Newton's Laws of Motion

Newton’s Third Law of Motion

The third law of motion describes what happens to the body when it exerts a force on another body.

The Newton’s 3rd law states that for every action there is an equal and opposite reaction.

When two bodies interact, they apply force on each other that are equal in magnitude and opposite in direction. To understand Newton’s third law with the help of an example, let us consider a book resting on a table. The book applies a downward force equal to its weight on the table. According to the third law of motion, the table applies an equal and opposite force on the book. This force occurs because the book slightly deforms the table; as a result, the table pushes back on the book like a coiled spring. Newton’s third law of motion implies the conservation of momentum.

Read More: Newton’s Third Law of Motion

Review Questions

1) While driving to work, Naveen always keeps his purse on the passenger seat.  By the time he gets to work, his purse would have fallen on the floor in front of the passenger seat. One day, he asks you to explain why this happens in terms of physics. What do you say?

We can explain this to Naveen in terms of Newton’s first law of motion. The purse due to inertia travels forward with the car until acted upon by another force, in this case, the force of the car floor.

2) Which of Newton’s laws best explains how a magician can pull a tablecloth from underneath dishes?
Newton’s first law of motion can explain how a magician pulls a tablecloth from underneath the dishes. A negligible horizontal force is applied during the process. As per Newton’s first law of motion, the dishes and glasses remain in their state of motion (rest); as a result, they remain undisturbed. The table cloth in the trick is made so slippery such that it doesn’t apply any frictional force on glasses and dishes.

Understanding laws of motion with engaging video lecture

Laws of Motion Summary

Laws of Motion Problems with Solution

1. Suppose a bike with a rider on it having a total mass of 63 kg brakes and reduces its velocity from 8.5 m/s to 0 m/s in 3.0 second. What is the magnitude of the braking force?

Solution:

The combined mass of the rider and the bike = 63 kg
Initial Velocity = 8.5 m/s
Final Velocity = 0 m/s
The time in which the bike stops = 3 s

The net force acting on the body is equal to the rate of change of momentum of the object.

\(F=\frac{\Delta p}{\Delta t}\)

The momentum of a body with mass m and velocity v is given by  p = mv

Hence, the change in momentum of the car is given by

\(\Delta p=mv-mu=m(v-u)\)

Hence, the net force acting on the car is given by 

\(F=\frac{m(v-u)}{\Delta t}\)

Substituting the value, we get

\(F=\frac{63\,kg\times (0-8.5\,m/s)}{3.0\,s}=1.8\times 10^2\,N\)

\(F=1.8\times 10^2\,N\)

2. Calculate the net force required to give an automobile of mass 1600 kg an acceleration of 4.5 m/s²

We calculate the force using the following formula

\(F=ma\)

Substituting the values in the equation, we get

\(F=100\,kg\times 4.5\,m/s^2=7200\,N\)

\(F=7200\,N\)

Frequently Asked Questions – FAQs

Who discovered the three laws of motion?

Sir Isaac Newton discovered the three laws of motion.

Why are the laws of motion important?

Newton’s laws are essential because they relate to everything that we do or see in everyday life. These laws tell us how things move or stay still, why we don’t float out of our bed or fall through the floor of our house.

What is Newton’s laws of motion all about?

Newton’s laws of motion imply the relationship between an object’s motion and the forces acting on it. In the first law, we come to understand that an object will not change its motion unless a force acts on it. The second law states that the force on an object is equal to its mass times its acceleration. And, finally, the third law states that for every action, there is an equal and opposite reaction.

What is the difference between Newton’s laws of motion and Kepler’s laws of motion?

Newton’s laws of motion are general and apply to any motion, while Kepler’s laws of motion apply only to planetary motion in the solar system.

What are some daily life examples of Newton’s 1st, 2nd and 3rd laws of motion?

• The motion of a ball falling through the atmosphere, or a model rocket being launched up into the atmosphere are both excellent examples of Newton’s 1st law.

Riding a bicycle is a good example of Newton’s 2nd law. In this example, the bicycle is the mass. The leg muscles pushing on the pedals of the bicycle is the force.

• You hit a wall with a certain amount of force, and the wall returns that same amount of force. This is an example of Newton’s 3rd law.

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