4-4 Action And Reaction

Action and Reaction Chapter 4, Section 4

Newton’s Third law of motion  Newton’s

Third Law of Motion states that if one object exerts a force on another object, then the second object exerts a force of equal strength in the opposite direction of the first object

Equal but Opposite  Although

the action and reaction forces will be equal and opposite, the same force acting on a greater mass results in a smaller acceleration (More massive objects move slower)

Action-Reaction in Action  Newton’s

3rd law in action-

 Walking  Birds

flying  Fish swimming  Do

Action-Reaction Forces cancel?

 Newton’s

3rd law refers to forces on two different

objects.  Action-reaction forces do NOT cancel: Action and reaction forces cannot be added together because they are acting on different objects

Momentum  Momentum

is the “quantity of motion”; the product of an object’s mass and its velocity  Momentum = mass x velocity  Momentum is (kg x m/s)  Momentum is in the same direction as its velocity  More momentum an object has, harder it is to stop  Momentum is influenced by mass and velocity

Conservation of Momentum  Law

of conservation of momentum states that the total momentum of the objects that interact does not change; the quantity of the total momentum is the same before and after they interact  The total momentum of any group of objects remains the same unless outside forces act on the object  Friction is an outside force

Two Moving Objects  Two

trains traveling in the same direction; Train X is traveling 10 m/s and Train Y is traveling 5 m/s in front of X  When train Y hits X, it slows to 5 m/s and X speeds to 10 m/s to conserve momentum

One Moving Object  Car

X moves down the road at 10 m/s and hits car Y, which is not moving  Car X no longer moves after the collision, but car Y does.  Momentum has been transferred from car X to car Y and conserved

Two connected objects  Two

trains are coupled together when they collide, yet momentum is still conserved  Velocity of first train is 10 m/s  Total momentum before the collision is 30,000 kg x m/s (both cars together)  After the collision, the train cars make an object with a total of 60,000 kg x m/s  Velocity is 5 m/s after collision; momentum is conserved  (60,000 kg x 10 m/s) + 0 = 120,000 kg x 5 m/s