Newton's Laws of Motion

Laws of Motion Questions

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

We can explain this to Naveen in terms of Newton’s first law of motion. The wallet, 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 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 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 Numericals

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 equals the rate of change of an object’s momentum.

\(\begin{array}{l}F=\frac{\Delta p}{\Delta t}\end{array} \)

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

\(\begin{array}{l}\Delta p=mv-mu=m(v-u)\end{array} \)

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

\(\begin{array}{l}F=\frac{m(v-u)}{\Delta t}\end{array} \)

Substituting the value, we get

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

\(\begin{array}{l}F=1.8\times 10^2\,N\end{array} \)

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.

\(\begin{array}{l}F=ma\end{array} \)

Substituting the values in the equation, we get

\(\begin{array}{l}F=100\,kg\times 4.5\,m/s^2=7200\,N\end{array} \)

\(\begin{array}{l}F=7200\,N\end{array} \)

Frequently Asked Questions – FAQs

Q1

Who discovered the three laws of motion?

Sir Isaac Newton discovered the three laws of motion.

Q2

Why are the laws of motion important?

Newton’s laws are essential because they relate to everything 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.

Q3

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 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 there is an equal and opposite reaction for every action.

Q4

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 apply only to planetary motion in the solar system.

Q5

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 an excellent 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.

Here is a fantastic episode of Mission JEE 2022! This session covers Newton’s Laws of Motion.

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