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Class 9 Science Chapter 6 How Forces Affect Motion Extra Questions
Class 9 Science Chapter 6 Extra Questions on How Forces Affect Motion
How Forces Affect Motion Class 9 Very Short Question Answer
Question 1.
What is a force?
Answer:
A force is a push or a pull that can change the state of rest or motion of an object, its speed, direction, or shape. It has both magnitude and direction. Its SI unit is newton (N).
Question 2.
What is the SI unit of force?
Answer:
The SI unit of force is newton, written with a small ‘n’. Its symbol is capital N.
1 N = 1 kg m/s2.
Question 3.
What are balanced forces?
Answer:
When two or more forces acting on an object produce zero net force, they are called balanced forces. Balanced forces do not change the state of motion of the object.
Question 4.
State Newton’s First Law of Motion.
Answer:
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 it.
Question 5.
Write the formula for Newton’s Second Law.
Answer:
F = ma, where
F is net force (in N),
m is mass (in kg), and
a is acceleration (in m/s squared).
Alternatively, a = \(\frac{F}{m}\).
Question 6.
State Newton’s Third Law of Motion.
Answer:
Whenever one object exerts a force on a second object, the second object simultaneously exerts an equal and opposite force on the first object. Action and reaction forces act on different objects.
Question 7.
Why does friction always act opposite to the direction of motion?
Answer:
Friction is a force that opposes relative motion between surfaces in contact. Since it opposes motion, it always acts in the direction opposite to the direction in which the object is moving or tends to move.
Question 8.
Name the instrument used to measure force.
Answer:
A spring balance is used to measure the magnitude of a force. When force is applied, the spring stretches, and the reading on the scale gives the magnitude of the force.
Question 9.
A book stays at rest on a table even after being pushed gently and not moving. Is the applied force zero or still present, and why?
Answer:
The force is still present but is balanced by friction and normal reaction. Since net force is zero, there is no motion.
Question 10.
Two students push a box in opposite directions with equal force but the box does not move. What hidden idea about force is shown here?
Answer:
It shows balanced forces and zero net force. Equal opposite forces cancel each other, so no acceleration occurs.
Question 11.
A stone thrown in space keeps moving even without continuous force. What does this suggest about force and motion?
Answer:
It suggests that force is not needed to maintain motion, only to change it. This demonstrates inertia.
Question 12.
Why does a heavier object require more effort to stop even if moving slowly?
Answer:
Because it has greater mass and hence greater inertia. More force is needed to change its state of motion.
How Forces Affect Motion Class 9 Short Question Answer
Question 1.
Why does a person sitting in a bus feel jerked backward when the bus suddenly starts?
Answer:
This is due to inertia. When the bus suddenly starts moving forward, the body of the person (due to inertia) tends to remain at rest. The lower part of the body (supported by the seat) moves forward with the bus, but the upper body lags behind due to inertia of rest. This gives the sensation of being jerked backward.
Question 2.
Explain why the force of friction between two surfaces depends on the nature of the surfaces.
Answer:
Friction arises due to the interlocking of irregularities (tiny bumps and grooves) on the surfaces in contact. A rougher surface has more and larger irregularities, leading to more interlocking and hence greater friction. A smoother surface has fewer irregularities, resulting in less friction. That is why a wooden surface has more friction than a polished marble surface.
Question 3.
What happens to the acceleration of an object if
(a) the force is doubled keeping mass constant, and
(b) the mass is doubled keeping force constant?
Answer:
From a = \(\frac{F}{m}\) ;
(a) If force is doubled (2F) and mass is constant :
a = \(\frac{2 F}{m}\) = 2 times the original acceleration.
Acceleration doubles.
(b) If mass is doubled (2 m) and force is constant:
a = \(\frac{F}{2 m}\) = half the original acceleration.
Acceleration becomes half.
Question 4.
Why do airbags in cars reduce injury during accidents?
Answer:
During a collision, the vehicle comes to a sudden stop. Without an airbag, the passenger’s body would decelerate very rapidly (large acceleration). Using F = ma, this large deceleration means a large force acts on the body, causing injury. The airbag inflates quickly to form a soft cushion. The passenger’s head pushes into the soft bag, increasing the stopping time. With more time, deceleration is less, and therefore the force on the passenger’s body is reduced, minimising injury.
Question 5.
Explain with an example how Newton’s Third Law helps us to walk.
Answer:
When we walk, we push the ground backwards with our feet. By Newton’s Third Law, the ground exerts an equal and opposite force on our feet in the forward direction. This forward force (which is actually friction) is what propels us forward. Without friction, such as on a very slippery surface, our feet would slip backward and we would be unable to walk forward effectively.
