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NCERT Class 9 Science Important Chapter 6 How Forces Affect Motion
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How Forces Affect Motion
Chapter – 6
| IMPORTANT QUESTION ANSWER |
Short Questions & Answers:
1. What effects can a force have on an object?
Ans: A force can make an object move from rest, change the speed and direction of a moving object, and even change the shape of an object.
2. Is force a scalar or a vector quantity?
Ans: Force is a vector quantity because you need to specify its direction along with its magnitude and unit.
3. What is the SI unit of force?
Ans: The SI unit of force is the newton, and its symbol is N.
4. What instrument is commonly used to measure the magnitude of a force, such as weight?
Ans: A spring balance is commonly used to measure the magnitude of a force.
5. What are balanced forces?
Ans: Two forces applied to an object that are equal in magnitude but completely opposite in direction are called balanced forces.
6. What is the force of friction?
Ans: It is a force that arises between surfaces in contact, always acting in a direction opposite to the direction of the object’s motion or the applied force.
7. What forces are acting on a stationary box pushed by a person on the floor?
Ans: The applied force (push), the force of friction, the gravitational force (weight) acting downwards, and the normal force acting upwards.
8. Who first argued through thought experiments that a body moves indefinitely on a horizontal plane if all impediments are removed?
Ans: Galileo Galilei argued this in the 17th century.
9. What term did Isaac Newton use to describe an object’s tendency to resist changes in its state of rest or motion?
Ans: Newton used the word ‘inertia’ to describe this tendency.
10. State Newton’s first law of motion.
Ans: An object at rest remains at rest, and an object in motion continues to move with a constant velocity, unless a net force acts upon the object.
11. State Newton’s second law of motion.
Ans: When a net force acts on an object, the object accelerates in the direction of the net force. The magnitude of the acceleration is proportional to the net force and inversely proportional to the object’s mass.
12. What is the mathematical formula for Newton’s second law?
Ans: The formula is F = ma, where F denotes force, m denotes mass, and a denotes acceleration.
13. How is one newton (1 N) of force defined?
Ans: One newton is defined as the force that produces an acceleration of 1 m s–2 on an object of mass 1 kg.
14. How is the momentum of an object defined?
Ans: The momentum of an object is defined as the product of its mass and its velocity.
15. Why does a cricket fielder pull their hands backwards while catching a fast-moving ball?
Ans: Pulling the hands back increases the time duration for the ball’s velocity to reduce to zero. This decreases the acceleration and reduces the force applied to the fielder’s hands, preventing injury.
16. State Newton’s third law of motion.
Ans: Whenever one object is exerting a force on a second object, the second object is simultaneously exerting an equal and opposite force on the first object.
17. Do the action and reaction forces in Newton’s third law act on the same object?
Ans: No, these equal and opposite forces always occur in pairs and act on two different interacting objects.
18. How does a person walk forward according to Newton’s laws?
Ans: A person pushes the ground backwards with their feet, and the ground applies an equal and opposite force of friction forward on their feet, enabling them to move.
19. Why is it difficult to climb a smooth tree trunk?
Ans: Climbing relies on friction between the tree trunk and the person’s legs pushing down, which produces an equal upward force on the person. Smooth trunks have less friction, making it harder to climb.
20. How does a rocket lift off into space?
Ans: The rocket’s engine expels gas downwards, and according to Newton’s third law, the exhaust gas exerts an equal and opposite upward force on the rocket, launching it into the air.
Fill in the Blanks:
1. The SI unit of force is the __________.
Ans: Newton (N).
2. Newton used the term __________ to describe an object’s tendency to resist changes in motion.
Ans: Inertia.
3. The mathematical formula for Newton’s second law is __________.
Ans: F = ma.
4. The momentum of an object is the product of its __________ and velocity.
Ans: Mass.
5. A __________ balance is commonly used to measure force.
Ans: Spring.
True or False:
1. Force is a vector quantity.
Ans: True.
2. Balanced forces always cause an object to accelerate.
Ans: False.
3. According to Newton’s third law, action and reaction forces act on different objects.
Ans: True.
4. Friction always acts in the same direction as the applied force.
Ans: False.
5. A rocket lifts off because the exhaust gases push it upward according to Newton’s third law.
Ans: True.
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