Newton’s 3 laws of motion Isaac Newton Physics 100 Chapt 3

1 st 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)

2 nd Law: F=ma T he 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 force Small acceleration a Large force Large acceleration

inversely proportional to mass a a Large mass Small acceleration small mass Large acceleration Bowling ball Beach ball

“Inertial” mass “Inertial” mass, m i, is the resistance to changes in the state of motion Objects with large m i are hard to get moving (& once started, hard to stop), Objects with small m i 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 much water! 10cm

Net force Tip-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 1 st 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 2 nd 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/m combine: set proportionality constant = 1: a = F/m  multiply both sides by m

Units of Force F=ma kg ms2ms2 Unit of force: 1 Newton = 1N = 1 kg m/s 2 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= WgWg Suppose I jump off a tqble

Two different aspects of mass Weight: W = m g Force of gravity is proportional to “gravitational” mass a = FmFm Inertia; resistance to changes in state is proportional to “inertial” mass mimi mggmgg Newton’s 2 nd law: Experiment shows: m g = m i

My mass My weight in pounds: W = 190 lbs 855 N 10 m/s 2 !!!!! Convert to Newtons: W = 190 lbs x 4.5 N/lb = 855 N m = W/g = = 85.5 kg

Newton’3 rd 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 push on 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 me to accelerate backwards This force tries to accel. the bus forward

Air-filled balloon action: balloon pushes on air reaction: air pushes on balloon

recoil action: gun exerts force F 2 on bullet making it accelerate reaction: equal but opposite force on the gun F 1 Produces a recoil

Rocket propulsion action: rocket engine pushes exhaust gasses out the rear reaction: rocket gets pushed in the opposite direction