 # Chapter 12: Forces Section 3.

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Chapter 12: Forces Section 3

Newton’s 1st law: Law of Inertia
An object at rest remains at rest and an object in motion stays in motion at constant speed unless an outside force acts on the object. Newton’s 2nd law: Law of acceleration The unbalanced force acting on an object equals the object’s mass times its acceleration. Newton’s 3rd Law: Law of Interactions For every action there is an equal, but opposite, reaction

Newton’s 3rd Law: Law of Interactions
Forces always occur in pairs! When a ball is dropped, it bounces when it comes into contact with the floor. What is the action? What is the reaction? What is the consequence?

Action and reaction forces, continued
The Action: Ball hits the floor. The Reaction: Floor hits the ball. The Consequence: Ball bounces back up. F = -F

Momentum = mass x velocity
Any object that has mass and is moving has momentum… Momentum = mass x velocity p = mv Measured in: kg m/s

What does that mean about the velocity of each?
Example Can a cruise ship and a bullet have the same momentum? Yes! Mass of the ship is MUCH GREATER than mass of the bullet What does that mean about the velocity of each? Equal momentum mass velocity Equal BIG small slow fast

How is momentum related to the newton’s laws?
To change an object’s momentum, a force has to be applied. Newton’s 2nd Law says that force is equal to an object’s mass times its acceleration. If there is acceleration or deceleration, by definition, velocity changes. If momentum is the product of mass and velocity, then momentum changes because velocity changes.

Law of Conservation of momentum
The total amount of momentum in an isolated system is conserved! Conserved means Accounted for Not lost or wasted Momentum can be transferred between objects by contact, but Momentum of all objects before contact Momentum of all objects after contact =

Collisions = contact Playing pool  the cue ball gives momentum to the others that are on the table The initial momentum on the cue ball comes from the person exerting a force using the stick. Car crash  the engine of the car is the force that gives the car momentum. If the car hits another car, whether stationary or moving, that car’s momentum changes. Two ice skaters push off of one another  one goes in one direction, one goes in the opposite direction. How much momentum did they have at the beginning?

How fast is the momentum change?
When you jump off the wall to the beach below at Narragansett Beach, why is it good to bend your knees when you land? Why do pitchers “follow through” on a pitch? Or in any sport for that matter? What are some safety measures that make car crashes safer?