Lakhmir Singh & Manjit Kaur Test: Forces Laws of Motion - Free 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. 

If no external force acts on a body, it will:

An object will either:

• Remain at rest if it is stationary.
• Continue moving at a constant speed in a straight line if it is already in motion.

This principle is known as the law of inertia. It states that an object maintains its state of motion unless acted upon by an external 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 Newton's Second Law, F = m × a. For the first body, F = m₁ × a. For the second body, F = m₂ × 4a. Since the same force F acts on both bodies, we can set the expressions equal: m₁ × a = m₂ × 4a. Dividing both sides by a gives m₁ = 4m₂, so m₂ = m₁/4. Therefore, the mass of the second body is one-fourth the mass of the first body.

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.

The magnitude of inertia of a body is determined by its:

The inertia of an object refers to its tendency to resist changes in its state of motion. This property is directly related to the object's mass. Here are the key points:

• Inertia is the natural resistance of an object to changes in its motion.
• The greater the mass of an object, the greater its inertia.
• Heavier objects require more force to change their motion compared to lighter ones.
• In simple terms, mass is a measure of an object's inertia.

Thus, the correct answer is that the magnitude of inertia is determined by the object's mass.

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.

The magnitude of a physical quantity is 8.5 Ns. The physical quantity is:

The physical quantity with a magnitude of 8.5 Ns is momentum. Here are some key points to understand:

• Momentum is defined as the product of an object's mass and its velocity.
• Its SI unit is kilogram metre per second (kg m/s), which can also be expressed as Newton second (Ns).
• Therefore, a quantity of 8.5 Ns indicates the momentum of an object.

The stone ball has more inertia than the rubber ball. This is because:

• Inertia is directly related to mass.
• The stone ball, being denser, typically has more mass than the rubber ball of the same size.
• Thus, the stone ball exhibits greater resistance to changes in motion.

When you shake a branch, some leaves fall off because of inertia of rest. Here's why:

• Inertia of rest is a property that keeps objects at rest until a force acts on them.
• Leaves initially stay still while the branch moves quickly.
• This sudden movement makes the leaves fall as they tend to stay in their original position.

When the driver of a fast moving car suddenly applies brakes, the passengers in the car:

When a car brakes suddenly, the passengers experience a shift in their bodies due to the principle of inertia. Here’s what happens:

• The car decelerates quickly, but the passengers' bodies tend to continue moving forward.
• This results in passengers feeling as if they are being thrown forward.
• Inertia causes the body to resist the change in motion, leading to this sensation.

Option a) Correct

• In space, there's no air, so no air resistance (friction). That means nothing is opposing the spaceship’s motion.

• This allows it to continue moving at a constant speed.

Option b) Incorrect

• This statement is fundamentally wrong.

• Mass is an intrinsic property of an object and doesn't change, even in space.

• While weight might become negligible or zero in microgravity, mass always stays the same.

• Mass and weight are not the same. Weight depends on gravity, but mass is constant everywhere.

Option c) Correct

• Due to the very weak force of gravitation, there is no external force on the spaceship, so the statement is true.

Option d) Correct

• There’s no Earth-based friction (like air drag) acting on the spaceship outside Earth’s atmosphere.

• So, it keeps moving without resistance.