Question 6.
A student says that if no force acts, a moving object must stop. Is this always correct in real life and why?
Answer:
This is not always correct because in real life friction acts continuously. In absence of external force and friction, motion continues due to inertia. So motion does not stop on its own.
Question 7.
Two identical carts are pushed with different forces, but both move. How can their motion still be different even if both are moving?
Answer:
Both move due to unbalanced forces, but acceleration depends on force magnitude. Greater force produces higher acceleration. Their speeds and motion patterns will differ.
Question 8.
A spring balance shows different readings on Earth and Moon for the same object. What does this indicate about mass and weight?
Answer:
It shows that weight depends on gravitational force, which changes with location. Mass remains constant, but weight changes because g is different on Moon.
How Forces Affect Motion Class 9 Long Question Answer
Question 1.
State Newton’s Second Law of Motion. Derive the relationship F = ma and explain with examples.
Answer:
Newton’s Second Law states:
When a net force acts on an object, the object accelerates in the direction of the net force. The acceleration is directly proportional to the net force and inversely proportional to the mass of the object. Derivation: Let a net force F act on an object of mass m, producing acceleration a.
From the law, a is directly proportional to F (for constant m): a ∝ F.
Also, a is inversely proportional to m (for constant F): a ∝ 1/m.
Combining: a ∝ F/m.
This gives us F = ma (where the constant of proportionality is 1 in SI units).
Example 1 :
A car of mass 1000 kg accelerates at 2 m/s2.
The net force = 1000 × 2 = 2000 N.
Example 2:
A cricket fielder pulling hands back while catching increases stopping time, reducing acceleration, and thus reducing the force on their hands. Airbags work on the same principle.
Example 3:
Cracking a coconut in one go. The coconut is brought down at high velocity and stopped in a very short time. The small stopping time creates a very large deceleration, and thus a very large force that breaks the shell.
Question 2.
Explain Newton’s Third Law of Motion with three examples. Also explain why action and reaction forces do not cancel each other.
Answer:
Newton’s Third Law states:
Whenever one object exerts a force on a second object, the second object simultaneously exerts an equal and opposite force on the first object.
Example 1 (Walking):
A person pushes the ground backwards with their foot. The ground pushes the person forward with friction. These equal and opposite forces make the person walk forward.
Example 2 (Rowing a canoe):
The canoeist pushes water backward with a paddle. The water pushes the paddle (and canoe) forward with an equal force. The canoe moves forward.
Example 3 (Rocket launch):
A rocket engine expels gases downward. These gases push the rocket upward with an equal and opposite force, launching it into the sky. Why action-reaction forces do not cancel: Action and reaction forces are always equal in magnitude and opposite in direction. However, they act on two different objects. Cancellation of forces only happens when two forces act on the SAME object. Since these forces act on different objects, they cannot cancel each other.
Question 3.
What is inertia? Describe Newton’s First Law and explain how Galileo’s idea contributed to it. Give two practical examples of inertia.
Answer:
Inertia is the natural tendency of an object to resist any change in its state of rest or state of uniform motion in a straight line. Greater the mass of an object, greater is its inertia.
Newton’s First Law:
An object at rest remains at rest and an object in motion continues to move with constant velocity, unless a net force acts upon it.
Galileo’s Contribution:
Before Galileo, people believed a continuous force was needed to keep objects moving. Galileo, through thought experiments, showed that on a perfectly smooth (frictionless) horizontal surface, an object would continue moving forever without any force. Newton built on this idea, calling the property ‘inertia’ and formalising it as his first law in 1687.
Example 1 (Passenger in a bus):
When a moving bus stops suddenly, passengers jerk forward. This is because the passengers’ bodies tend to continue moving forward (inertia of motion) even though the bus stops.
Example 2 (Shaking a tree branch):
When a fruit tree’s branch is shaken, fruits fall off. The branch starts moving when shaken, but the fruits tend to remain at rest (inertia of rest), so they get separated and fall.
Question 4.
Explain the concept of forces on a system of objects using the example of two boxes connected by a string. How is Newton’s Second Law applied to a system?
Answer:
Consider two boxes of masses m1 and m2 placed on a frictionless surface, connected by a string. An external force F pulls m1 (Box 1) to the right.
Method 1:
For Box 1, the forces are F (right) and Tension T (left).
Net force on Box 1 = F – T = m1 × a.
For Box 2, only Tacts to the right. T = m2 × a.
Solving these equations gives acceleration and tension.
Method 2 (System approach):
Treat both boxes and the string as one system. The tension is an internal force and cancels out. Only the external force F matters. The total mass of the system is m1 + m1.
