Newton’s �Laws of Motion

I Law of Inertia

II F=MA

III Action-Reaction

Newton’s Laws of Motion

• 1^st Law – An object at rest will stay at rest, and an object in motion will stay in motion at constant velocity, unless acted upon by an unbalanced (outside) force. Motion leads to motion, rest leads to rest.
• 2^nd Law – Force equals mass times acceleration.
• 3^rd Law – For every action there is an equal and opposite reaction.

1^st Law of Motion�(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 by an unbalanced (outside) force. Motion leads to motion and rest leads to rest.

1^st Law

• Inertia is the tendency of an object to resist changes in its velocity: whether in motion or motionless.

A pile of pumpkins will remain at rest unless acted upon by an unbalanced (outside) force.

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An elephant has more inertia than a mouse because it has more mass.

1^st Law

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• The moon will keep revolving around the earth forever, unless acted on by an unbalanced force.

1^st Law

• Unless acted upon by an unbalanced force, this golf ball would sit on the tee forever.

Newtons’s 1^st Law and You

Don’t let this be you. Wear seat belts.

Because of inertia, objects (including you) resist changes in their motion. When the car going 80 km/hour is stopped by the brick wall, your body keeps moving at 80 km/hour.

2^nd Law

2^nd Law

The net force of an object is equal to the product of its mass and acceleration, or F=ma.

2^nd Law

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• When mass is in kilograms and acceleration is in m/s/s (m/s²), the unit of force is in newtons (N).
• One newton is equal to the force required to accelerate one kilogram of mass at one meter/second/second.

2^nd Law (F=MxA)

• How much force is needed to accelerate a 1400 Kilogram car 2 meters per second/per second?
• Write the formula
• F = M x A
• Fill in given numbers and units
• F = _______kg x ___ meters per second/second
• Solve for the unknown
• ______ kg-meters/second/second or 2800 N

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Newton’s 2^nd Law proves that different masses accelerate to the earth at the same rate, but with different forces.

• We know that objects with different masses accelerate to the ground at the same rate.
• However, because of the 2^nd Law we know that they don’t hit the ground with the same force.

F = ma

98 N = 10 kg x 9.8 m/s/s

F = ma

9.8 N = 1 kg x 9.8 m/s/s

Check Your Understanding

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 1. How much force is needed to accelerate a 66-kg skier 1 m/s²?

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2. What is the force on a 1000-kg elevator that is falling freely at 9.8 m/s²?

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3^rd Law

• For every action, there is an equal and opposite reaction.

3^rd Law

According to Newton, when you sit in your chair, your body exerts a downward force on the chair and the chair exerts an upward force on your body. These two forces are called action and reaction forces.

3^rd Law

Flying gracefully through the air, birds depend on Newton’s third law of motion. As the birds push down on the air with their wings, the air pushes their wings up and gives them lift.

Other examples of Newton’s Third Law

• The baseball forces the bat to the left (an action); the bat forces the ball to the right (the reaction).

Test your knowledge!

• Identify which law of motion each situation is describing below.
• A book lies undisturbed on a desk.
• A comet passing Jupiter is accelerated by Jupiter’s great gravity.
• A rocket soars skyward as its engine fires.
• A baseball hits the catcher’s mitt when it is missed by the batter.

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5. A car does not move until its engine is started and the car is put in gear.

6. A chair with wheels moves back when the secretary pushes on the desk.

7. A hit baseball accelerates past the pitcher who threw it.

8. A falling object accelerates as it falls.

9. An open air-filled balloon flies around the room.

10. A gust of wind blows an arrow in flight off target.