Lakhmir Singh & Manjit Kaur Test: Forces & Laws of Motion - Class 9 MCQ

Force is a phenomenon which is responsible for the change in state or shape or size of a body either from rest to motion or motion to rest.

The correct option is C.
Balanced force:Forces acting on an object which does not change the state of rest or of uniform motion of it are called balanced forces. Unbalanced force:The resultant of two opposite forces acts on an object and brings it in motion. These opposite forces are called unbalanced forces.

Correct Answer :- B

Explanation :- When two forces of unequal magnitudes act in opposite directions on an object simultaneously then the object moves in the direction of the force with larger magnitude forces. These forces acting on the object are known as unbalanced forces.

According to the law of inertia or Newton's first law of motion, a body in state of rest or of uniform motion remains in state of rest or of uniform motion unless it is acted upon by an external force. Hence a body in uniform motion required no force to keep them in uniform motion.

The Second Law of Motion states that if an unbalanced force acts on a body, that body will experience acceleration ( or deceleration), that is, a change of speed.

 One can say that a body at rest is considered to have zero speed, ( a constant speed). So any force that causes a body to move is an unbalanced force. Also, any force, such as friction, or gravity, that causes a body to slow down or speed up, is an unbalanced force. 

The gravitational force for a falling object is along the direction of motion. Hence, its speed increases continuously till  it touches the ground.

F = m × a 
Where, F = force
m = mass
a = acceleration

When the mass of a body increases while keeping the force constant, the acceleration of the body is likely to decrease. This is because acceleration is inversely proportional to mass when force is constant, as described by Newton's second law of motion (F = ma).

- Explanation:
- Increase: Incorrect, as mass and acceleration are inversely proportional when force is constant.
- Remain same: Incorrect, as acceleration changes with mass.
- Decrease: Correct. With increased mass, acceleration decreases.
- Sometimes increases and sometimes decreases: Incorrect; acceleration consistently decreases with increased mass.

According to the Newton’s Second Law of Motion, the change in momentum of a body per unit time is directly proportional to the unbalanced force acting on the body and the change in momentum takes place in the direction of the unbalanced force of the body. It can be represented as F = ma. From the equation we can see that mass is inversely proportional to acceleration so if the acceleration of the body increases by 4 times, the mass of the body will reduce by 4 times.

Law of Inertia or First law of motion.
As the body tends to be in its original state when the horse suddenly stops, The lower part of the rider stops with it but the upper part remains in motion. So, the rider falls forward.

Mass ad velocity both are considered to be quantities which defines motion.

The combination of both is called quantity of motion. It is momentum.

P = mv

If we consider the situation when force is equivalent to the rate of change of momentum. I.e Force = (change in momentum)(time interval)
 then, change in momentum = Force * (time interval).
Therefore, the unit for momentum can be Newton - second (Ns)

The intrinsic property of material body which resists a change in its state of rest or of uniform
motion along a straight line is called inertia.
Inertia is a natural tendency of the body. Mass is taken as a quantitative measure of inertia of a
material body. The larger the mass of a body the greater the difficulty in bringing about a change in its
state of motion, i.e. the mass is a measure of inertia.

Momentum is a vector quantity. For a particle with mass, the momentum equals mass times velocity, and velocity is a vector quantity while mass is a scalar quantity. A scalar multiplied by a vector is a vector. A moving body would be a particle with a mass.