Laws of Motion Notes for NEET Exam
1. Force and Inertia
Force: A push or pull that changes or tends to change the state of rest or uniform motion of an object. Force is a vector quantity with both magnitude and direction.
Inertia: The tendency of an object to resist any change in its state of rest or uniform motion. It is directly related to the mass of the object. The greater the mass, the greater the inertia.
Diagram: Inertia in Daily Life
Example: When a bus suddenly stops, passengers tend to fall forward due to inertia.
PYQ 1 (NEET 2022): Which of the following best describes inertia?
A) The force acting on an object
B) The tendency of an object to resist changes in its state of motion
C) The acceleration of an object
D) The energy possessed by an object
Answer: B) The tendency of an object to resist changes in its state of motion
Explanation: Inertia is the property of an object to resist changes in its state of rest or uniform motion.
PYQ 2 (NEET 2020): A body of mass 5 kg is moving with a constant velocity of 10 m/s. What is the force acting on the body?
A) 0 N
B) 5 N
C) 10 N
D) 50 N
Answer: A) 0 N
Explanation: Since the body is moving with constant velocity, the net force acting on it is zero (Newton’s First Law).
2. Newton’s First Law of Motion
Statement: An object remains in a state of rest or uniform motion in a straight line unless acted upon by an external force.
Also known as: Law of Inertia.
Key Points:
- If the net force on an object is zero, its velocity remains constant.
- Inertia is a measure of an object’s resistance to changes in its state of motion.
PYQ 3 (NEET 2021): Which of the following is an example of Newton’s First Law?
A) A ball rolling on the ground comes to rest due to friction
B) A rocket propelling forward
C) A car accelerating on a road
D) A book resting on a table
Answer: A) A ball rolling on the ground comes to rest due to friction
Explanation: The ball comes to rest because of the external force of friction, which opposes its motion.
3. Momentum and Newton’s Second Law
Momentum (p): The product of an object’s mass and velocity (\( p = mv \)). It is a vector quantity with direction the same as velocity.
Newton’s Second Law: The rate of change of momentum of an object is directly proportional to the applied force and occurs in the direction of the force (\( F = ma \)).
Key Points:
- Force is directly proportional to acceleration (\( F \propto a \)).
- The SI unit of force is Newton (N), where \( 1 \, \text{N} = 1 \, \text{kg} \cdot \text{m/s}^2 \).
PYQ 4 (NEET 2023): A force of 20 N acts on a body of mass 4 kg. What is the acceleration produced?
A) 5 m/s²
B) 10 m/s²
C) 15 m/s²
D) 20 m/s²
Answer: A) 5 m/s²
Explanation: Using \( F = ma \), \( a = \frac{F}{m} = \frac{20}{4} = 5 \, \text{m/s}^2 \).
4. Impulse
Impulse (J): The product of force and the time for which it acts (\( J = F \Delta t \)). It is equal to the change in momentum (\( J = \Delta p \)).
Key Points:
- Impulse is a vector quantity with the same direction as the force.
- SI unit of impulse is Newton-second (Ns) or kg·m/s.
PYQ 5 (NEET 2020): A force of 50 N acts on a body for 2 seconds. What is the impulse imparted to the body?
A) 25 Ns
B) 50 Ns
C) 100 Ns
D) 200 Ns
Answer: C) 100 Ns
Explanation: Impulse \( J = F \Delta t = 50 \times 2 = 100 \, \text{Ns} \).
PYQ 6 (NEET 2019): A cricket ball of mass 150 g is moving with a velocity of 20 m/s. If it is stopped by a fielder in 0.1 seconds, what is the impulse applied by the fielder?
A) 3 Ns
B) 6 Ns
C) 9 Ns
D) 12 Ns
Answer: A) 3 Ns
Explanation: Impulse \( J = \Delta p = m(v_f - v_i) = 0.15 \times (0 - 20) = -3 \, \text{Ns} \). The negative sign indicates the direction of the impulse.
5. Newton’s Third Law of Motion
Statement: For every action, there is an equal and opposite reaction.
Key Points:
- Action and reaction forces act on different objects.
- These forces are equal in magnitude but opposite in direction.
Diagram: Action-Reaction Pair
Example: When you push a wall, the wall pushes back with an equal force.
PYQ 7 (NEET 2021): Which of the following is an example of Newton’s Third Law?
A) A book resting on a table
B) A rocket propelling forward
C) A car accelerating on a road
D) A ball falling freely
Answer: B) A rocket propelling forward
Explanation: The rocket exerts a force on the exhaust gases, and the gases exert an equal and opposite force on the rocket, propelling it forward.
