Newton’s 3 Laws

To explain all nature is too difficult a task for any one man or even for any one age. `Tis much better to do a little with certainty, and leave the rest for others that come after you, than to explain all things. Quoted in G Simmons Calculus Gems (New York 1992). Quotes by Newton

[His epitaph:] Who, by vigor of mind almost divine, the motions and figures of the planets, the paths of comets, and the tides of the seas first demonstrated. If I have been able to see further, it was only because I stood on the shoulders of giants. Letter to Robert Hooke

I know not what I appear to the world, but to myself I seem to have been only like a boy playing on the sea-shore, and diverting myself in now and then finding a smoother pebble or a prettier shell, whilest the great ocean of truth lay all undiscovered before me. Quoted in D Brewster, Memoirs of Newton Numero pondere et mensura Deus omnia condidit

Newton stats: Born: 4 Jan 1643 in Woolsthorpe Lincolnshire, England Died: 31 March 1727 London, England

Newton’s Biography

Newton’s First Law The Law of Inertia

The Law of Inertia : A body at rest will stay at rest and a body in motion will stay in motion, moving in a straight line path at constant speed unless a net force acts on it.

In other words, a body will keep doing what it is already doing.

A net force is the sum of all the forces acting. It is also known as a resultant or resultant force.

All objects have forces acting on them. When the forces balance out (= zero) we say that a state of equilibrium exists.

If there is equilibrium, an object will not change its state of motion.

Let’s look at some inertia demonstrations:

The amount of inertia is related to mass. You might say that mass is a measure of an object’s inertia..

Please define the following: Inertia Force Newton Net Force Equilibrium Friction Weight Newton’s 1 st Law Normal Force Support force

Inertia – tendency of an object to continue in its original state of motion Force – a push or pull Newton – the SI unit of force. Force needed to give a mass of 1 kg an acceleration of 1 m/s 2. (symbol – N) Net Force – the sum of all the forces acting on an object. Equilibrium – when all the forces acting on an object are balanced. Net force = 0, so acceleration = 0.

Friction – force that occurs when two objects are in contact. It opposes motion. Is a dissipative force. Weight – the force of gravity acting on an object. Newton’s 1 st Law – an object will move in a straight path with constant speed or remain at rest unless a net (unbalanced) force acts on it. Normal Force – force that an object exerts perpendicular to a surface. Support force – the force that balances an object at rest.

Net Force Combining Forces

Force is a vector quantity, which means that it has a magnitude and direction. F

All vectors combine (can be added together) according to vector math. Parallel vectors in same direction - ADD Parallel vectors in opposite directions - SUBTRACT

8 N, RIGHT8 N, LEFT 0 NNet force =

8 N, RIGHT 16 N, RIGHTNet force =

Newton’s Second Law The Law of Acceleration

A body will accelerate in direct proportion to the net force applied and inversely proportional to its mass. (The acceleration will be in the direction of the net force.) F = m a N kgm/s 2 =

A Newton is the unit of Force: it is a Derived unit – Derived units are a combination of fundamental units N = kg m/s 2

A special case of F = ma : The force of gravity acting on a body is weight (W). W= mg where ““g” is the acceleration due to gravity (10 m/s 2 ) So, W = 10 times mass or Mass = W/10 Where weight is in Newtons (N) and Mass is in kilograms (kg)

Find the weight: 200 kg 3 kg 700 g 4000 g 85 kg 9 kg 200kg x 10m/s 2 = 2000N 3kg x 10m/s 2 = 30N 0.7kg x 10m/s 2 = 7N 4kg x 10m/s 2 = 40N 85kg x 10m/s 2 = 850N 9kg x 10m/s 2 = 90N

Find the mass: 600 N 1000 N 45 N 9 N W = mg or m = W/g 600N / 10m/s 2 = 60 kg 1000N / 10m/s 2 = 100 kg 45N / 10m/s 2 = 4.5 kg 9N / 10m/s 2 = 0.9 kg

What force is needed to accelerate a 30 kg wagon at a rate of 4 m/s 2 ? F = ma F = 30kg x 4 m/s 2 F = 120 kg m/s 2 F = 120 N

What acceleration is produced when Pete, 300 kg, is pushed with a constant force of 60N? F = ma 60N = 300kg (a) 60N/300kg = a 0.2 m/s 2 = a

If a net force of 50 N produces an acceleration of 5 m/s 2 on a cart, what is the mass of the cart? F = ma 50N = m (5 m/s 2 ) 50N/ 5 m/s 2 = m 10 kg = m

Ezekiel pushes his 50 kg recliner closer to his tiny flat screen TV. Friction from the rug opposes Ezekiel’s attempt until he pushes with a force of 200 N, when the recliner starts to slide slowly across the rug. Ezekiel stops to take a breath and then decides to push harder. If Ezekiel now pushes with a force of 300 N, at what rate will the recliner accelerate? 200N300N 50 kg F net = 300 – 200 = 100 N F = ma 100 N = 50 kg x a 100 N/50 kg = a 2 m/s 2 = a