with an example.
Answer & Explanation:
Newton’s Third Law states that for every action, there is an equal and opposite reaction.
Example: When a person jumps off a boat, the boat moves backward while the person moves forward.
Example: When a person jumps off a boat, the boat moves backward while the person moves forward.
📘 Exam Preparation Tip:
Thoroughly understand the three laws of motion with real-life examples. Practice numericals based on the second law (F=ma) and the conservation of momentum. Differentiate between balanced and unbalanced forces.