Laws of Motion is a fundamental topic in NEET Physics with a high percentage weightage in the syllabus. At least 1-3 questions are asked in NEET 2022 every year on the topics of Law of Motion. In this discussion, we have prepared important notes you need to memorise on this topic to aid your NEET 2022 preparation. Some of the major topics in the Chapter on Laws of motion are Force, Circular Motion, and Newton’s laws. We will be discussing all of those in detail.
What are Force and Inertia?
Force: Force can be defined as a push or pull that alters or tends to alter the state of rest or motion of a physical object or a body. A force has both magnitude and direction, so it is a vector quantity.
Inertia: Inertia can be defined as the nature of a physical object due to which it opposes any changes in itself or its state. The inertia of an object can be measured by measuring its mass. Greater force is required to change the state of a heavier object.
Newton’s Laws of Motion
- Newton’s First Law of Motion: If the total external force on a physical object is equal to zero, that object will remain at rest, or it will move with a constant velocity. This law is the basis of the concept of inertia, therefore, also referred to as the law of inertia.
- Newton’s Second Law of Motion: It defines the force. According to this law, the acceleration of a physical object is directly proportional to the net force applied or acting on it and inversely proportional to its mass.
- Newton’s Third Law of Motion: States that there is an equal and opposite reaction to every action. If object A applies a force on object B, object B applies a force of equal magnitude on object A but in the opposite direction.
What are Momentum and Impulse?
- Momentum is represented by p. Its formula is p = mv, where m is the mass of the body or the physical object and v is the velocity of the object. Therefore, the momentum of an object is defined as the product of its mass and velocity.
- Impulse is the total effect generated when a large amount of force is applied or acts for an extremely short period of time.
Conservation Law of Momentum
The Conservation Law of Momentum states that the net or total change in linear momentum is zero without an external force. Given below are applications of Conservation Law of Momentum
- The Recoil of a Gun: The gun and the working bullet system easily define the conservation law of momentum. Before firing, the gun and bullet are at rest; therefore, the system’s momentum is zero. However, on firing the bullet, it moves forward while the recoil moves backwards. Because the mass of a gun is very large compared to a bullet, the recoil is small compared to the velocity of a bullet.
- The Motion of Rocket: Rockets also define the conservation law of momentum. The gases in the rocket burn with large momentum, causing the rocket to gain an equal and opposite momentum, enabling the rocket and its engine to propel with a very high velocity.
- The Man and the Boat: Suppose a man is standing still on a boat, so the momentum of both the man and the boat is zero. If the man jumps from the boat towards the shore, he gains momentum in the forward direction. As stated by the conservation law of momentum, the boat gains an equal momentum to move in the backward or opposite direction, so it slightly moves.
What is friction, and what are its types?
Friction is a contact force experienced by a surface or an object when it moves relative to another surface or object. Friction is self-adjusting in nature. Suppose an object is placed on a plane surface and force F is applied to it, then F is the frictional force where N is the normal force, and μ is the coefficient of frictional force. Given below are the types of friction:
- Static Friction: It is the type of frictional force that acts on a body or a physical object when no other relative motion is there between that object and the surface. Here “μs” is the coefficient of static friction, and N is the Normal force.
- Kinetic Friction: It is also called sliding friction and is the type of frictional force that resists the correlative motion between two surfaces in contact. Here “μk” is the coefficient of kinetic friction, and N is the Normal force.
- Rolling Friction: It is also called sliding friction and can be defined as the frictional force that resists the motion of the body when a body is rolling on a surface. Here “μr” is the coefficient of rolling friction, and N is the Normal force.
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Example of multiple-choice question based on the concept of Friction
Question: Choose the correct statement true in case of Friction
- Rolling friction is higher than sliding friction
- Rolling friction is lower than sliding friction
- Rolling friction is equal to sliding friction
- Rolling friction and sliding friction are one and the same thing
Answer: Rolling friction is lower than sliding friction
Example of Sample Question on Recoil of Gun
Before Firing, Total Mass= m1 + m2, Velocity v=0, because the system was at rest
Therefore, Momentum = mass X velocity = 0
After Firing, Total Mass= m1 +m2, Velocity v= 150m/s
Therefore, Momentum = 3 + 2v
What is Circular Motion?
