Newton’s Laws Inertia

What is a Force?  The idea the forces caused motion was professed by Aristotle in the 4 th century B.C.  Force – a push or pull that one body applies to another  A force is a vector, it has magnitude and direction

What is a Force?  A force can cause an object’s motion to change  When two or more forces combine at the same time, they create a net force

Net Forces  Multiple forces create a net force – that means they act as a single force  Balanced forces are equal is size but opposite in direction  Balanced forces act as if there is no force

Example of Balanced Forces

Example of Unbalanced Forces

Inertia  Galileo professed the first modern ideas about motion  The inclined plane experiment

Inertia  Inertia – an object’s resistance to any change in motion  All objects have inertia  Objects with a greater mass have a greater inertia

Newton’s 1 st Law Newton built on Galileo’s ideas by stating: An object moving with a constant velocity keeps moving with that velocity unless a net force acts on it; an object at rest will stay at rest unless a net force acts on it.

Mass  Inertia depends on mass  Mass is the amount of matter an object contains  Newton’s laws are based on mass

Weight  Weight is a force that results from gravity  Weight is not the same as mass  Weight equals mass times acceleration - mg

SI and English, a Comparison SIEnglish KilogramsSlugs SIEnglish KilogramsSlugs NewtonsPounds

Newton’s Laws F=ma

Forces and Motion are Connected  An object will have greater acceleration if a greater force is applied to it  Which will accelerate faster, a car with one person pushing it or the same car with eight people pushing it?

Forces and Motion are Connected  The mass of an object also affects acceleration  Which will accelerate faster, a car with two people pushing it or the same two people pushing a fully loaded gravel truck?

Newton’s Second Law of Motion  Connects force mass and acceleration in the equation acceleration equals net force divided by mass: A = F/m F = ma

Quick Review  1 st Law: Things at rest stay at rest, things in motion continue to move with the same velocity unless acted on by a net force  2 nd Law: F = ma

Summary Things will speed up, change direction or stop only if acted on by a net force and the rate of change of the object’s motion is governed by the size of the net force and the amount of inertia the object has

Newton’s Laws Friction

Question: Why does my car slow down when I take my foot off of the gas? Answer: Friction

Friction  Friction is a force that opposes motion between two surfaces that are touching  Friction changes the energy of motion into heat energy – rub your hands together and feel the heat!

Types of Friction  Static – the friction between two surfaces that are in contact but not moving  Sliding – The friction that opposes the motion of two surfaces that are in contact and sliding past each other

Types of Friction  Rolling – friction between an object rolling and the surface it is rolling on  Fluid Friction - the resistance of a gas or liquid as an object passes through it

Calculating Friction  Static and kinetic friction depend on the surfaces in contact and the force between the surfaces Ff=μFnFf=μFn

More Impediments to Motion  Air resistance – the opposition to something moving through the air  Air resistance depends on shape, size, and speed. A good portion of the gas your car burns is to overcome air resistance

Extreme Air Resistance  Terminal Velocity – an objects weight matches the air resistance

Newton’s Laws ActionReaction

Newton’s Third Law of Motion  To every action force, there is an equal and opposite reaction force  Action – reaction forces act on different objects and differ from balanced forces

Examples of Action Reaction Pairs  Rocket propulsion is based on Newton’s third law  When I throw a ball, my hand pushes against the ball and accelerates the ball. The ball actually pushes back just as hard on my hand

Simple but Tricky  When I jump up, my feet push against the earth. Since the earth is so massive, it has a lot of inertia and doesn't move. However, the earth pushes back just as hard and throws me into the air.