By Newton’s Second Law:
a = \(\frac{F}{\left(m_1+m_2\right)}\)
Both methods give the same acceleration, which demonstrates the power of the system approach. It simplifies calculations significantly. Internal forces (tensions between parts of a system) do not affect the overall acceleration of the system. Only external forces determine the system’s acceleration.
How Forces Affect Motion Class 9 Numerical Questions
Question 1.
What is the weight of an object of mass 5 kg? (Use g = 10 m/s2)
Answer:
W= mg
= 5 kg × 10 m/s2 = 50 N.
The weight acts in the downward direction.
Question 2.
A force of 5 N acts on a 2 kg object initially at rest. Find the acceleration and the velocity after 3 seconds.
Answer:
Using Newton’s Second Law: a = \(\sqrt{\frac{F}{m}}\) = \(\sqrt{\frac{5}{2}}\) = 2.5 m/s2.
Initial velocity u = 0.
Using v = u + at ;
v = 0 + 2.5 × 3 = 7.5 m/s.
The object accelerates at 2.5 m/s2 and reaches a velocity of 7.5 m/s after 3 seconds
Question 3.
Two forces of 8 N and 5 N are acting on a block. Find the net force when:
(i) both act in the same direction and
(ii) they act in opposite directions.
Answer:
(i) When both act in the same direction:
Net force = 8 + 5 = 13 N, acting in the direction of both forces.
(ii) When they act in opposite directions:
Net force = 8 – 5 = 3 N, acting in the direction of the larger force (8 N side).
Question 4.
The velocity-time graph of a hall of mass 30 g moving along a straight line on a long table is given in following figure. How much force does the table exert on the ball to bring it to rest?
Answer:
Given that,
Mass of the ball, m = 30 g = 0.03 kg
The velocity-time graph shows that the velocity of the ball at t = 0 is 25 cm s-1 = 0.25 ms-1
That is, initial velocity of the ball u = 0.25 ms-1.
The velocity of the ball at t = 6 is zero.
That is final velocity of the ball v = 0.
Time taken to come to rest t = 6 s
Acceleration of the ball,
a = v – \(\frac{u}{t}\)
= 0 – \(\frac{0.25}{6}\)
= – 0.042 m s-2
-ve sign indicated the ball is retarded.
Now Newton’s second law of motion,
F = ma = 0.03 × – 0.042
= – 0.00126 N = – 1.26 × 10-3 N
Question 5.
The velocity-time graph of a body of mass 50 g is shown in figure. Study this graph and answer the following questions:
(i) Calculate the force acting on the object in the time interval 0.3 s.
(ii) Calculate the force acting on the object in the time interval 0.10 s.
(iii) Is there any time interval in which no force acts on the body? Justify your answer.
Answer:
Given, Mass of body m = 50 g = 0.05 kg
(i) During time interval 0.3 s
Acceleration of the body, a = v – \(\frac{u}{t}\)
= 120 – \(\frac{0}{3}\) = 40 ms-2
Force, F = ma = 0.05 × 40 = 2 N
(ii) During time interval 6 – 10 s (Acceleration is negative; exact value depends on graph slope from 120 m/s to 0 m/s over 4s → a = – 30 ms-2; Force = -1.5 N)
(iii) Yes, between 3 s to 6 s the velocity is constant (horizontal line on graph), so acceleration a = 0 and therefore no force acts on the body (F = ma = 0).
How Forces Affect Motion Class 9 Case Based Questions
Case I.
Rahul and his friends were playing tug of war. Initially, both teams were pulling with equal force and the rope was not moving. Then Rahul’s team started pulling harder. The rope began to move towards Rahul’s team.
Answer the following questions:
Question 1.
What kind of forces were acting when the rope did not move?
Answer:
Balanced forces were acting. Both teams were pulling with equal force in opposite directions, so the net force was zero and the rope remained stationary.
Question 2.
What changed when Rahul’s team pulled harder?
Answer:
Rahul’s team applied a force of larger magnitude. The forces became unbalanced, with a net force in Rahul’s direction. This unbalanced net force caused the rope to accelerate in Rahul’s direction
Question 3.
Which law of motion explains the movement of the rope?
Answer:
Newton’s Second Law explains this. A net (unbalanced) force acts on the rope, producing acceleration in the direction of that net force (towards Rahul’s team). Newton’s First Law also applies: the rope was at rest when forces were balanced, and began moving only when an unbalanced force acted.
Case II.
During a cricket match, a fast bowler bowled a ball at 150 km/h. The batsman hit the ball and it went straight back to the bowler. The fielder caught the ball by pulling their hands backwards as the ball arrived.
Answer the following questions:
Question 1.
What force caused the hall to change direction when hit by the bat?
