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 the vault, the vault against the gymnast. The of the force arrows are the but the direction is PUSHES PUSHES BACK LENGTH SAME OPPOSITE

Force Pair The forces two objects apply to each other. Force pairs will always act in The girls feet act on The boat acts on OPPOSITE DIRECTIONS THE BOAT 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 cancel out because they are acting on DO NOT 2 DIFFERENT OBJECTS

FORCE PAIR If one of the pairs is much more you will only see the object Example – When you push down on the Earth, you won’t see the Earth move, only you jumping in the air MASSIVE LESS MASSIVE MOVE

ACTION AND REACTION FORCE PAIR called the ACTION FORCE In a one force is called the and the other force is called the For every there is a that is in strength, but in direction. Action – Reaction forces don’t cancel because they FORCE PAIR ACTION FORCE REACTION FORCE ACTION FORCE REACTION FORCE EQUAL OPPOSITE 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 = x P = x Momentum is a vector quantity Units: Mass = Velocity = Momentum = MASS VELOCITY m v kg m/s kg * m/s

Momentum More = more More = more More = needed to an object. Which object would have more momentum? MASS MOMENTUM VELOCITY MOMENTUM MOMENTUM GREATER FORCE STOP

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 because it has mass and velocity. When it hits the other balls , the cue balls decrease. The other balls start moving (and have mass) which mean they now have MOMENTUM VELOCITY & MOMENTUM 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 t Force (F) = N Change in time ( t) = s Impulse (I) = N s

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 We could also look at this as: And if we were to manipulate this formula by multiplying both sides by time it would end up looking like: 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? 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: Elastic collision – Inelastic collision – The amount of momentum involved before the collision will When colliding objects bounce off each other When objects collide and stick together. always be the same after the collision.

Collisions with 2 moving objects When two objects are moving in the and a occurs, the momentum of the slower object and the momentum of the faster object SAME DIRECTION COLLISION SPEEDS UP SLOWS DOWN BEFORE COLLISION 4 m/s 2 m/s AFTER COLLISION

Collisions with 1 moving object When object is moving and into a nonmoving object, all the is to the object. ONE COLLIDES MOMENTUM TRANSFERRED NONMOVING BEFORE COLLISION 4 m/s 0 m/s AFTER COLLISION

Collisions with connected objects When one object is moving and , but to a nonmoving object, the gets evenly split between the COLLIDES CONNECTS MOMENTUM TWO MOVING OBJECTS BEFORE COLLISION 4 m/s 0 m/s AFTER COLLISION