Physical Science

Moodule 6

Understanding Newton’s First Law of Motion

Objectives:

After going through this lesson, you are expected to:

1. state Newton’s first law of motion;

2. define what is inertia;

3. recognize the relationship of mass and inertia

4. explain the subtle distinction between Newton’s first Law of Motion and Galileo’s assertion that force is not necessary to sustain horizontal motion; and

5. apply Newton's first law to understand the effect of applying a force on an objects.

Let’s Review

Review

1. Who proved that all falling objects fall with uniform acceleration?

a. Newton

b. Aristotle

c. Galileo

d. Plato

Review

2. It causes the object in vacuum fall at the same time

a. due to air resistance

b. due to no air resistance

c. due to air in space

d. due to mass of object

Review

3. He proved that heavier object falls faster than lighter object.

a. Newton

b. Aristotle

c. Galileo

d. Plato

Review

4. He is well known in his experimentation of dropping a cannon ball in the Leaning Pisa Tower

a. Newton

b. Aristotle

c. Galileo

d. Plato

Review

5. According to Aristotle, which falls, faster heavier or lighter object ?

a. heavier object

b. lighter object

c. fall at the same time

d. no data at all

Have you ever wondered why people, objects keep on moving?

How do we keep at rest and keep our feet on the ground?

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How do objects stay in place and displaced?

1. Force :
1. Force is an external effort which may move a body at rest or stop a moving body or change the speed of a moving body or change the direction of a moving body or change the shape and size of a body.
2. Effects of force :
1. Force can move a body at rest.
2. Force can stop a moving body.
3. Force can change the speed of a moving body.
4. Force can change the direction of a moving body.
5. Force can change the shape and size of a body.

NEWTON’S LAWS OF MOTION

In 1667, he developed 3 laws of motion that described movement of objects in terms of forces.

These laws of motion still hold true today.

Newton’s Laws of Motion

1^st Law of Inertia–

an object at rest will stay at rest, and an object in motion will stay in motion at constant velocity, unless acted upon an unbalanced force.

Examples of inertia :-

1. If a striker hits a pile of coins on a carrom board, the lowest coin moves out and due to inertia of rest, the other coins fall down.
2. If a coin placed on a playing card over a tumbler is flicked with the finger, due to inertia of rest, the coin falls down into the tumbler.
3. When we travel in a car and the driver applies the brakes suddenly, we tend to fall forward due to inertia of motion.
4. When we are standing in a bus and the bus begins to move suddenly, we tend to fall backward because our feet in contact with the floor moves forward but the upper part of the body continues to remain at rest due to inertia of rest.

Inertia and mass

• The inertia of a body is measured by the magnitude of force required to change the state of the body.
• The force required to change the state of a heavier body is more than the force required to change the state of the lighter body.
• This is because the mass of the heavier body is more than the mass of the lighter body.

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• So 'The mass of a body is a measure of its inertia."

Momentum of a body :

• The momentum of a body is the product of its mass and velocity.
• Momentum = mass x velocity

p=mv

• where p is the momentum of a body
• m is the mass of the body
• v is the velocity of the body
• If a body is at rest its velocity is zero and so its momentum is also zero.
• The SI unit of momentum is kilogram metre per second or kg m/s or kg ms-¹

• Eg:-A truck moving at a very low speed can kill a person standing in its path because of the heavy mass of the truck.

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A bullet of small mass when fired from a gun can kill a person because of the large velocity of the bullet.

So the impact of a body depends upon its mass and velocity.

QUIZ TIME

Modified True or False. Read each statement carefully. Write True if the statement is correct and Write false if the statement is incorrect. Write your answer on a separate sheet of paper.

_____1. Inertia is the tendency of an object to resist motion.

____2. Newton’s first law of motion is also called the law of acceleration.

____ 3. If an object is at rest, inertia will keep it at rest.

____4. The inertia of an object is determined by its speed

____5. The speed of an object changes only when it is acted on by an unbalanced force.

_____6. A stationary object resists movement only because of gravity.

_____7. The tendency of an object to resist a change in motion depends on its mass.

_____8. If the net force acting on an object is zero, its inertia is also zero.

_____9. When you are moving at a high rate of speed, inertia makes it to stop.

_____10. Newton’s first law of motion applies to objects that are already moving and at rest.

1. True
2. False / Inertia
3. True
4. False/ Mass
5. True
6. False/ Friction
7. True
8. True
9. False/ Friction
10. True

References:

https://www.slideshare.net/lashikamadaan/laws-of-motion-classxi

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Thank You

for your attention