III. Defining Force Force Newton’s First Law Friction Forces & Motion III. Defining Force Force Newton’s First Law Friction

A. Force Force a push or pull that one body exerts on another What forces are being exerted on the football? Fkick Fgrav

*All Push or Pull on an Object Types of Forces Magnetic Gravitational Centrifugal Friction Electro-static Elastic Chemical *All Push or Pull on an Object

A. Force Balanced Forces forces acting on an object that are opposite in direction and equal in size no change in velocity

A. Force Net Force unbalanced forces that are not opposite and equal velocity changes (object accelerates) Fnet Ffriction Fpull N W

B. Newton’s First Law Newton’s First Law of Motion An object at rest will remain at rest and an object in motion will continue moving at a constant velocity unless acted upon by a net force.

B. Newton’s First Law Newton’s First Law of Motion “Law of Inertia” tendency of an object to resist any change in its motion increases as mass increases

1st Law Once airborne, unless acted on by an outside (net) force this golf ball would never stop!

1st Law Or…. unless acted upon by an outside force, this golf ball would sit on the tee forever.

It’s a force we sometimes cannot see – friction. Why then, do we observe every day objects in motion slowing down and becoming motionless seemingly without an outside force? It’s a force we sometimes cannot see – friction.

Friction! There are four main types of friction: What is this unbalanced force that acts on an object in motion? Friction! There are four main types of friction: Sliding friction: ice skating Rolling friction: bowling Fluid friction (air or liquid): air or water resistance Static friction: initial friction when moving an object

C. Friction Friction force that opposes motion between 2 surfaces depends on the: types of surfaces force between the surfaces

C. Friction Friction is greater... between rough surfaces when there’s a greater force between the surfaces (e.g. more weight) Pros and Cons?

Objects on earth, unlike the frictionless space the moon travels through, are under the influence of friction.

The Unit of Force The unit of force is the Newton One Newton = 1 kg-meter/sec2 It is the force required to accelerate a 1 kg mass 1 meter/s every second *More on this when we go over Newton’s 2nd law: F = ma

Taken from “The Physics Classroom” © Tom Henderson, 1996-2001. ConcepTest 1 TRUE or FALSE? The object shown in the diagram must be at rest since there is no net force acting on it. True! If there is no net force There is no change in motion. A net force causes a change in motion, or acceleration. Taken from “The Physics Classroom” © Tom Henderson, 1996-2001.

ConcepTest 2 You are a passenger in a car and not wearing your seat belt. Without increasing or decreasing its speed, the car makes a sharp left turn, and you find yourself colliding with the right-hand door. Which is the correct analysis of the situation? ...

ConcepTest 2 1. Before and after the collision, there is a rightward force pushing you into the door. 2. Starting at the time of collision, the door exerts a leftward force on you. 3. both of the above 4. neither of the above 2. Starting at the time of collision, the door exerts a leftward force on you.

Newtons’s 1st Law and You Don’t let this be you. Wear seat belts. Because of inertia, objects (including you) resist changes in their motion. When the car going 80 km/hour is stopped by the brick wall, your body keeps moving at 80 m/hour.