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NEWTON’S LAWS OF MOTION

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NEWTON’S LAWS OF MOTION. Isaac Newton. NEWTON’S LAWS OF MOTION. 1 st Law – An object at rest will stay at rest, and an object that is moving will keep on moving at the same speed and in the same direction, unless acted upon by an unbalanced force (Law of inertia).

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## NEWTON’S LAWS OF MOTION

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### Isaac Newton

NEWTON’S LAWS OF MOTION
• 1st Law – An object at rest will stay at rest, and an object that is moving will keep on moving at the same speed and in the same direction, unless acted upon by an unbalanced force (Law of inertia).
• 2nd Law – Force equals mass times acceleration.
• 3rd Law – For every action there is an equal and opposite reaction.
NEWTON’S LAWS OF MOTION
• 1st Law – An object at rest will stay at rest, and an object that is moving will keep on moving at the same speed and in the same direction, unless acted upon by an unbalanced force (Law of inertia).
• 2nd Law – Force equals mass times acceleration.
• 3rd Law – For every action there is an equal and opposite reaction.
What is Inertia?
• Inertia is the tendency to resist any change in motion
• The larger the mass of an object, the greater its inertia and the harder it is to change its motion
• E.g. if at rest  tends to stay at rest
• E.g. if moving at constant speed  tends to stay at constant speed
• E.g. if moving in one direction  tends to stay in the same direction
1st Law of Motion (Law of Inertia)
• Newton stated his first law of motion as, "An object in motion wants to stay in motion, and an object at rest wants to stay at rest.“
• Objects keep doing what they are doing unless acted upon by an unbalanced force.

Your pencil case has inertia, it stays stationary. What happens when you apply force to it?Unless acted upon by an unbalanced force, your book would keep on moving with the same speed and direction – forever!

• Why does it stop then?
Friction
• Objects on earth, unlike the frictionless space the moon travels through, are under the influence of friction.
• There are four main types of friction:
• Sliding friction: ice skating
• Rolling friction: bowling
• Fluid friction (air or liquid): air or water resistance
• Static friction: initial friction when moving an object
Application of Newton’s 1st Law
• 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.
• Airbags and seatbelts restrain your body so that you come to a stop with the car (provides the unbalanced forced)
• Headrests are placed in cars to prevent whiplash injuries during rear-end collisions
• To dislodge ketchup from the bottom of a ketchup bottle?
Video
• http://www.youtube.com/watch?v=Q6dmvnlrLZ8
Newton’s 2nd Law
• An object will accelerate in the direction of a net force acting upon it.
• The net force of an object is equal to the product of its mass and acceleration, or F=ma

F = m x a

F = m a
• If mass remains constant, doubling the acceleration, doubles the force.
• If force remains constant, doubling the mass, halves the acceleration.
• EXAMPLE:
• Full paper roll = more mass
• Assume force is the same, acceleration should be faster
2nd Law (F = m x a)
• How much force is needed to accelerate a 1400 kg car to 2/m/s2?
• Write the formula
• F = m x a
• F = 1400 kg x 2/m/s2

=2800 kg/m/s2 or 2800 N

Check Your Understanding
• 1. What acceleration will result when a 12 N net force applied to a 3 kg object?
• 2. A net force of 16 N causes a mass to accelerate at a rate of 5 m/s2. Determine the mass.
Check Your Understanding
• 1. What acceleration will result when a 12 N net force applied to a 3 kg object?

12 N = 3 kg x 4 m/s/s

• 2. A net force of 16 N causes a mass to accelerate at a rate of 5 m/s2. Determine the mass.

16 N = 3.2 kg x 5 m/s/s

Newton’s 3rd Law

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

Newton’s 3rd Law

According to Newton, whenever objects A and B interact with each other, they exert forces upon each other.

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.

The action and the reaction force are of the same size, but act in different directions

Demonstrations
• Look at your book
• Does the book exert a force on the table?
• Does the table exert a force on the book?
• Push against wall
• Are you exerting force on the wall?
• Is the wall exerting force on you?
Application of Newton’s 3rd Law
• Wings of birds push air downwards and air pushes bird upwards
• A fish uses its fins to push water backwards. In turn, the water reacts by pushing the fish forwards, propelling the fish through the water.
• Wheels of vehicles spin, grip and push the road backwards, the road pushes the wheels forward
NEWTON’S LAWS OF MOTION
• 1st Law – An object at rest will stay at rest, and an object that is moving will keep on moving at the same speed and in the same direction, unless acted upon by an unbalanced force (Law of inertia).
• 2nd Law – Fnet = ma
• 3rd Law – For every action there is an equal and opposite reaction.