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# Newton’s First Law of Motion

Newton’s Laws of motion were first proposed by Sir Issac Newton in the late 17th century. Newton’s First Law of Motion basically states that a body always opposes its change in the state of motion. Newton’s First Law of Motion finds its importance in various other laws and it is one of the fundamental laws of physics. It also finds its importance in various real-life examples. In this article, we will learn about Newton’s First Law of Motion, examples, and others in detail.

## What Is Newton’s First Law of Motion?

According to Newton’s first law of motion,

“A body in the state of rest stays at rest and a body in the state of uniform motion stays at uniform motion until and unless a net external force is applied.”

Simply, we can say that an object which is at rest will continue to stay at rest and an object which is in uniform motion continues to stay in uniform motion until an external force is applied. The first law of motion is also called as Law of Inertia.

### Two Conditions of Newton’s First Laws

The two important conditions on which Newton’s First Laws of motion depend are,

• For Objects at Rest: If an object is at rest, i.e. there, velocity and acceleration are zero. The object will remain at rest until an external force is applied.
• For Objects in Motion:  If an object is in a state of uniform motion, i.e. it has an initial velocity. The object will remain in a state of uniform motion until an external force is applied.

### What is Inertia?

The natural tendency of an object to resist a change in its state of rest or in its state of uniform motion is called Inertia. It is the fundamental property of any object. We can observe inertia in our daily life as, when a bus starts from rest we tend to lean backward, this is because of the Inertia of rest.

### What Is an External Force?

The force applied on an object externally that produces an acceleration in the body is called the external force. For an object of mass ‘m’ and the external force applied is ‘F’ then the acceleration produced is ‘a’ the relation between them is,

F = m×a

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

External force can be categorized as,

• Balanced Forces: The force is 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.

### Let Us Understand the First Law of Motion by an Example

Now, Let us understand the First Law of Motion by various examples that we can observe in our daily life. The image given below shows a football that is placed on the ground it will not move until a net external force is applied to it.

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

## Newton’s First Law of Motion Example in Daily Life

In our daily life, we came across various examples which support Newton’s First Law of Motion. Some of the examples which support this law are,

• 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 vehicle.
• 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 an 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 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.

## Free Body Diagrams

Free body diagrams also called FBD are very useful to solve various problems of mechanics. FBDs show all the force that is being applied to any object in the proper directions. To solve problems on Newton’s Laws of Motion using FBD use the steps given below,

### Steps to Solve Problems on Newton’s Laws of Motion

In a fixed wedge a block of mass m is kept now we have to find the acceleration of the block then,

Step 1: Draw the F.B.D of the block,

Step 2: Write all the components of the force acting on the block in the x and y components.

Force acting along incline plane Fincline = mg sin45°

Force acting along normal Fnprmal = mg cos45°

Step 3: Find the acceleration using Newton’s law of motion.

ma = mg sin45°

a = g sin45°

## Constraint Equations

Constraint Equations are formed in such motions where the motion of one body is dependent on the motion of another body i.e. motion of one body affects the motion of another body.

In the above pulley system, it is evident that the motion of blocks M1 and M2 are interdependent on each other and hence constraint equations are formed.

M1g – T = M1a1

M2a2 = 2T – M2g

We have two equations and three unknowns, thus a constraint equation is required to solve the above equations.

## Solved Examples on Newton’s First Law of Motion

Example 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 broke, so the elevator falls. Determine the normal force acted by the elevator’s floor on 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.

Example 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.

Example 3: A person is traveling in an aeroplane at a constant speed of 500 mph. Another person is travelling 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.

Example 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 = 20 N upwards.

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

F = mg

m = 110 N/ 10 m/s2

= 11 kg

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

F = ma

a = 20N /11kg

= 1.81 m/s2

Example 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.

Example 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.

Example 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, in 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.

## FAQs on Newton’s First Law of Motion

### Question 1: State Newton’s first law of motion and give one example.

Newton’s First Law of motion states that an object at rest stays at rest or an object at motion continues its state of motion until an external force is applied. For example, a ball rolling on a frictionless surface tends to roll infinitely until an external force is applied.

### Question 2: What is the difference between Newton’s First Law and Inertia?

Inertia is the property of any object that it acquires either in a state of rest or in a state of motion.  Newton’s first law is a law which states that an object always maintains its Inertia of rest or Inertia of motion.

### Question 3: Why do objects slow down?

In general, an object moving with a constant velocity slows down because of friction.