How Forces Affect Motion Class 9 Extra Questions and Answers Science Chapter 6

Students can keep Class 9 Science Extra Questions and Class 9 Exploration Chapter 6 Extra Question Answer How Forces Affect Motion handy for quick reference during exams.

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 balanced force?
Answer:
Forces acting on a body that cancel each other and do not change its state of motion.

Question 2.
What is an unbalanced force?
Answer:
Forces that cause a change in the state of motion of a body.

Question 3.
What is force?
Answer:
A push or pull acting on an object.

Question 4.
What is friction?
Answer:
A force that opposes the motion of an object.

Question 5.
State Newton’s First Law of Motion.
Answer:
A body remains at rest or in uniform motion unless acted upon by an unbalanced force.

Question 6.
What is momentum?
Answer:
The momentum of an object is the product of its mass and velocity and has the same direction as that of the velocity. The S. I. unit is kg m/s. (p = mv)

Question 7.
State Newton’s Second Law of Motion.
Answer:
Newton’s Second Law of Motion, which states that the magnitude of the acceleration is proportional to the magnitude of the net force and is inversely proportional to the mass of the object.

Question 8.
State Newton’s Third Law of Motion.
Answer:
For every action, there is an equal and opposite reaction.

Question 9.
What happens when unbalanced forces act on a system of objects?
Answer:
They change the motion of the system (cause acceleration).

Question 10.
Give one example of action and reaction forces.
Answer:
Walking: feet push the ground backward, and the ground pushes the body forward.

How Forces Affect Motion Class 9 Short Question Answer

Question 1.
A truck of mass Mis moved underaforce F. If the truck is then loaded with an object equal to the mass of the truck and the driving force is halved, then how does the acceleration change?
Answer:
Acceleration can be given as follow:
F = ma OR a = \(\frac{F}{m} \) Or a₁ = \(\frac{F}{M} \)
When mass is doubled and force is halved;
a₂ = \(\frac{F}{4 M}\)
OR
\(\frac{a_2}{a_1}=\left(\frac{F}{4 M} \div \frac{F}{M}\right)=\frac{1}{4} \)
So, acceleration becomes one-fourth.

Question 2.
Define momentum. Calculate the momentum of a 6 kg object moving at 3 m/s.
Answer:
Momentum is the product of mass and velocity (p = mv).
p = 6 x 3 = 18 kg m/s
Momentum 18 kgm/s

Question 3.
State Newton’s Third Law of Motion. A man pushes a wall with 50 N force. What is the reaction force?
Answer:
For every action, there is an equal and opposite reaction.
Reaction force = 50 N in opposite direction

Question 4.
What is friction? A force of 20 N moves a box, but friction is 5 N. Find net force.
Answer:
Friction is a force that opposes motion between surfaces.
Net force = (20 – 5 = 15 N)
Net force = 15 N

Question 5.
What are balanced and unbalanced forces? A 4 kg body has net force 8 N4. Find acceleration.
Answer:
Balanced forces cancel out; unbalanced forces change motion.
a = \(\frac{F}{m}=\frac{8}{4}\) = 2 m/s²
Acceleration = 2 m/s²

Question 6.
Two balls of the same size but of different materials, rubber and iron, are kept on the smooth floor of a moving train. The brakes are applied suddenly to stop the train. Will the balls start rolling? If so, in which direction? Will they move with the same speed? Give reasons for your answer.
Answer:
Once the train stops suddenly, the balls would tend to remain in motion. Due to this, both the balls would move in forward direction. Since masses of both the balls are different, therefore inertial force will also be different. So, they will move at different speeds.

Question 7.
Water sprinkler used for grass lawns begins to rotate as soon as the water is supplied. Explain the principle on which it works.
Answer:
When water comes out of the nozzle of the sprinkler, the Third Law of Motion comes into effect. Due to this, the sprinkler begins to rotate.

