12.1 Forces Objectives: M 4-1 Describe examples of force and identify appropriate SI units used to measure force. 10 M 4-2 Explain how the motion of an object is affected when balanced and unbalanced forces act on it. 7 M 4-3 Compare and contrast the four kinds of friction. 15 M 4-4 Describe how gravity and air resistance affect falling objects. 8

FORCES FORCE: a push or pull that acts on an object Measured in Newtons (N) = a vector: shows direction and magnitude Net force: is the overall force acting on an object after all forces have been combined. Desk is pushing up on books right now. They are pushing down on seats and seats are pushing up on them. Balanced forces = no motion, unbalanced forces= motion. Types of forces: friction, nuclear force, magnetic force, electromagnetic force, etc. Identifying forces in different scenarios.

Balanced and Unbalanced Forces Balanced forces: a net force of 0. *No Motion Unbalanced forces: net force not equal to 0. * Causes motion

Examples of Forces Electromagnetic Forces (between magnets, protons/electrons) Nuclear Forces (strong force) (holds the nucleus of an atom together, nuclear energy, atomic bomb). Centripetal Forces (changes the direction of an object and causes it to move in a circle) Friction Gravity

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: Static, Sliding, Rolling, and Fluid.

Friction between two surfaces

Static Friction Acts on stationary objects. 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 Ex. Moving a piece of furniture on carpet.

Static Friction

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

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

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.

Gravity Force of attraction that acts between any two masses. Pulls objects together The magnitude of the force is proportional to the mass of the objects. Ex. Gravitational pull between sun and earth Gravitational pull between satellites and the earth.

Gravity on earth Pulls objects towards the earth at a rate of 9.8 m/s2 Gravity acts equally on ALL objects, pulling them at the same rate towards the earth. Objects fall towards the earth at different rates due to air resistance (fluid friction).

Falling objects TWO Forces acting on a falling object: 1. Gravity (pulling downward) 2. Air resistance (acts in opposition to gravity, reduces acceleration) Terminal velocity: when air resistance = force of gravity The object is no longer accelerating. V= a x t speed of a falling object Myth or fact: if you throw a penny from the empire state building, it will have the potential to kill someone below if it hits them.

Puzzle… Myth or fact: If you drop two basketballs with the same shape but different masses (2 kg and 20kg), they will hit the ground at the same time. True or False, and why?