Newton’s 1st Law

Inertia

Force

Anything capable of changing an object’s state of motion

Any push or pull Causes object to speed up, slow down, or

change direction (acceleration)

Galileo’s Experiment

Inertia

Galileo reasoned without friction ball would keep going

Also reasoned that ball initially at rest would stay that way unless something caused it to move

Inertia - what Galileo called the tendency of an object to maintain its initial state

Law of Inertia

Object at rest will remain at rest or object in motion will remain in uniform motion in a straight line

Newton

Related mass to inertia More massive objects resist change more Newton’s 1st Law – in the absence of an

unbalanced force, a body at rest remains at rest and a body in motion with a constant velocity

Force(s) acting must be external and must not total zero

Summary

Newton’s 2nd Law

Free Body Diagram

Used to show all net forces acting on an object What can an object with a net force of zero be

doing? Draw the free body diagram for a box being

acted on by all the following forces: 5N (W), 3N(S), 8N (N), and 7N(E)

Can it be at rest? Can it be at a constant speed? What is it doing?

Force on Object

Objects acted on by a net unbalanced force will accelerate in the direction of the force

This means they will speed up, slow down, or change direction

Newton’s 2nd Law

The acceleration of an object is directly proportional to the net force acting on it and inversely proportional to the mass.

The direction of acceleration is the same as the direction of the force

a = ∑F / m

∑F = ma

Force Redefined

Force can be redefined as an action capable of accelerating an object

Unit of force is the Newton (N) One Newton is the force needed to

accelerate a 1 kg mass at 1 m/s2

1 N = 1 kg m/s2

Examples

What force is needed to accelerate a 1000kg car at ½ g?

What is the net force required to bring a 1500 kg car to rest from 100 km/hr within a distance of 55 m?

A 50 kg go-cart starts from rest and is accelerated by a 550 N force. If the cart experiences a 280 N friction force and 35 N of air resistance, find

a) it’s acceleration

b) it’s velocity after 3.0 s

c) its displacement after 3.0 s

What force must be exerted on a 500 kg crate in order to move it from rest to 3.0 m/s in 1.0 s?