Newton’s Laws of Motion

A force is a “push or pull” on an object. It usually causes a change in in the state of motion of an object, sometimes causing a deformation (change in shape) instead.Forces have a magnitude and direction – they are vector quantities.Most common method of measuring force magnitude involves a spring scale.The units of force are kg·m/s2, otherwise known as the newton.

Force

Mass is measured using a balance, comparing the mass of an unknown object to that of a known object.The official SI unit for mass is the kilogram (2.2 lbs).

Common Conversions1 kilogram = 1000 grams1 kg = 1000 g

Mass

Recall that an object’s density (ρ) is defined as the ratio of its mass to its volume:

Mass is not the identical to volume, which is the amount of physical space an object takes up.

Mass vs. Volume

Au

Density of Gold: 19.3 g/cm3

Cu

Density of Copper: 8.96 g/cm3

As such it is written as Fg, or sometimes W, and like all forces, its SI unit is the kg·m/s2 (newton).

Weight

( )g mF Wg= =

The weight of an object at (or very near) the surface of the earth may be calculated as follows:

Weight is a measure of how strongly Earth’s gravity pulls on a mass – it is a force.

Mass vs. Weight

Mr. Almeida’s Instant Weight-Loss Diet†

On the Moon, the gravitational force is approximately 1/6th what it is on Earth. Objects therefore weigh 1/6th as much on the moon as they do on Earth.

Mass is a property of an object – it is immutable.Weight is a measure of the gravitational force on an object – it varies with the location of the object

† Sorry there is nothing I can do about your mass.

Net Force, or resultant force, or sum of forces, refers to the overall force acting on a mass.The effect of the single net force is equal to the combined effect of all the individual forces.

Fnet

WS

Fnet

WS

Collinear Forces

Fnet

WS

Non-Collinear Forces

Every body continues in its state of rest or uniform speed in a straight line, unless it is compelled to change that state

by a net force acting on it.

Newton’s First Law of Motion

WS

This tendency to maintain one’s state of motion (whether at rest or moving) is called inertia, which is another name for mass.

More mass means more inertia, or a greater resistance to change.

Hence, Newton’s First Law of Motion is commonly called “The Law of Inertia.”

Key words and phrases:• “at rest”• “constant (or uniform) speed”• “in a straight line”• “net force

Equilibrium refers to the balancing of the forces (Fnet = 0).

Static equilibrium – forces balanced, stationary object Dynamics equilibrium – forces balanced, moving object

A Visual Take on Newton’s First Law of Motion

Forces are Balanced

Object at Rest(v = 0 m/s)

Object in Motion(v ≠ 0 m/s)

Stay at Rest Stay in Motion(same speed, direction)

ifif

thenthen

net 0=F

“Equilibrium”See:

Think:

Example

Example

Equilibrium refers to the balancing of the forces.Two kinds of equilibrium: Static equilibrium – forces balanced, stationary object Dynamics equilibrium – forces balanced, moving object

Static / Dynamic Equilibrium

net 0=F“Equilibrium”See: Think:

The single most important thing you will memorize understand in this class all year:

Newton’s Second Law of Motion

A net Force applied to a mass causes that object to acceleration.

net m=F a

WS

A net force of 80 N causes the 16 kg boy to accelerate down the waterslide. What is his acceleration? If he starts from rest at the top of the slide and it takes him 3 seconds to reach the bottom, what is his speed at the bottom of the slide?

Example

Newton’s 2nd Law and Free-Fall

Newton’s Third Law of Motion“Whenever one object exerts a force on a second object, the second object exerts a force (equal in magnitude, in

the opposite direction) back on the first.”

Newton’s Third Law of Motion

Newton’s Third Law of Motion

Newton’s Third Law of Motion

Newton’s Third Law of Motion