1. Chapter 12: Forces and Motion
Section 12.1
Forces

2. Forces A push or pull that acts on an object
Can cause an object to start moving or it can change an object’s acceleration
Measured in Newtons (N)
Force is a vector ~ shows direction and magnitude
Net force is the overall force acting on an object after all forces have been combined.

3. Balanced and Unbalanced Forces
Balanced forces combine to form a net force of 0.
There is no change in the object’s motion.
Unbalanced forces combine to form a net force not equal to 0.
Unbalanced forces cause objects to accelerate.

4. Friction
A force that opposes the motion of objects that touch each other
Friction happens at the surface of objects (where they touch)
4 main types of friction

5. Static Friction Acts on objects that are not moving
Always acts in the direction that is opposite of an applied force
Keeps an object from being moved across a surface
Stops acting on an object once the object is moving

6. Sliding Friction Acts on an object once the object is moving
Goes against the direction in which the object is moving as it slides on a surface
Occurs when two objects rub together
Less of a force than static friction

7. Rolling Friction Force that acts on rolling (round) objects
Acts on the surface and at the point where the object is touching something else (the ground for example)
Allows the object to touch the ground and not slip
Slows rolling objects down

8. Fluid Friction
Force that opposes an object’s motion through a fluid (liquid or gas)
Force increases as the object’s speed through the fluid increases
When fluid friction acts on objects moving through the air, it is called air resistance.

9. Gravity Downward force that acts between any two masses
An attractive force ~ pulls objects together
Acts over large distances (unlike friction)

10. Falling objects Forces acting on a falling object:
Gravity (pulling downward)
Air resistance (acts in opposition to gravity, reduces acceleration)
Terminal velocity is reached when the force of air resistance equals the gravitational force.
The object is no longer accelerating.
V= a x t speed of a falling object

11. Projectile Motion
The motion of a falling object (called the projectile) after it has been given a forward velocity (example: throwing something forward)
Air resistance and gravity act on the object.
Because of the forward velocity and the downward gravitational force, the object will follow a curved path.