How Forces Affect Motion Class 9 Long Question Answer

Question 1.
Two blocks of masses 4 kg and 6 kg are tied together and pulled by a force of 20 N. Calculate (i) acceleration (ii) tension in the string.
Answer:
Total mass = 10 kg
Acceleration, a = \(\frac{20}{10}\) = 2 m/s²
Tension = force needed to move 4 kg block =
4 × 2 = 8N
Acceleration = 2 m/s², Tension = 8 N

Question 2.
Explain balanced and unbalanced forces with examples. How do they affect the motion of an object?
Answer:
Balanced forces are equal in magnitude and opposite in direction, so they cancel each other and do not change the state of motion (e.g., a book resting on a table). Unbalanced forces cause a change in motion, such as acceleration, deceleration, or change in direction (e.g., pushing a stationary box to move it).

Question 3.
Derive the relation F = ma from Newton’s Second Law of Motion.
Answer:
According to the second law, force is proportional to the rate of change of momentum.
Momentum, p = mv, so change in momentum = m (v – u).
Rate of change = \(\frac{m(v-u)}{t} \) = ma.
Thus, F = ma when proportionality constant is 1.

Question 4.
A 10 kg object moving at 2 m/s is acted upon by a force for 5s and reaches 12 m/s.
Calculate:
(i) acceleration
(ii) force. Explain the result.
Answer:
Acceleration, a = \(\frac{v-u}{t}=\frac{12-2}{5}\) = 2 m/s²
Force F = ma = 10 × 2 = 20 N
This shows that a constant force produces uniform acceleration in the direction of motion.

Question 5.
Why do passengers fall forward when a moving bus stops suddenly? Explain.
Answer:
When a moving bus stops suddenly, the lower part of the passenger’s body stops with the bus. However, the upper part continues to move forward due to inertia of motion.

According to Newton’s First Law, a body resists any change in its state of motion.
This forward motion of the upper body causes the passenger to fall forward. Thus, inertia is responsible for this effect in daily life situations.

Question 6.
Why is it easier to push a moving object than a stationary one? Explain.
Answer:
A stationary object experiences static friction, which is higher than other types of friction. To start motion, a larger force is needed to overcome this static friction. Once the object starts moving, sliding or rolling friction comes into play.

These are smaller compared to static friction, so less force is required. Therefore, it is easier to push a moving object than a stationary one.

Question 7.
Why do athletes run before jumping in long jump? Explain.
Answer:
Athletes run to increase their speed before taking a jump. Higher speed means greater momentum (momentum = mass × velocity).

Greater momentum helps the athlete travel a longer distance in air. This is because more forward motion is maintained during the jump. Thus, running helps in achieving a longer jump distance.

Question 8.
A large truck and a car, both moving with a velocity of magnitude u, have a head-on collision and both of them come to a halt after that. If the collision lasts for 1 s:
(a) Which vehicle experiences the greater force of impact?
(b) Which vehicle experiences the greater change in momentum?
(c) Which vehicle experiences the greater acceleration?
(d) Why is the car likely to suffer more damage than the truck?
Answer:
(a) During head on collision, forces applied by truck and car are action-reaction forces. Hence both vehicles experience same (equal) force of impact.
(b) Here initial velocity of both car and truck is same equal to y, and final velocity of both is zero. But mass of truck is much more than that of car, hence change in momentum of truck is more than change in momentum of car.
(c) For same force of impact, the acceleration of car will have greater magnitude because its mass is less.
(d) Car suffers more damage than the truck, as acceleration of car is more, its velocity falls to zero in a shorter time and consequently, its momentum changes in a shorter time.

Question 9.
(a) Two balls of the same size but of different materials, rubber and iron are kept on the smooth floor of a moving train. The brakes are applied suddenly to stop the train. Will the balls start rolling? If so, in which direction? Will they move with the same speed? Give reasons for your answer.
(b) A horse continues to apply a force in order to move a cart with a constant speed. Explain why?
Answer:
(a) Once the train stops suddenly, the balls would tend to remain in motion. Due to this, both the balls would move in forward direction. Since masses of both the balls are different, therefore inertial force will also be different. So, they will move at different speeds.

(b) When a cart is moving on the road, it has to encounter friction. To maintain a constant speed, some force needs to be applied continuously to overcome the friction. Hence, the horse needs to continuously apply a force in order to move the cart with a constant speed.

