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Newton’s First Law of Motion
  • Last Updated : 07 Apr, 2021

In physics, the change in the position of an object with respect to its surroundings in a given interval of time is defined as motion. The motion of a body with mass m can be described in terms of mathematical terms such as distance, displacement, speed, velocity, time, and acceleration. On the basis of observations of Galileo Galilei’s experiment Newton presented three laws of motion. These laws are known as Newton’s Laws of Motion. Before, explaining Newton’s Laws of motion first discuss some basic terms like force, types of forces, inertia as:

Force

It is defined as a push or pulls on an object that produces an acceleration in the body on which it acts. A force can change the speed, direction, and shape of the body on which it acts. 

Mathematically, it is defined as the product of mass (m) and acceleration (a) as:

F = m a

The S.I. unit of force is Newton (N).



Mainly, there are two types of forces: Balanced and Unbalanced forces.

  • Balanced Forces: The forces are said to be balanced forces if they nullify one another and their resultant force is equivalent to zero.
  • Unbalanced forces: When two opposite forces acting on a body, move a body in the direction of the greater force or forces which bring a motion in a body are called unbalanced forces.

Inertia

The natural tendency of an object to resist a change in its state of rest or of uniform motion is called inertia.

Newton’s Laws of Motion

Sir Isaac Newton gave the three laws of motion in the 17th century. These laws are widely used in the field of classical mechanics. They are valid almost everywhere. The laws are briefly stated below.

Newton’s 1st law states that a body at rest or uniform motion will continue to be at rest or uniform motion until and unless a net external force acts on it.

Newton’s 2nd law states that the acceleration of an object as produced by a net force is directly proportional to the magnitude of the net force, in the same direction as the net force, and inversely proportional to the mass of the object.

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

Here in the present article, only the first law of motion is properly explained. This law tells about the relationship between the motion of an object and the force acting on it.

Newton’s First Law of Motion

The first law of motion states that a body continues to be in the state of rest or uniform motion in a straight line unless acted upon by an external unbalanced force. It is also called the Law of Inertia.



In simple words, we can say that objects cannot start, stop, or change direction on their own. They require some external force to cause such a change. This tendency of objects to resist certain changes in their state of motion or rest is known as inertia. 

The first law of motion depends on the following two conditions:

  • Objects at rest: If an object is in the state of rest, its velocity and acceleration are having zero magnitude. This enables the object to stay in rest.
  • Objects in motion: If an object is in motion, its velocity is not equal to zero whereas acceleration is equal to zero. Therefore, the object will continue to be in uniform motion.

How Newton’s first law of motion applicable in everyday life?

  • When the brakes of a vehicle are applied quickly, the passenger will be thrown forward due to the presence of inertia. Inertia tries to keep the passenger moving. This is the reason why it is recommended to wear seat belts while traveling by vehicles.
  • A roller coaster uses the principle of inertia. It continues to move in the same direction at a constant speed until the tracks act as external force that changes its direction. 
  • If you slide a hockey puck on ice, eventually it will stop. This is because of friction on the ice or if it hits something, like a player’s stick or a goalpost. 
  • A book lying on the table remains at rest as long as no net force acts on it.
  • A marathon runner continues to run several meters beyond the finish line due to the inertia.
  • If pulled quickly, a tablecloth can be removed from underneath the dishes. The dishes remain still unless the friction from the movement of the tablecloth is not too high in magnitude.
  • Men in space find it more difficult to stop moving because of a lack of gravity acting against them.
  • Inertia enables ice skaters to glide on the ice in a straight line motion.

Sample Problems 

Problem 1: A person is in an elevator that moving upward at a constant velocity. The weight of the person is 800 N. Immediately the elevator rope is broke, so the elevator falls. Determine the normal force acted by elevator’s floor to the person just before and after the elevator’s rope broke.

Solution:

Given that, 

The weight of the person, W = 800 N.

Before the elevator’s rope broke:

When the person is in the elevator, weight acts on the person downwards. A normal force acts on the person upwards and the magnitude of the normal force is equal to the weight. Because the person is at rest in the elevator and the elevator moves at a constant speed with no acceleration, so there is no net force acting on the person.

∑F = 0

N – w = 0



N = w

N = 800 N

After the elevator’s rope broke:

The elevator and the person free fall together, where the magnitude and the direction of their acceleration(a) is equal to acceleration due to gravity(g). There is no normal force on the person.

Hence, the normal force acted by elevator’s floor to the person just before and after the elevator’s rope broke are 800 N and 0 N respectively.

Problem 2: What net force is required to keep a 100 kg object moving with a constant velocity of 10 m/s?

Solution:

Newton’s first law states that an object in motion tends to stay in motion unless if acted upon by a net force. This means that if friction is not present, there is no net force required to keep an object moving if it’s in motion. 

A net force is only required to change an object’s motion. The 100 kg object is moving at a constant velocity i.e. there is no net force acting on the object. 

So, the net force is equal to 0 N.

Problem 3: A person is traveling in an airplane at a constant speed of 500 mph. Another person is traveling in their car at a constant speed of 50 mph. Determine who experiences a larger acceleration in both cases?

Solution:

Since both the persons are traveling at a constant speed, the acceleration of both the persons is zero. 

Thus, neither of the person experiences any acceleration. 

Since the acceleration is zero, then there is no net force acting on both the persons.

Problem 4: A passenger in an elevator has a mass that exerts a force of 110N downwards. He experiences a normal force upwards from the elevator’s floor of 130N. What direction is he accelerating in, if at all, and at what rate? Use g=10 m/s2

Solution:

Here, the net force is equal to (130 – 120) N = 20N upwards. 

To find the mass of the passenger, use the following formula:

F = mg

m = 110 N / 10 m/s2 

    = 11kg

Then, to find the net acceleration, use Newton’s second law.

F = ma

a = 20N /11kg

  = 1.81 m/s2

Problem 5: A 1500 kg spaceship travels in the vacuum of space at a constant speed of 600 m/s. Ignoring any gravitational forces, what is the net force on the spaceship?

Solution:

In a vacuum, there is no friction due to air resistance. Newton’s first law states that an object in motion stays in motion unless acted upon by a net force. Thus, the spaceship will travel at the constant speed of  600 m/s and the net force on the spaceship must be zero as acceleration is also zero. 

Since, 

F = ma

Therefore,

F = (1500kg)(0m/s2)

  = 0 N.

Problem 6: A ball rolls off the back of a train going 50 mph. Neglecting air friction, what is the horizontal speed of the ball just before it hits the ground?

Solution:

Newtons first law states than an object in motion tends to stay in motion unless acted upon by an external force. 

Neglecting air friction, there is no external force to slow the ball down in the horizontal direction after it falls off the train. 

The acceleration of gravity would only affect the ball in the vertical direction. 

So, the horizontal speed of the ball is 50 mph.

Problem 7: A van is driving around with a bowling ball in the back, free to roll around. The van approaches a red light and must decelerate to come to a complete stop. As the van is slowing down, which direction is the bowling ball rolling?

Solution:

According to Newton’s First Law of Motion, an object that is in motion will stay in motion unless acted on by another force. 

When the van slows down, the ball will want to continue moving forward, and the friction between it and the floor of the van is not strong enough to keep the ball back. 

So, the bowling ball rolls to the front of van.

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