Report

Chapter 2 – Lesson 4 Newton’s Third Law of Motion When one object exerts a force on a second object, the second object exerts an equal force in the opposite direction on the first object. ANOTHER WAY TO SAY IT…. For every action there is an equal and opposite reaction. Think about it… How is the gymnast able to flip over? When the gymnast PUSHES the vault, the vault PUSHES BACK against the gymnast. The LENGTH of the force arrows are the SAME but the direction is OPPOSITE Force Pair The forces two objects apply to each other. Force pairs will always act in OPPOSITE DIRECTIONS The girls feet act on THE BOAT The boat acts on THE GIRLS FEET Do Action-Reaction Forces Cancel? If 2 equal act in opposite directions they are balanced and cancel each other out. There is no movement Action – Reaction forces DO NOT cancel out because they are acting on 2 DIFFERENT OBJECTS FORCE PAIR If one of the pairs is much more MASSIVE you will only see the LESS MASSIVE object MOVE Example – When you push down on the Earth, you won’t see the Earth move, only you jumping in the air ACTION AND REACTION In a FORCE PAIRone force is called the ACTION FORCE and the other force is called the REACTION FORCE For every ACTION FORCE there is a REACTION FORCE that is EQUAL in strength, but OPPOSITE in direction. Action – Reaction forces don’t cancel because they ACT ON DIFFERENT OBJECTS How do Action-Reaction forces work when you are… JUMPING • When you jump, you push down on the ground. • The ground then pushes up on you. It is this upward force that pushes you into the air. How do Action-Reaction forces work when… A rocket launches *When the rocket fuel is ignited, a hot gas is produced. As the gas molecules collide with the inside engine walls, the walls exert a force that pushes them out of the bottom of the engine. • The action force is - THE DOWNWARD PUSH •The reaction force is - THE UPWARD PUSH ON THE ROCKET ENGINE BY GAS MOLECULES Why doesn’t the bowling ball move backward if the pin is pushing on it equally? The bowling ball has more MOMENTUM Momentum The measure of how hard it is to stop a moving object. Momentum = MASS x VELOCITY P= m x v Momentum is a vector quantity Units: kg Mass = m/s Velocity = kg * m/s Momentum = Momentum More MASS = more MOMENTUM More VELOCITY = more MOMENTUM More MOMENTUM = GREATER FORCE needed to STOP an object. Which object would have more momentum? Practice Problem 1 What is the momentum of a bird with a mass of 0.018 kg flying at 15 m/s? Practice Problem 2 A golf ball travels at 16m/s, while a baseball moves at 7 m/s. The mass of the golf ball is .045 kg and the mass of the baseball is 0.14 kg. Which has greater momentum? Law of Conservation of Momentum The total momentum of a group of objects stays the same unless outside forces act on the objects. The cue ball has MOMENTUM because it has mass and velocity. When it hits the other balls , the cue balls VELOCITY & MOMENTUM decrease. The other balls start moving (and have mass) which mean they now have MOMENTUM Stopping an object with momentum To stop an object we need to apply a force over a certain period of time. We call this impulse Impulse – the force acting on an object in a specific amount of time. IMPULSE = F x Force (F) = N Change in time ( t) = s Impulse (I) = N s t Using Newton’s 2nd Law to determine how hard it is to stop a moving object. Newton’s 2nd Law tells us that F=ma fv iv We could also look at this as: F m t And if we were to manipulate this formula by multiplying both sides by time it would end up looking like: F t m ( fv iv) or F t m v Impulse = change in momentum Why does an egg break or not break? An egg dropped on a tile floor breaks, but an egg dropped on a pillow does not. Why? FΔt= mΔv In both cases, m and Δv are the same. If Δt goes up, what happens to the force? Right! Force goes down. When dropped on a pillow, the egg starts to slow down as soon as it touches it. A pillow increases the time the egg takes to stops. Practice Problem A net force of 100 N is applied to a 20kg cart that is already moving at 3m/s. The final speed of the cart was 8m/s. For how long was the force applied? F t m v 100N t 20kg 5m /s 100N t 100kg m /s Identify the variables Mass = 20kg Δvelocity = 8m/s-3m/s = 5m/s Time = ? Force = 100N Practice Problem A .057 kg tennis ball falls on a tile floor. The ball changes velocity from -1.2 m/s to +1.2 m/s in 0.02 s. What is the average force on the ball? Identify the variables: Mass = 0.057 kg Δvelocity = +1.2 – (-1.2) = 2.4 m/s Time = 0.02 s using FΔt= mΔv F x (0.02 s) = (0.057 kg)(2.4 m/s) F= 6.8 N Collisions Objects collide in two different ways: 1. Elastic collision – When colliding objects bounce off each other 2. Inelastic collision – When objects collide and stick together. The amount of momentum involved before the collision will always be the same after the collision. Collisions with 2 moving objects When two objects are moving in the SAME DIRECTION and aCOLLISION occurs, the momentum of the slower object SPEEDS UP and the momentum of the faster object SLOWS DOWN BEFORE COLLISION 4 m/s AFTER COLLISION 2 m/s Collisions with 1 moving object When ONE object is moving and COLLIDES into a nonmoving object, all the MOMENTUM is TRANSFERRED to the NONMOVING object. BEFORE COLLISION 4 m/s AFTER COLLISION 0 m/s Collisions with connected objects When one object is moving and COLLIDES , but CONNECTS to a nonmoving object, the MOMENTUM gets evenly split between the TWO MOVING OBJECTS BEFORE COLLISION 4 m/s AFTER COLLISION 0 m/s