CBSE Class 9 Science Force and Laws of Motion MCQs Set C

Question: A water tanker filled up to two-thirds of its tank with water is running with a uniform speed. When the brakes are suddenly applied, the water in its tank would :
a) move backward
b) move forward
c) rise upwards
d) remain unaffected
Answer: b

Question: If we release a magnet held in our hand, it falls to the ground. The force which makes the magnet fall down is an example of :
a) balanced force
b) unbalanced force
c) magnetic force
d) muscular force
Answer: b

Question: When a rubber balloon held between the hands is pressed, its shape changes. This happens because :
a) balanced forces act on the balloon
b) unbalanced forces act on the balloon
c) frictional forces act on the balloon
d) gravitational forces act on the balloon
Answer: a

Question: ‘When a hanging carpet is beaten with a stick, the dust particles start coming out of it’. This phenomenon
can be best explained by making use of :
a) Newton’s third law of motion
b) Newton’s law of gravitation
c) Newton’s first law of motion
d) Newton’s second law of motion
Answer: c

Question: When a toothpaste tube is squeezed, its shape changes. The force responsible for this is an example of :
a) balanced forces
b) centripetal forces
c) unbalanced forces
d) centrifugal forces
Answer: a

Question: Which of the following effect cannot be produced by an unbalanced force acting on a body ?
a) change in speed of the body
b) change in shape of the body
c) change in direction of motion of the body
d) change in state of rest of the body
Answer: b

Question: A passenger in a moving train tosses a coin which falls behind him. This shows that the motion of train is
a) accelerated
b) uniform
c) retarded
d) along circular track
Answer: a

Question: The inertia of a moving object depends on :
a) momentum of the object
b) speed of the object
c) mass of the object
d) shape of the object
Answer: c

Question: The inertia of an object tends to cause an object :
a) to increase its speed
b) to decrease its speed
c) to resist a change in its state of motion
d) to decelerate due to friction
Answer: c

Question: When we talk of a force acting on a body, it usually means :
a) electrical force
b) balanced force
c) unbalanced force
d) nuclear force
Answer: c

Question: Name the physical quantity whose unit is kg.m/s.
Answer: Momentum

Question: State whether momentum is scalar or vector.
Answer: Vector

Question: Name the property of bodies (or objects) to resist a change in their state of rest or of motion.
Answer: Inertia

Question: What is the other name of Newton’s first law of motion ?
Answer: Galileo’s law of inertia

Question: What name is given to the product of mass and velocity of a body ?
Answer: Momentum

Question: When a ball is dropped from a height, its speed increases gradually. Name the force which causes this change in speed.
Answer: Force of gravity

Question: State whether force is a scalar or a vector quantity.
Answer: Vector

Question: With which physical quantity should the speed of a running bull be multiplied so as to obtain its momentum ?
Answer: Mass (of bull)

Question: Name the unbalanced force which slows down a moving bicycle when we stop pedalling it.
Answer: Force of friction

Question: Name the physical quantity which is considered to be a measure of the quantity of motion of a body.
Answer: Momentum

Question: The mass of object A is 6 kg whereas that of another object B is 34 kg. Which of the two objects, A or B, has more inertia ?
Answer: b

Question: What is the total momentum of the bullet and the gun before firing ?
Answer: Zero

Question: What is the usual name of the forces which cannot produce motion in a body but only change its shape ?
Answer: balanced forces

Question: State whether the following statement is true or false :
Unbalanced forces acting on a body change its shape.
Answer: false

Fill in the following blanks with suitable words :

Question: ……………….. is a measure of the inertia of a body.
Answer: Mass

Question: Newton’s first law of motion is also called Galileo’s law of …………………
Answer: inertia

Question: When a stationary car starts suddenly, the passengers are jerked …………..
Answer: backward

Question: If there were no unbalanced force of …………… and no ………….. resistance, a moving bicycle would go on moving for ever.
Answer: friction ; air

Question: When a running car stops suddenly, the passengers are jerked …………
Answer: forward

KEY CONCEPTS

1. Balanced and Unbalanced Forces

Balanced Forces The net force is when two or more forces are applied on the same object and at the same time. The applied forces combined are called the net force. = 0 25 N 25 N

Balanced Forces The force I apply in one direction plus the force you apply in the opposite direction are added together. 25 N -25 N = 0 Because the forces are equal and balanced…just like a balanced scale…this is an example of balanced forces. Unbalanced Forces What does it mean to have something unbalanced? Unequal, not the same, different How could we have unbalanced forces?

Unbalanced Forces A force is applied in one direction and either another smaller or larger force is applied in the opposite direction or no force is applied at all in the opposite direction.

