Newton's First Law of Motion

Before the revolutionary ideas of Galileo and Newton, people commonly believed that objects naturally slowed down over time because it was their inherent nature. This assumption stemmed from everyday observations, where things like friction, air resistance, and gravity seemed to slow moving objects.

⇒However, what those early thinkers didn't realize was that they were missing the larger picture: the role that forces, such as friction and gravity, play in changing an object's motion on Earth.

⇒Had they been able to observe an object in the vacuum of space, far from any forces that might influence its movement—like gravity or atmospheric drag—they would have seen something radically different.

⇒In such a setting, an object would keep moving at a constant speed in a straight line unless an external force acted on it to change its motion.

⇒If the object were stationary, it would stay at rest unless something pushed or pulled on it. This realization, articulated by Newton in his First Law of Motion, revealed a key truth about the natural world: objects don't inherently want to stop moving—they only change their motion when a force makes them do so.

Newton’s First Law of Motion, also known as the law of inertia, states that a body always opposes its change in the state of motion.

⇒Newton's Laws of Motion were first proposed by ''Sir Isaac Newton'' in the late 17^th century.

⇒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 holds significant importance in various real-life examples.

Here, we will learn about Newton’s First Law of Motion, its examples, real-life applications of Newton's First Law of Motion, Newton's First Law of Motion equations, and some practice problems on Newton's First Law of Motion.

What is Newton’s First Law of Motion?

Newton’s first law of motion states that,

"An object at rest will remain at rest, and an object in motion will remain in motion with a constant velocity, unless acted upon by an external force."

In simple words, 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.

Newton's First Law of Motion Class 9

Newton's First Law of Motion, often introduced in class 9 physics, is a fundamental concept if a body is at rest or moving at a constant speed in a straight line, it will stay at rest or continue moving in the same way unless a force acts on it.

This law not only applies to physical objects in our everyday life but also forms a basis for more complex principles in physics. Understanding this law paves the way for comprehending the forces at play in the universe and is a critical step for students in class 9 to grasp the broader concepts of physics.

First Law of Motion Formula,

The formula for Newton's First Law of Motion is:

F_net=0

• This means if the total (net) force acting on an object is zero, the object will either stay at rest or keep moving in a straight line at a constant speed.

• If an object is at rest: It will stay at rest until a force makes it move. This means that if an object is already in motion, it will continue moving in the same direction and with the same speed, and if it is at rest,  it will remain at rest. This condition is fundamental to understanding the behavior of objects in motion and the effect of forces on their motion.

• If an object is moving: It will keep moving in a straight line at the same speed until a force changes its speed or direction. An inertial reference frame is a reference frame that is either at rest or moving with a constant velocity relative to other reference frames. The use of an inertial reference frame ensures that the observation of the object's motion is consistent and not affected by the motion of the observer. If the observer is not in an inertial reference frame, then the observation of the object's motion will be distorted by the observer's motion.

• In simple terms, an object will not change its motion (i.e., it won't accelerate) unless a net external force acts on it. This principle, known as inertia, is key to understanding how objects behave when different forces are applied to them.

Also Read,

• Newton's Second Law of Motion: Derivation, Formula
• Newton's Third Law of Motion: Definition, Formula

Understanding Force and Net Force

Force refers to a push or pull that can cause an object to change its state of motion or shape.

• It can be applied in various forms, such as gravity pulling an object downward, or a person pushing a box across the floor. Forces are measured in units called Newtons (N).
• Net force is the total force acting on an object after all individual forces have been combined. It is the vector sum of all the forces. If multiple forces are acting in different directions, the net force determines the overall effect on the object’s motion.

For example, if a person pushes a box with 10 N of force to the right, and another person pushes with 5 N to the left, the net force would be 5 N to the right. If forces cancel each other out, the net force would be zero, meaning no change in motion would occur.

• Person 1 pushes the box with a force of 10 N to the right.
• Person 2 pushes the box with a force of 5 N to the left.

• To the right: Positive direction (+10 N).
• To the left: Negative direction (-5 N).

⇒Net Force= 10N(right)+(−5N)(left)

⇒Net Force=10N−5N=5N

⁛The net force acting on the box is 5 N to the right.

Examples of Newton's First Law of Motion

Now, Let us understand the First Law of Motion by various examples that we can observe in our daily life. Newton's first law of motion examples can be observed in numerous everyday situations:

Example 1: A Stationary Car

A car parked on a level surface will remain stationary until an external force, such as a push, is applied, In this picture a person applied a force on car but the car didn't move.

Example 2: A Rolling Ball

A ball rolling on a flat surface will continue to roll in a straight line unless friction or another force stops it.

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.

Example 2: A Passenger in a Car

When a car suddenly stops, passengers lurch forward because their bodies continue to move forward due to inertia.

Applications of Newton's First law of Motion

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 travelling 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.

Concept of Inertia

Newton's First Law of Motion Describes the Concept of Inertia, Inertia is the resistance of an object to any change in its state of motion. It is a fundamental property of matter that describes how objects tend to maintain their current state of motion.

⇒An object at rest tends to stay at rest, and an object in motion tends to stay in motion with the same speed and direction unless acted upon by an external force. This property of matter was first described by Sir Isaac Newton in his first law of motion, which is also known as the law of inertia.

