Unit 01 “Forces and the Laws of Motion” Introduction to Forces

Newton’s 3 Laws of Motion 1st Law: Inertia 2nd Law: F=ma 3rd Law: Action/Reaction

Newton’s 1st Law of Motion “An object at rest will stay at rest and An object in constant velocity motion will stay in constant velocity motion Unless acted on by an unbalanced force.” Demonstrations Tablecloth Trick Car Accidents Skateboarder 1st Law can also be written as: An object in equilibrium will stay in equilibrium unless acted on by an outside force. Or More mass, more inertia: Harder to change an object’s motion. Less mass, less inertia: Easier to change an object’s motion.

Skateboarding In object in constant velocity motion will stay in constant velocity motion unless acted on by an outside force! Before, Donald and the skateboard are in motion. After, the skateboard has a force on it and stops. Donald has no force and he continues moving.

Skateboarding Before, Donald and the skateboard are in motion. After, Donald has a force on him and stops. The skateboard has no force and it continues moving.

Answer: Before: What was moving. After: What stopped Answer: Before: What was moving? After: What stopped? What continued moving? Why? What had a force on it? What didn’t have a force on it? What Law # is it?

Example: Tablecloth Trick

Newton’s 2nd Law: F=ma “Force is equal to the mass of an object multiplied by its acceleration.” Force = mass x acceleration Demonstrations Skateboarders Basketball and Tennis Ball Earth and People 2nd Law can also be written as: The acceleration of an object is directly proportional to the force on the object and inversely proportional to the object’s mass.

F a F a m a m a Basketball and Tennis Ball Trick The basketball F and a are directly proportional m and a are indirectly proportional F a F a m a m a Basketball and Tennis Ball Trick The basketball has more mass, so it accelerates less. The tennis ball has less mass, so it accelerates more.

Example: Two Ice Skaters

A cool calculation… When you fall to the ground, you pull the Earth with an equal and opposite force as Earth pulls you with! But we don’t see Earth move! This is because Earth is so massive its acceleration is too small to see! The Earth does move, but we can’t feel it! Question: What if EVERY person on the Earth jump up and hit the Earth at the same time in the same place … would the Earth move enough to notice

A cool calculation… F = ma Mass = 70kg * 7,000,000,000 = 490,000,000,000kg Acceleration = -9.8m/s2 FEarth on People = ? FPeople on Earth= Mass = 6,000,000,000,000,000,000,000,000kg Acceleration = ? F = ma

Newton’s 3rd Law: Action/Reaction “For every force applied to an object, the object applies an equal and opposite force back.” Demonstrations Rollerblader Force sensors Punch you teacher 3rd Law can also be written as: For every action there is an equal and opposite reaction.

Action-Reaction Pairs I push the chair The wind (air molecules) push the flag. The Earth pulls a ball. The chair pushes me The flag pushes the air molecules The ball pulls the Earth. With equal and opposite force!!!!

Equal and Opposite Forces Action: Your hand push Ms Bucci’s Face. Reaction: Ms Bucci’s face pushes your hand. If you punch Ms Bucci’s face, her face hits your hand back with an equal and opposite force. When you get in a fight, it is easy to get the “last hit” as soon as they hit you, you hit them back, according to Newton’s 3rd Law!

Equal and Opposite Forces The BIG truck and the SMALL truck hit each other with equal and opposite forces. Action: The big truck hits the small truck. Reaction: The small truck hits the big truck… with an equal and opposite force!

Equal and Opposite Forces Action: The bug hits the windshield. Reaction: The windshield hits the bug. The bug hits the windshield with the SAME force that the windshield hits the bug with!

Example: Pushing a Table Why is it wrong to say “I pushed myself off the table”?