Newton’s 3 laws of motion

Isaac Newton 1642 --- 1727

Physics 100 Chapt 3

1st Law: Law of Inertia

A body at rest tends to stay at rest, a body in motion tends to keep moving along at a constant speed and in a straight-line path unless interfered with by some external forces.

example

Motorcycle crash dummy

Another example(watch the ladder)

2nd Law: F=ma

The acceleration of a body is directly proportional to the net force acting on it and inversely proportional to its mass.The direction of the acceleration is in the direction of the applied force.

Directly proportional to Force

a

Small forceSmall acceleration

a

Large force

Large acceleration

inversely proportional to mass

aa

Large mass

Small acceleration

small mass

Large acceleration

Bowling ball

Beachball

“Inertial” mass

“Inertial” mass, mi, is the resistance to changes in the state of motion

Objects with large mi

are hard to get moving

(& once started, hard to stop),

Objects with small mi

easier to get moving

(& easier to stop),

Units of mass (we cant avoid them!)

Mass: basic unit = 1kilogram = 1kg mass of 1 liter of water

This muchwater! 10cm

10cm

10cm

Net forceTip-to-tail method for adding vector

Slide tail of one to tip of the other (keep directions

fixed)

Net force is the vector from the tail of the 1st to the tip of the

2nd. (0 in this case).

Tip-to-tail method

Slide tail of one to tip of the other (keep directions

fixed)

Net force points down the hill

Newton’s 2nd law F=ma

a F

a 1/m

a is proportional to F:

direction of a = direction of F: a F

a is inversely proportional to m:

a F/mcombine:

set proportionalityconstant = 1: a = F/m

multiply both sides by m

Units of Force

F=makgm

s2

Unit of force: 1 Newton = 1N = 1 kg m/s2

1 pound =1lb = 4.5 N

What is your mass?Weight = force of Earth’s gravity on you

W=mg

W

a=g

F=ma

m=Wg

Suppose I

jump offa tqble

Two different aspects of mass

Weight: W = m

gForce of gravity is

proportional to “gravitational”

mass

a =

Fm

Inertia; resistance to changes in

state is proportional to “inertial” mass

mi

mgg

Newton’s 2nd law:

Experiment shows: mg =

mi

My mass

My weight in pounds: W = 190 lbs

855 N10 m/s2

!!!!!

Convert to Newtons:

W = 190 lbs x 4.5 N/lb = 855 N

m = W/g = = 85.5 kg

Newton’3rd Law: action-reaction

Whenever one object exerts a force on a second object, the second object exerts an equal in magnitude but opposite in direction force on the first.

action: I pushon the canoe

reaction: the canoe pushes

me forward

Action Reaction

I push on the bus

F

v= 0

But I accelerate

Newton: The bus exerted an “equal but opposite” force on me.

v

Look again

F

-F

All forces come in pairs!

This force causes meto accelerate backwards

This force tries to accel.the bus forward

Air-filled balloon

action: balloonpushes on air

reaction: airpushes on balloon

recoilaction: gun exertsforce F2 on bullet

making it accelerate

reaction: equal butopposite force on the gun F1

Produces a recoil

Rocket propulsion

action: rocket enginepushes exhaust

gasses out the rear

reaction: rocketgets pushed

in the oppositedirection