Chapter 3 Notes Forces and Motion

Newton’s 3 Laws of Motion 1. An object at rest will stay at rest unless acted on by an unbalanced force. The law of inertia. 2. F = m x a 3. For every action, there is an equal and opposite reaction.

Force:  Things tend to keep doing what they are doing until something comes along to change it.  A force has the ability to cause a change. It does not mean a force MUST cause motion just that it can.  2 units used for force. English system-pounds (lb), metric system-newtons (N) 4.48 lb = 1N

Mass and Force  1N = 1 kg  m/s 2  Force (weight) and mass have different units. Mass is in grams (g) or kilograms (kg),  Mass is the amount of “stuff” (matter) in the object.  On Earth 2.2 lb = 1 kg. Mass does not depend on gravity. The mass of a person is the same anywhere. Weight is different depending on the location of the object. It does depend on gravity.

Mass and Inertia Inertia-when an object resists the change in motion. It wants to keep doing what it is doing. Depends on mass. The more mass = the more inertia. Think of which is harder to start moving? The only way to change mass is to physically change the object. Ex: cutting off your arm

Mass and Inertia F = m x a F = Force (N), m = mass (kg), a = acceleration (m/s 2 ) Forces cause acceleration.

Balanced and Unbalanced Forces The motion of an object depends on the total of all forces on the object- net force One direction is + and the opposite is – Balanced Force-net force = zero, no change in motion. Unbalanced force-net force is NOT zero.

Gravity All object have gravity between them. You feel the pull toward the center of the Earth because it is the largest. It depends on how much mass the objects have. The more mass = the more gravity. Gravity also depends on how close together the 2 objects are.

This shows a feather and an apple falling to Earth in a non-air chamber. Both fall at the same rate.

Mass and Weight Weight changes depending on your location. Your acceleration toward the Earth is 9.8 m/s2. Weight is a force so we can plug it into Newton’s 2 nd law. F w = m x a Where the a = 9.8 m/s 2.

Newton’s 3 rd Law Deals with a pair of objects. All forces come in pairs. Action/reaction forces act on 2 different objects, NOT on the same object.

Friction Friction always works AGAINST motion. (In the opposite direction.) Friction comes from 2 surfaces moving against each other. Air friction-also known as air resistance, air moving around an object. Viscous friction-objects in water or other fluid create this. Sliding friction-2 surfaces rub together. Rolling friction-one object rolls over another.

Friction and motion A car rolling down a hill has gravity pulling it down and friction pushing back toward the top of the hill. All true (real) machine have friction and eventually everything will stop. Gravity Friction

If you are on a skateboard and throw a ball forward, you will go backward. The heavier the ball or the faster you throw it, the quicker you will go backward. P = m x v, P = momentum (kg  m/s), m = mass (kg), v = velocity (m/s) Law of Conservation of Momentum- momentum cannot be created or destroyed in a group of objects NOT effected by an outside force. Momentum usually has + and – to indicate direction. This is how a jet moves. Air is pushed back through the engine and it propels the plane forward.

Air in Air Out Jet moves forward