How Forces Affect Motion Class 9 Quiz Questions

Question 1.
Why does a ball rolling on grass slow down and stop?
Answer:
Due to friction between the ball and grass, which opposes motion.

Question 2.
What will happen to the motion of an object if no external force acts on it?
Answer:
It will remain at rest or move with uniform velocity.

Question 3.
Why do we lean forward while walking?
Answer:
To maintain balance and overcome inertia of rest.

Question 4.
Why is it difficult to stop a moving heavy truck than a bicycle?
Answer:
Because the truck has more mass and greater inertia.

Question 5.
What is the direction of friction force?
Answer:
Opposite to the direction of motion.

Question 6.
A 68 kg runner exerts a force of 59 N. What is the acceleration of the runner?
Answer:
m=68kg,F = 59N, a = ?
F = ma
59 = 68a
a = \(\frac{59}{68} \) = 0.87 m/s²

Question 7.
What causes a rocket to move upward?
Answer:
Explosion of gases downward produces an upward reaction force.

Question 8.
Why do athletes use spiked shoes while running?
Answer:
To increase friction and get better grip on the ground.

Question 9.
If two equal forces act in opposite directions, what will be the result?
Answer:
The net force will be zero, so no change in motion.

Question 10.
A hammer is hitting a nail. Compare the force on the nail with the force on the hammer.
Answer:
The forces acting on nail and on hammer are equal in magnitude (strength) but opposite in direction.

How Forces Affect Motion Class 9 Skill Based Questions

Question 1.
Observe the following figures. (a) has no cloth on the table while (b) has cloth on it. Now answer the following questions.
(i) In which case the pencil cell covers more distance?
(ii) Give the reason of covering different distances in both cases.

Answer:
(i) In case (a), pencil cell covers more distance than case (b).
(ii) In case (a), there is less friction due to smooth surface, while in case
(b), cloth makes the surface rough due to which more frictional force is exerted on the pencil cell.

Question 2.
Observe the following figure and answer the questions:

(i) What are the bullocks doing?
(ii) What types of forces are applied by bullocks?
(iii) Is such type of force a contact or non-contact force?
Answer:
(i) The two bullocks are pulling a cart having load.
(ii) The bullocks applied the muscular force.
(iii) This type of force is a contact force because the bullocks are in direct contact with the cart.

How Forces Affect Motion Class 9 Case Based Questions

Question 1.
One day, Rahul noticed that his toy car gradually came to a stop. Anaya explained that this happened because of friction, a force that opposes motion between two surfaces in contact. Later, both Rahul and Anaya observed how some forces can act without physical contact. They experimented with a magnet attracting iron nails and a charged plastic comb pulling small bits of paper. Anaya told Rahul that these were examples of magnetic and electrostatic forces, which act at a distance.

While they were talking, a mango fell from a tree nearby. Anaya pointed to it and said, “That’s the force of gravity-it pulls everything towards the Earth.”
I. Which force made the toy car stop?
II. Why did the mango fall down?
III. Which force pulled the paper to the balloon?
OR
Name the forces which act without contact.
Answer:
I. Friction
II. Due to gravity
III. Electrostatic force
OR
Magnetic force and electrostatic force

Question 2.
Tina and Aryan used a spring balance to weigh a water bottle. Tina said the bottle weighs 5 newton. Aryan asked if its mass would change on the Moon. Tina replied, “No, only its weight changes because gravity is less on the Moon.
I. What instrument did Tina and Aryan use to measure weight?
II. Will the mass of the bottle change on the Moon?
III. Why does the weight change on the Moon?
OR
What is the relation between weight and gravity?
Answer:
I. Spring balance
II. No, mass remains the same everywhere.
III. Because gravity on the Moon is less than on the Earth.
OR
Weight depends on the force of gravity acting on an object’s mass.

Question 3.
Velocity versus time graph of a ball of mass 50 g rolling on a concrete floor is shown in the given figure.

