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

http://www.newton.cam.ac.uk/newtlife.html

Newton’s Biography

Newton’s First LawThe 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:

http://video.google.com/videoplay?docid=3493831395462934634&q=inertia&hl=en

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:

InertiaForceNewtonNet ForceEquilibriumFrictionWeightNewton’s 1st LawNormal ForceSupport 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/s2. (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 1st 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 ForceCombining 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

8 N, RIGHT

16 N, RIGHTNet force =

Newton’s Second LawThe 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 kg m/s2=

A Newton is the unit of Force: it is a Derived unit –

Derived units are a combination of fundamental units

N = kg m/s2

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/s2)

So, W = 10 times mass orMass = W/10

Where weight is in Newtons (N) andMass is in kilograms (kg)

Find the weight:

200 kg

3 kg

700 g

4000 g

85 kg

9 kg

200kg x 10m/s2 = 2000N

3kg x 10m/s2 = 30N

0.7kg x 10m/s2 = 7N

4kg x 10m/s2 = 40N

85kg x 10m/s2 = 850N

9kg x 10m/s2 = 90N

Find the mass:

600 N

1000 N

45 N

9 N

W = mg or m = W/g

600N / 10m/s2 = 60 kg

1000N / 10m/s2 = 100 kg

45N / 10m/s2 = 4.5 kg

9N / 10m/s2 = 0.9 kg

What force is needed to accelerate a 30 kg wagon at a rate of 4 m/s2?

F = maF = 30kg x 4 m/s2

F = 120 kg m/s2

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/s2 = a

If a net force of 50 N produces an acceleration of 5 m/s2 on a cart, what is the mass of the cart?

F = ma 50N = m (5 m/s2)50N/ 5 m/s2 = 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?

200N 300N50 kg

F net = 300 – 200 = 100 N

F = ma100 N = 50 kg x a100 N/50 kg = a2 m/s2 = a