6. Law of Conservation of Linear Momentum
Statement: The total momentum of a system remains constant if no external force acts on it.
Applications:
- Collisions (elastic and inelastic).
- Explosions.
- Rocket propulsion.
PYQ 8 (NEET 2023): Two objects of masses 2 kg and 3 kg collide and stick together. If the initial velocity of the 2 kg object is 4 m/s and the 3 kg object is at rest, what is their final velocity?
A) 1.6 m/s
B) 2.4 m/s
C) 3.2 m/s
D) 4.0 m/s
Answer: A) 1.6 m/s
Explanation: Using conservation of momentum: \( m_1 u_1 + m_2 u_2 = (m_1 + m_2) v \).
\[ (2 \times 4) + (3 \times 0) = (2 + 3) v \]
\[ 8 = 5v \Rightarrow v = 1.6 \, \text{m/s} \]
7. Equilibrium of Concurrent Forces
Definition: When multiple forces act on a body at the same point (concurrent forces), the body is said to be in equilibrium if the vector sum of all forces is zero (\( \sum F = 0 \)).
Conditions for Equilibrium:
- The sum of all horizontal forces must be zero (\( \sum F_x = 0 \)).
- The sum of all vertical forces must be zero (\( \sum F_y = 0 \)).
Diagram: Equilibrium of Forces
Example: A book resting on a table is in equilibrium because the gravitational force is balanced by the normal force.
PYQ 9 (NEET 2022): A body is in equilibrium under the action of three forces. Which of the following is true?
A) The forces must be equal in magnitude
B) The forces must be collinear
C) The forces must form a closed triangle when represented vectorially
D) The forces must act in the same direction
Answer: C) The forces must form a closed triangle when represented vectorially
Explanation: For equilibrium, the vector sum of the forces must be zero, which means they should form a closed triangle when represented vectorially.
8. Friction
Definition: Friction is the force that opposes the relative motion or tendency of motion between two surfaces in contact.
Types of Friction:
- Static Friction: Acts when the object is at rest.
- Kinetic Friction: Acts when the object is in motion.
- Rolling Friction: Acts when an object rolls over a surface.
Laws of Friction:
- Friction is independent of the area of contact.
- Friction is proportional to the normal force (\( f = \mu N \)).
Diagram: Types of Friction
Example: Static friction prevents a box from sliding on an inclined plane.
PYQ 10 (NEET 2024): Which of the following is true about friction?
A) It depends on the area of contact
B) It is independent of the normal force
C) It is proportional to the normal force
D) It is always greater than the applied force
Answer: C) It is proportional to the normal force
Explanation: Friction is directly proportional to the normal force, as given by \( f = \mu N \).
PYQ 11 (NEET 2021): A block of mass 5 kg is placed on a horizontal surface. If the coefficient of static friction is 0.4, what is the minimum force required to move the block?
A) 10 N
B) 15 N
C) 20 N
D) 25 N
Answer: C) 20 N
Explanation: The minimum force required to overcome static friction is \( F = \mu_s N = \mu_s mg \).
\[ F = 0.4 \times 5 \times 10 = 20 \, \text{N} \]
9. Dynamics of Uniform Circular Motion
Definition: When an object moves in a circular path at constant speed, it is said to be in uniform circular motion.
Centripetal Force: The force that keeps an object moving in a circular path (\( F_c = \frac{mv^2}{r} \)).
Applications:
- Vehicle on a Level Circular Road: Friction provides the centripetal force.
- Vehicle on a Banked Road: Normal force and friction together provide the centripetal force.
Diagram: Centripetal Force
Example: A car turning on a curved road requires centripetal force to maintain its path.
PYQ 12 (NEET 2023): A car of mass 1000 kg moves on a circular track of radius 50 m with a speed of 10 m/s. What is the centripetal force acting on the car?
A) 1000 N
B) 2000 N
C) 3000 N
D) 4000 N
Answer: B) 2000 N
Explanation: Using \( F_c = \frac{mv^2}{r} \):
\[ F_c = \frac{1000 \times (10)^2}{50} = 2000 \, \text{N} \]
PYQ 13 (NEET 2020): A stone tied to a string is whirled in a horizontal circle. If the string breaks, the stone will:
A) Move radially outward
B) Move radially inward
C) Move tangentially to the circular path
D) Stop immediately
Answer: C) Move tangentially to the circular path
Explanation: When the string breaks, the centripetal force disappears, and the stone moves tangentially due to inertia.
