Notes: Newton’s Laws of Notes: Newton’s Laws of MotionMotion

CW: Action-Reaction ForcesCW: Action-Reaction Forces

Newton’s Newton’s Laws of MotionLaws of Motion

March 18, 2009

ObjectivesObjectives

1.1. Explain the three laws of motionExplain the three laws of motion

11stst Law of Motion Law of Motion (Law of Inertia) (Law of Inertia)

An object at rest will stay An object at rest will stay at rest, and an object in at rest, and an object in motion will stay in motion motion will stay in motion traveling in a straight line, traveling in a straight line, unless acted upon by an unless acted upon by an unbalanced force.unbalanced force.

11stst Law Law

Once in motion, Once in motion, unless acted on unless acted on by an by an unbalanced force unbalanced force it would never it would never stop! stop!

11stst Law Law

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

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

Why then, do we observe Why then, do we observe every day objects in motion every day objects in motion slowing down and becoming slowing down and becoming motionless seemingly without an motionless seemingly without an outside force?outside force?

There are four main types of friction:There are four main types of friction: Sliding friction: Sliding friction: ice skating Rolling friction: Rolling friction: bowling Fluid friction: Fluid friction: air or water resistance Static friction: Static friction: initial friction when

moving an object

22ndnd Law Law

22ndnd Law Law

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

22ndnd Law Law

The unit of force is in newtons (N).The unit of force is in newtons (N).

One N is equal to the force required One N is equal to the force required to accelerate one kg of mass at one to accelerate one kg of mass at one m/s/s.m/s/s.

Newton’s 2nd 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 2nd Law we know that they don’t hit the ground with the same force.

F = maF = ma

98 N = 10 kg x 9.8 m/s/s98 N = 10 kg x 9.8 m/s/s

F = maF = ma

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

33rdrd Law Law

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

Forces always act in pairs

Newton’s 3rd Law in NatureNewton’s 3rd Law in Nature

A fish uses its fins A fish uses its fins to push water to push water backwards. The backwards. The water water reactsreacts by by pushing the fish pushing the fish forwards, forwards, propelling the fish propelling the fish through the water.through the water.

33rdrd Law Law

As the birds As the birds push down on push down on the air with the air with their wings, the their wings, the air pushes their air pushes their wings up and wings up and gives them lift.gives them lift.

Action-reaction force pairs make it Action-reaction force pairs make it possible for birds to fly.possible for birds to fly.

Other examples of Newton’s Other examples of Newton’s Third LawThird Law

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

33rdrd Law LawVarious fuels are Various fuels are burned in the burned in the engine, engine, producing hot producing hot gases. gases.

As the gases As the gases move downward, move downward, the rocket moves the rocket moves in the opposite in the opposite direction.direction.