When an object or a physical body moves in a circular path, its motion in that path is called circular motion. While moving, it experiences a force acting towards the radius of the circular path. This forces the centripetal force, which helps the object or body move on a circular path. Given below are types of circular motion:
- Non-Uniform Circular Motion: As the name suggests, an object is said to have a non-uniform motion when moving in a circular path, but its speed is not constant. When moving in a non-uniform circular motion, the net acceleration of the moving body has two components—radial and tangential. Radial acceleration is due to the changes in the direction of the velocity, while the tangential acceleration is due to the changes in the magnitude of the velocity.
- Uniform Circular Motion: An object is said to be in a uniform circular motion when it follows the circular path with a constant speed.
Tips to Solve Questions on Laws of Motion
- Question: Suppose there is a body with a mass of 10kg and an acceleration of 5m/s2. The net force experienced by it will be?
Tip to Solve the Question: The questions where mass and acceleration are given can be solved by using the Newton’s Second Law of Motion
- Question: Suppose there is a car with a mass of 1100 kg, which accelerates on a level road from rest to 2m/s in 14s. The steepest slope this car can climb is?
Tip to Solve the Question: Read the question carefully and pay attention to every detail. Note that the plane is smooth.
Solution: The steepest slope this car can climb is 0.83 degrees.
- Question: A body with a mass of 100 grams is tied at the end of a string of length 3 m and rotates in a vertical circle. It is just able to complete the circle. If the tension in the string is at its lowest point, which is 3.7 N, what will be its angular velocity? Also, consider g = 10 m/s2
Tip to Solve the Question: We can understand from the question that the body is moving in the circle. So, to calculate the velocity, you can use the concept of Circular motion to get the answer.
Solution: 3 rad/s
Important Formulas to Memorise in Laws of Motion
|Force, F||F = m X a, where m is mass and a is the acceleration|
|Newton’s first law of motion||F = 0|
|Newton’s second law of motion||F = m X a|
|Newton’s third law of motion||F1 = – F2|
|Momentum, p||p = mv, where m is mass and v is velocity|
|Friction, F||F = μN|
Strategies for Solving Questions on Laws of Motion
- Start by creating a rough sketch or diagram of the question mentioning all the information given in the question like force, mass, acceleration, etc. It will help you correlate between the information given and create a better understanding of that question.
- Identify the principles involved in the problem.
- Before solving the question, determine if it includes Newton’s laws of motion or not. If it doesn’t, you can approach the question from a different point of view. If it does, then the unknown value can be determined with the help of the laws of motion
After going through the discussion, you will now be well acquainted with the key points you need to remember related to it in your Physics exam. Physics can be a challenging subject, but with hard work and willpower, you can master it. Keep pushing hard; success is near.
1. What is motion?
Motion is the change in a body's stance in correspondence with time. To understand the concept of motion, consider the example of cars in a race. Suppose there is a car A and a car B, and both are running simultaneously in the race. Both the cars are in a high-speed motion compared to the audience, but they are at rest with respect to each other as both are moving at the same speed. Similarly, consider the example of a moving bus. If two persons A and B are sitting on the bus, person A is at rest with respect to person B and vice versa, but both of them are in motion with respect to person C, who is waiting for the bus at the bus stop. Hence, motion is a relative quantity as it is considered with respect to the position of the observing body. Every physical body is in motion, either fast or slow.
2. Why are the laws of motion important?
Laws of motion are the pillars of classical mechanics, one of the most fundamental branches of Physics. Applications of the laws of motion are seen in many activities of everyday life. For example, when a person jumps, their feet exert a force on the ground, and the ground exerts an opposite force of equal magnitude that pushes the person into the air. This example is based on the principle of Newton's third law of motion.
3. What are some real-life examples of Newton's third law of motion?
There are innumerable real-life examples of Newton's third law of motion. When a person is swimming, he pushes the water back with his feet. This exerts a force on the water, which then exerts an opposite force of the same magnitude back on the person propelling him forward. Also, when a bullet is fired, the gun exerts a force on it to propel it in the forward direction. The bullet also exerts a force of equal magnitude on the gun to push it in the backward direction.
4. What is a real example of Newton's second law of motion?
If you throw a ball, the acceleration with which it moves is directly proportional to the force with which it was thrown. Therefore, the harder the ball is thrown, the faster it moves. Also, if you push a car and a truck with the same force, you will observe that the car accelerates more than the truck. This is because it is lighter and has a lower mass. Both these examples demonstrate Newton's second law of motion.