Answer the following questions based on the above information.
(a) Calculate the acceleration of the ball.
(b) Calculate the frictional force of the floor on the ball.
(c) The momentum of ball at time t = 8 sec is
(i) 0.4 kg ms^-1
(ii) 0
(iii) 0.5 kg ms^-1
(iv) cannot predict
(d) Name the type of force which acts on the body when the resultant force is not equal to zero.
(i) Balanced force
(ii) Unbalanced force
(iii) Frictional force
(iv) External force
Answer:
(a) u = 80 m/s. v = 0, t = 8s
a = \(\left(\frac{v-u}{t}\right)=\left(\frac{0-80}{8}\right)\) =-10ms^-2

(b) F = ma = \(\left(\frac{50}{1000}\right) \) kg × (-10) ms^-2
a = -0.5Newton.
Frictional force of the floor acting on the ball = 0.5 Newton.
(c) (ii) 0
(d) (ii) Unbalanced force

Suggested Projects and Activities

Question 1.
Friction in Daily Life
Observe different daily life situations such as walking, writing, driving, and sliding objects. Identify where friction is useful and where it is harmful. Explain how friction helps in walking and writing but causes wear and tear in machines. Suggest methods to increase or reduce friction in any two situations. Write your conclusion based on
your observations. How does friction affect our daily life activities?

2. Newton’s Laws in Transportation
Study different situations in vehicles like sudden start, sudden stop, and uniform motion. Explain how Newton’s First Law causes passengers to lean backward or forward. Describe how seat belts help in reducing injury using inertia. Give examples of Newton’s Third Law in rocket or car movement. Conclude how Newton’s laws are
important for road safety.

3. Motion of Connected Objects (System of Objects)
Take two or mùre objects connected by a string or placed together. Apply a force and observe their motion. Explain why all objects move together as a system. Discuss how internal forces cancel out and only external force causes motion. Give real-life examples like train coaches or loaded carts. How does this concept help in understanding system motion?

How Forces Affect Motion Extra Questions for Practice

Multiple Choice Questions

Question 1.
The SI unit of force is
(a) Joule
(b) Newton
(c) Watt
(d) Pascal

Question 2.
An object moving with constant velocity means
(a) unbalanced force is acting
(b) net force is zero
(c) acceleration is increasing
(d) mass is changing

Question 3.
Friction always acts
(a) in the direction of motion
(b) perpendicular to motion
(c) opposite to motion
(d) along gravity

Question 4.
Read the given statements and select the correct option.
Assertion (A): Friction acts opposite to the direction of motion.
Reason (R): Gases have large interparticle spaces between their particles.
(a) Both A and R are true but R is not the correct explanation of A.
(b) Both A and R are true and R is the correct explanation of A.
(c) A is true but R is false.
(d) A is false but R is true.

Question 5.
Match Column A with Column B.

Column A	Column B
(i) Force	(p) Opposes motion
(ii) Friction	(q) Newton’s IIIrd Law
(iii) Action-reaction	(r) Push or pull

(a) (i) – (r), (ii) – (q), (iii) – (p)
(b) (i) – (r), (ii) – (p), (iii) – (q)
(c) (i) – (q), (ii) – (p), (iii) – (r)
(d) (i) – (p), (ii) – (r), (iii) – (q)

Very Short Answer Type Questions

Question 6.
What is force?

Question 7.
What can be measure from the spring balance?

Question 8.
Draw position-time graph when the object is at rest.

Question 9.
What is friction?

Question 10.
What is net force?

Short Answer Type-I Questions

Question 1.
Differentiate between balanced and unbalanced forces.

Question 2.
State Newton’s First Law of Motion.

Question 3.
Why does friction oppose motion?

Question 4.
What is momentum?

Question 5.
Give one example of Newton’s Third Law.

Short Answer Type-II Questions

Question 1.
State the effect of force.

Question 2.
Explain Newton’s Second Law of Motion.

Question 3.
Why is friction both useful and harmful?

Question 4.
Explain forces acting on a system of objects.

Question 5.
Why do objects slow down on their own?

Long Answer Type Questions

Question 1.
Explain why it is difficult to walk on ice and how friction helps in walking on normal ground. Suggest methods to improve walking on slippery surfaces.

Question 2.
Why do heavy vehicles take longer distance to stop compared to light vehicles? Explain using the concept of momentum.

Question 3.
Explain why rockets move upward even though gases are pushed downward. How does Newton’s Third Law apply here?

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