Forces  A force is a PUSH or a PULL.  Described by: 1. Its strength 2. The direction in which it acts  Measured in: Newtons (N)  Measured by: Spring Scale  Represented by: An arrow (direction)

Forces Acting in the Same Direction Unbalanced Motion Occurs 5N We can add two forces together to produce a larger net force. 5N + 5N = 10 N

Forces Acting in the Opposite Direction With Motion Unbalanced Motion Occurs 5N Forces can subtract to produce net force in the direction of the larger force. 5N + 5N 5N + 5N +5N + 5N = 5 N

Forces Acting in Opposite Direction Without Motion No Motion Occurs 5N Forces may cancel each other and produce no net force. 5N + 5N = 0 N

Friction – A Force Friction is the force two surfaces exert on each other when they rub against each other. The strength of the force of friction depends on two things: 1. How hard the surfaces push together. 2. The types of surfaces involved.

Four Types of Friction  The friction that acts on objects that are not moving.  To move a stationary object, you must overcome static friction. Static Friction

Four Types of Friction  The friction that occurs when two solid surfaces slide over one another.  Examples: Brakes on a bicycle Sliding on ice  Less friction than static friction Sliding Friction

Four Types of Friction  The friction between something that rolls and the surface it is rolling on.  Examples: Roller skates Ball bearings  Less friction than static or sliding friction Rolling Friction

Four Types of Friction  The friction that occurs when a solid object moves through a fluid. (water, air, oil, etc)  Examples: Surfing When objects rub against air Meteors in the atmosphere  Least of all 4 types of friction Fluid Friction

Gravity  Newton concluded that a force acts to pull objects straight down toward the center of the Earth.  Gravity – a force that pulls objects together  Gravity acts everywhere, not just on Earth  Gravity = G where G = (9.8 m/s 2 ) (mass of object)

Two Factors That Affect Gravitational Pull  Two factors that affect the gravitational attraction between objects Mass Distance ○ Mass, Force (more mass, more attraction) ○ Distance, Force (more distance, less attraction)

Law of Universal Gravitation  The force of gravity acts between all objects in the universe.

Weight vs. Mass  Weight changes if gravitational pull changes.  Mass does not depend on gravitational pull; it stays the same regardless of location.  Weight – a force of gravity on a person or object at the surface of a planet.  Mass – the measure of the amount of matter in an object. What’s the difference?

Why does an object move? Why does an object stop moving?  An object in motion will remain in motion or an object at rest will remain at rest unless acted upon by an outside force.  Inertia – the tendency of an object to resist change in motion  Newton’s First Law is sometimes called the law of inertia.  Ex. Earth going around the Sun Newton’s 1 st Law

What happens to the acceleration of an object if I increase its mass? What happens to the acceleration if I increase the force applies to the object?  The amount of acceleration is dependent upon the mass of the object and the amount of force applied to the object.  Acceleration = net force/mass  F = ma where F = force, m = mass, a = acceleration  Increasing mass decreases acceleration  Increasing force increases acceleration Newton’s 2 nd Law

What happens when two objects collide?  For every action, there is an equal and opposite reaction. Newton’s 3 rd Law

Free-body Diagram Four Forces: Gravity; Force pushing against gravity (Newton’s 3 rd Law); Friction; Applied Force What is the net force? Calculate the acceleration using that force.