Answer:
The bat applied a large contact force on the ball. By Newton’s Third Law, the ball also exerted an equal and opposite force on the bat (which is why the batsman feels the impact). The large force from the bat changed the ball’s direction and speed.
Question 2.
Why did the fielder pull their hands backwards?
Answer:
By pulling hands backwards, the fielder increased the time over which the ball’s high velocity reduced to zero.
From Newton’s Second Law, a = \(\frac{\text { change in velocity }}{\text { Time }}\)
Larger time means smaller deceleration (acceleration).
From F = ma, smaller acceleration means smaller force on the hands, reducing pain and risk of injury.
Question 3.
Which law of motion is demonstrated by the ball returning to the bowler after being hit?
Answer:
Newton’s Third Law is best demonstrated here. The bat exerts a force on the ball, and the ball exerts an equal and opposite force on the bat. Additionally, Newton’s Second Law explains how the force changes the ball’s velocity and direction.
Case III.
A rocket is launched from Earth. Its engine burns fuel and expels hot gases downward at high speed. The rocket rises upward. Once in space, the engine can be fired in any direction to change the rocket’s velocity.
Answer the following questions:
Question 1.
Which law of motion explains why the rocket rises when gases are expelled downward?
Answer:
Newton’s Third Law of Motion. The engine applies a force on the exhaust gases, pushing them downward. The exhaust gases simultaneously apply an equal and opposite force on the rocket in the upward direction. This upward force lifts the rocket.
Question 2.
For the rocket to lift off the ground, what condition must be met?
Answer:
The upward force generated by the expelled gases (thrust) must be greater than the weight of the rocket (gravitational force acting downward). Only then will there be a net upward force, and by Newton’s Second Law, the rocket will accelerate upward and lift off.
Question 3.
How can the spacecraft slow down in space by firing its engine in the direction of motion?
Answer:
If the engine fires in the direction of motion, gases are expelled forward. By Newton’s Third Law, the reaction force on the spacecraft is backward (opposite to motion). This backward force decelerates the spacecraft. This is exactly how the Vikram lander of Chandrayaan-3 slowed down for a soft landing on the Moon.
How Forces Affect Motion Extra Questions for Practice
Very Short Answer Type Questions
Question 1.
Define Newton’s Second Law of Motion.
Question 2.
What is the difference between balanced and unbalanced forces?
Question 3.
State one practical application of Newton’s Third Law.
Question 4.
A 10 kg object experiences a net force of 50 N. What is its acceleration?
(Answer: Acceleration = 5 m/s2)
Question 5.
Why does a bullet fired from a gun move much faster than the gun recoils?
Short Answer Type Questions
Question 1.
A player kicks a football of mass 0.5 kg and it accelerates at 20 m/s2. What is the force applied on the ball?
Question 2.
Explain with Newton s laws why it is easier to push an empty cart than a loaded one.
Question 3.
Why is it dangerous to jump out of a fast moving bus?
Question 4.
Two boxes of masses 3 kg and 5 kg are connected by a string on a frictionless surface. A force of 16 N pulls the 5 kg box. Find the acceleration of the system and the tension in the string. (Acceleration of system = 2 m/s2 Tension in string = 6 N)
Long Answer Type Questions
Question 1.
A sports car of mass 1200 kg starts from rest and reaches a velocity of 30 m/s in 10 seconds. Calculate:
(i) the acceleration of the car,
(ii) the net force acting on the car.
Also explain which Newton’s law is used. (Acceleration = 3 m/s2, Net force = 3600 N)
Question 2.
Describe in detail how friction acts in everyday life. Give three examples where friction is helpful and explain how Newton s laws help us understand friction.
Question 3.
Explain the three laws of Newton s motion with one real-life example for each. Which scientist’s work inspired Newton s first law?
Case/Source Based Questions
I. Priya is riding a bicycle at 10 m/s on a flat road. She stops pedalling. The bicycle gradually slows down and stops after covering 25 metres. The total mass of Priya and the bicycle is 60 kg.
Answer the following questions:
Question 1.
What force is responsible for the bicycle slowing down?
Question 2.
Calculate the average deceleration of the bicycle.
Question 3.
Using Newton s laws, explain why the bicycle eventually stops.
II. During a school science fair, students demon-strated Newton’s Third Law using a balloon rocket. They attached a straw to an inflated balloon with tape, threaded a horizontal string through the straw, and when they released the balloon, it shot across the room.
Answer the following questions:
Question 1.
Explain the forces involved using Newton s Third Law.
Question 2.
Would this balloon rocket work in outer space? Justify your answer.
Question 3.
What would happen if the mass of the balloon was doubled but the same amount of air was released? How would the acceleration change?