Introduction:

If you’re preparing for the BPSC exam and looking for simple, student-friendly notes, then Chapter 2: Force and Laws of Motion (BPSC Science Notes) is a must-read. This chapter is very important for Bihar competitive exams because it explains basic physics concepts like force, motion, inertia, and Newton’s laws in easy language. Questions are often asked directly from this topic in the BPSC Prelims and Mains exams. In this article, we’ll cover all key topics like types of forces, Newton’s three laws, mass and weight, momentum, friction, and circular motion with examples, formulas, and MCQs. These notes are specially designed for quick revision and better understanding.

1. Force – Basic Concept

• Force is a push or pull on an object.
• It changes:
  • The speed of an object
  • The direction of motion
  • The shape or size of an object

Note: Force is a vector quantity – it has both magnitude and direction.

Types of Force:

1. Contact Forces:
   • Require physical contact
   • Examples: Muscular force, Friction, Normal force, Tension
2. Non-contact Forces:
   • Act without physical contact
   • Examples: Gravitational force, Magnetic force, Electrostatic force

2. Newton’s Laws of Motion

First Law of Motion (Law of Inertia):

“An object remains at rest or in uniform motion unless acted upon by an external unbalanced force.”

• Explains inertia: the tendency of a body to resist change in motion
• Example: A book remains on a table unless pushed

Inertia Types:

• Inertia of Rest
• Inertia of Motion
• Inertia of Direction

Second Law of Motion:

“The rate of change of momentum is directly proportional to the applied force.”

Formula:
F = ma

• F = Force
• m = mass
• a = acceleration

Momentum (p):
p = mv

• p = momentum
• m = mass
• v = velocity

SI unit of Force: Newton (N)
1 N = 1 kg·m/s²

Third Law of Motion:

“For every action, there is an equal and opposite reaction.”

• Action and reaction forces act on different objects
• Example: Gun recoils when bullet is fired

3. Balanced and Unbalanced Forces

• Balanced Force:
  • Equal forces in opposite directions
  • No change in motion
  • Example: Book resting on table
• Unbalanced Force:
  • Causes motion
  • Example: Pushing a stationary box

4. Frictional Force

• Opposes motion
• Depends on:
  • Nature of surfaces
  • Normal force

Friction increases with roughness.

5. Mass and Weight

• Mass:
  • Amount of matter
  • Scalar
  • Constant
  • SI unit: kg
• Weight:
  • Force due to gravity
  • Vector
  • Changes with location
  • Formula: W = mg
    • g = 9.8 m/s² (on Earth)
    • SI unit: Newton

6. Circular Motion & Centripetal Force

• In circular motion, an object needs a force to move in a circle.
• This force is called centripetal force.
• Always acts towards the center.

7. Real-Life Applications of Newton’s Laws

• Seatbelt in car (1st law)
• Kicking a football (2nd law)
• Rocket propulsion (3rd law)

20 MCQ Practice Set – Force and Laws of Motion

Q1. Which of the following is a contact force?
a) Gravitational
b) Electrostatic
c) Magnetic
d) Friction
Ans: d)

Q2. Unit of force is:
a) kg.m/s
b) kg
c) m/s²
d) Newton
Ans: d)

Q3. Which law explains the sudden jerk felt in a car when brakes are applied?
a) First Law
b) Second Law
c) Third Law
d) Law of Gravitation
Ans: a)

Q4. Momentum depends on:
a) Force
b) Mass and Velocity
c) Weight
d) Power
Ans: b)

Q5. Action and reaction act on:
a) Same body
b) Same direction
c) Different bodies
d) Same point
Ans: c)

Q6. Force is a:
a) Scalar
b) Constant
c) Vector
d) None
Ans: c)

Q7. Friction is:
a) A contact force
b) A non-contact force
c) Always helpful
d) None
Ans: a)

Q8. Newton’s second law gives:
a) Definition of force
b) Value of inertia
c) Explanation of gravity
d) None
Ans: a)

Q9. SI unit of momentum is:
a) N
b) kg·m/s²
c) kg·m/s
d) m/s
Ans: c)

Q10. Force responsible for circular motion:
a) Gravitational
b) Centripetal
c) Magnetic
d) Inertia
Ans: b)

Q11. When two balanced forces act on a body, it:
a) Accelerates
b) Changes direction
c) Remains at rest or constant motion
d) Falls down
Ans: c)

Q12. Mass of a body is measured in:
a) Newton
b) Joule
c) Kg
d) Watt
Ans: c)

Q13. Example of third law of motion:
a) Rocket launch
b) Car braking
c) Sitting on chair
d) Turning a fan
Ans: a)

Q14. Inertia of motion means:
a) Stationary body remains stationary
b) Moving body keeps moving
c) Body starts moving on its own
d) None
Ans: b)

Q15. Friction always acts:
a) Along motion
b) Opposite to motion
c) Perpendicular
d) No direction
Ans: b)

Q16. Weight is the force of:
a) Friction
b) Contact
c) Gravity
d) Motion
Ans: c)

Q17. Which of the following changes with gravity?
a) Mass
b) Weight
c) Inertia
d) Volume
Ans: b)

Q18. Which of the following is a non-contact force?
a) Tension
b) Friction
c) Magnetic
d) Muscular
Ans: c)

Q19. What is the acceleration when force is zero?
a) Zero
b) Constant
c) Infinity
d) 9.8
Ans: a)

Q20. The force needed to accelerate 2 kg mass at 3 m/s² is:
a) 6 N
b) 3 N
c) 2 N
d) 1.5 N
Ans: a)