⇒The inertia of an object depends on its mass. The greater the mass of an object, the greater its inertia, and the more difficult it is to change its state of motion.

For example, a massive object like a boulder requires more force to set it in motion than a lighter object like a pebble. Similarly, a moving car requires more force to bring it to a stop than a bicycle moving at the same speed.

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.

Read More: Balancedand Unbalanced Forces

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,

F_incline = mg sin45°

Force acting along normal,

F_normal = 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.

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

M₁g - T = M₁a₁

M₂a₂ = 2T - M₂g

Solved Examples - 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.

• Once the elevator's rope breaks, both the person and the elevator are in free fall. This means both are accelerating downward due to gravity at the same rate, ggg.
• When an object is in free fall, the normal force from the floor is zero because there is no longer any force exerted by the floor on the person. The person is essentially "weightless" in this scenario because both the person and the elevator are accelerating together at the same rate.

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

   = 1.82 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.

Practice Questions

1. A space probe is drifting to the right at a constant velocity in deep interstellar space, far from any external forces. Two rocket thrusters are activated, exerting equal and opposite forces leftward and rightward. What happens to the motion of the probe?

a. The space probe would continue with constant velocity.
b. The space probe would speed up.
c. The space probe would slow down and eventually stop.
d. The space probe would immediately stop.

Correct Answer: (a)
Explanation: According to Newton's First Law of Motion, an object will continue in its state of motion unless acted upon by an unbalanced force. Since the forces from the two thrusters cancel each other out, there is no net force, and the probe continues to move at a constant velocity.

2. An elevator is moving upward at a constant velocity. How does the upward force exerted by the cable compare to the downward force of gravity on the elevator?

a. The upward force is greater than the downward force.
b. The upward force is equal to the downward force.
c. The upward force is smaller than the downward force.
d. The upward force could be larger or smaller than the downward force depending on the mass of the elevator.

Correct Answer:( b)
Explanation: When the elevator is moving with constant velocity, the net force must be zero according to Newton’s First Law. For the forces to cancel each other out, the upward force exerted by the cable must be exactly equal to the downward force of gravity.

3. A space probe is drifting to the right at constant velocity in deep interstellar space. A rocket thruster briefly fires in a direction perpendicular to the probe's motion, then turns off. What path does the probe follow after the thruster is turned off?

a. The probe will move downward in a curved path.
b. The probe will curve slightly downward and to the right.
c. The probe will move in a straight line to the right.
d. The probe will move diagonally downward and to the right.

Correct Answer: (c)
Explanation: Once the thruster is turned off, there is no net force acting on the probe. According to Newton's First Law, the probe will continue in a straight line at constant velocity. The force from the thruster only altered the vertical velocity, but with no further forces, the horizontal velocity remains constant, so the probe moves in a straight line.

4. A space probe in deep space is traveling with a constant velocity. What happens if no external forces are acting on the probe?

a. The probe will eventually stop due to internal friction.
b. The probe will change direction due to gravitational influences.
c. The probe will continue moving at the same velocity and in the same direction.
d. The probe will accelerate in the direction of its motion.

Correct Answer: (c)
Explanation: In the absence of external forces, the probe will continue to move at the same velocity and in the same direction. This is a direct consequence of Newton's First Law of Motion, which states that an object in motion will stay in motion unless acted upon by an external force.

5. A car is moving at a constant speed on a straight road. The engine is providing a force to move the car forward, but friction and air resistance are acting against the motion. What is the net force acting on the car?

a. The net force is greater than zero because the car is moving.
b. The net force is zero because the car is moving at constant velocity.
c. The net force is zero because the forces from the engine and friction cancel each other out.
d. The net force is in the direction of the engine’s force.

Correct Answer: (b)
Explanation: Since the car is moving at a constant speed, there is no acceleration, meaning the forces must balance out. The forward force from the engine is exactly canceled by the friction and air resistance. Therefore, the net force is zero.

7. A rocket in space is initially at rest. It fires its thrusters in one direction for a short burst and then turns them off. What will happen to the rocket once the thrusters are off?

a. The rocket will continue moving in the direction it fired its thrusters.
b. The rocket will stop moving.
c. The rocket will move in the opposite direction of the thrusters.
d. The rocket’s speed will decrease gradually.

Correct Answer: (a)
Explanation: According to Newton’s First Law, once the thrusters are turned off, the rocket will continue moving in the direction it was propelled. This is because there are no external forces acting to change its motion, so it maintains its velocity.

Conclusion - Newton's First Law of Motion

Newton's First Law of Motion, or the law of inertia, states that an object will remain at rest or move at a constant speed in a straight line unless acted upon by an unbalanced force. This principle highlights an object's resistance to motion changes, known as inertia, which is proportional to its mass. The law is essential for understanding motion and analyzing physical systems through free body diagrams and constraint equations. It forms the foundation of how we study and control movement in the world.

Important Physics Related Links:

• Law of Inertia
• Velocity
• Force
• Acceleration
• Exploring Forces
• Interaction of Forces
• Laws of motion questions: numericlas and FAQs
• Friction
• Types of friction
• Static and KInetic friction
• Factors affecting friction
• Mass and Inertia
• Momentum formula
• Real life applications of momentum
• Linear Momentum
• Uniform Accelerated Motion
• Examples of Newton's Third Law
• Solved Numericals of Physics Class 10
• All Derivations Physics Class 12 PDF Download