Newton’s Laws of Motion Lecture

1. Three laws of motion: Isaac Newton developed these to describe how motion behaves A. First law: an object at rest will remain at rest, an object in motion will stay in motion… until an outside forces acts to change it (thus a net force must occur: unbalanced forces: demo?) 1. also known as the law of inertia (?) 2. inertia: to resist a change in motion: to be lazy (either to a start or a stop of motion) 3. the greater the mass, the greater the inertia: -push a Prius or Hummer? -stand in front of a flying golf ball or paper airplane? 4. friction acts as an outside force: if no friction then object will not stop: what environment like this already exists ? 5. examples: coin/card, -thanksgiving dinner -jumping in a bus, -rocket in space

2. Second law: the net force of an object is due to its mass and how much it is accelerating A. Force = Mass x Acceleration F = M x A (mass in kg, accel in m/s 2 ) (FeMA) A FALLING OBJECT: Acceleration (m/s 2 ) Force (N) 1kg What is the formula for this graph (rise/run)? Where would a 2kg line go? -the heavier the object requires more what to start or stop?

Newton’s first two laws of motion show the relationship between force and motion. -The First Law explains what happens when there is NO net force on an object: -what is the law? -The Second Law explains what happens when there IS a net force: -what is the law? -thus the two laws are closely related

3. Third law: -every action has an equal and opposite reaction -thus, forces come in pairs: acting on TWO DIFFERENT objects: -the 1) action object and the 2) reaction object 1. Newton’s cradle: demo 2. walking: push against floor and the floor pushes you 3. bouncing ball: force down creates a force up 4. rocket: gases forced out creates a thrust into the rocket

4. Momentum A. describes how much MOTION (not force) an object has (the inertia in motion, what’s inertia?) B. it is the inertia (mass) of motion (velocity) C. can be transferred to another object: pool balls, dominoes D. momentum = mass x velocity P = M x V units are kg x m/s ie: a bowling ball vs golf ball traveling at the same velocity, which has a greater motion/momentum? 5. remember: the only way an object can START motion is for the forces acting on it are unbalanced. The opposite is also true (?)

Collisions When a large truck hits a small car, the collision forces are equal, but… The small mass car experiences a much greater change in velocity much more rapidly than the big truck. So… Wear your seat belt!! Which vehicle ends up with more damage?

Momentum The law of conservation of momentum states as long as the interacting objects are not influenced by outside forces (like friction) the total amount of momentum is conserved: does not change: P 1 = P 2 m 1 v 1 = m 2 v 2

Momentum The result of a skateboarder throwing a 1-kg ball at a speed of 20 m/sec is that he and the skateboard with a total mass of 40 kg move backward at a speed of +0.5 m/sec (if you ignore friction). Here’s why….. left: m 1 v 1 = m 2 v 2 :right 1kg 20m/sec = 40kg v 2 v 2 = 0.5m/sec (NOTE: We use positive and negative velocity to show opposite directions, not value)

Solving Problems If an astronaut in space were to release a 2 kilogram wrench at a speed of 10 m/s, the astronaut would move backward at what speed if the astronaut’s mass is 100 kilograms? SHOW YOUR WORK: know, ?know, formula, cancel, circle answer

Known: velocity 1 = 10 m/s mass 1 = 2 kg mass 2 = 100 kg unknown: velocity 2 (of the astronaut backward) formula: (law of conservation of momentum) m 1 v 1 = m 2 v 2  m 1 v 1 = v 2 m 2 Work: (Draw and label the free body diagram) 2kg x 10m/s = v 2 100kg 0.2 m/s = v 2

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