Unbalanced Forces If I have a chair and I push on one side of it with a force of 50 N and you push on the other side, with a force of 25 N, will the chair move? Which way will it move? The direction in which the most force is applied. What is the net force? 50 N 25 N.

Unbalanced Forces 50N -25N = 25N These forces are unequal so the forces are considered unbalanced forces. 50 N 25 N = 25 N

Unbalanced Forces If I push the chair in one direction with 25 N force and you push the chair in same direction with 25 N force, will the chair move? Why? Because the applied net force is UNBALANCED!

Unbalanced Forces 25 N 25 N = 50 N The result would be the chair moving in the direction it was pushed with a combined force of 50 N.

Test Yourself

An object of 5 kg is acted upon by two forces, 70 N each in opposite directions. What is its acceleration?

Why does an object accelerate during free fall?

Laws of Motion

Newton's First Law

1st Law – An object at rest will stay at rest, and an object in motion will stay in motion at constant velocity, unless acted upon by an unbalanced force.

An object at rest will stay at rest, and an object in motion will stay in motion at constant velocity, unless acted upon by an unbalanced force.

Newton's Second Law

"If the net force on an object is not zero, the object will accelerate. The direction of the acceleration is the same as the direction of the net force. The magnitude of the acceleration is directly proportional to the net force applied, and inversely proportional to the mass of the object."

The Effect of Mass

A force applied to an automobile will not have the same effect as the same force applied to a pencil. An automobile resists accelerating much more than a pencil does, because it has more inertia, or mass.

The acceleration of an object depends not only on how hard you push on it, but also on how much the object resists being pushed. What is the effect of mass on acceleration? This, too, turns out to be quite simple (I wonder why…). For the same force, an object with twice the mass will have half the acceleration. If it had three times the mass, the same force will produce one-third the acceleration. Four times the mass gives one-fourth of the acceleration, and so on.

This type of relationship between quantities (double one, get half the other) is called an inverse proportion or inverse variation. In other words, then:

Newton’s Second Law of Motion

The acceleration of an object is dependent upon both force and mass. Thus, if the colliding objects have unequal mass, they will have unequal accelerations as a result of the contact force which results during the collision.

Newton's Third Law

Newton's Third Law is stated as:

For every action there is an equal and opposite reaction.

"action…reaction" means that forces always occur in pairs. (Forces are interactions between objects, like conversations are interactions between people.)

Single, isolated forces never happen. The two forces involved are called the "action force" and the "reaction force."

These names are unfortunate for a couple of reasons :

Either force in an interaction can be the "action" force or the "reaction" force

The action and reaction forces exist at the same time. "equal" means

Both forces are exactly the same size. They are equal in magnitude. Both forces exist at exactly the same time. They both start at exactly the same instant, and they both stop at exactly the same instant. They are equal in time. "opposite" means that the two forces always act in opposite directions -exactly 180o apart.

Newton's third law of motion In every interaction, there is a pair of forces acting on the two interacting objects. The size of the force on the first object equals the size of the force on the second object. The direction of the force on the first object is opposite to the direction of the force on the second object. Forces always come in pairs - equal and opposite actionreaction force pairs.

Newton's third law of motion applied to collisions between two objects. In a collision between two objects, both objects experience forces which are equal in magnitude and opposite in direction. Such forces cause one object to speed up (gain momentum) and the other object to slow down (lose momentum). According to Newton's third law, the forces on the two objects are equal in magnitude.

Test Yourself

1. Can action reaction balance each other?

2. What does a force do?

3. Inertia and Mass

Inertia is the tendency of an object to resist any change in its motion. An object will continue to move at the same speed in the same direction unless acted upon by an unbalanced force. Inertia & Mass Inertia & Mass of a bowling ball rolled down the road would eventually come to a stop. Friction is an unbalanced force that causes the ball to stop or slow down. Without friction, the ball would keep going.

Inertia & Mass of a bowling ball and a tennis ball have the same inertia. Inertia & Mass If you had a tennis racket and I threw tennis ball at you, what would happen? If you had a tennis racket and I threw a bowling ball at you, what would happen? Why could you change the motion of the tennis ball but not the motion of the bowling ball?

Mass is the amount of matter in an object. A bowling ball has more mass than a tennis ball. The greater the mass of an object the greater its inertia. Mass is the measurement of inertia.

Test Yourself

Why do we fall forward if we alight from a moving bus?

2. Why does an athlete run for some distance before long jump?

4. Conservation of Momentum

Law of Conservation of Momentum

In a closed system, the vector sum of the momenta before and after an impact must be equal.

      Before                    After
m1v1 +m2v2  =   m1v1’ + m2v2’

Internal and External Force