Chapter 6 Newton’s 3rd Law For every action, there is an equal and opposite reaction If you push something, something must push back Forces are not a thing within itself, but rather all forces happen as a result of interactions between objects There is never a single isolated force in nature. If you look closely, there is always an oppositely-directed counterpart

Newton’s 3 rd Law Does a stick of Dynamite contain a force? A) Yes B) No This stick of dynamite does not contain a force, but it does contain energy, And that energy can be used to create a force remember the definition of Force is a push or pull on an object

Newton’s 3 rd Law A car accelerated along a road, what is the force that moves the car forward? A)The engine B)The road C)The tires D)The gasoline What direction is the tire pushing the road? If the tire pushes back on the road, the road must push on the tire.

Newton’s 3 rd Law If a swimmer pushes the water backward, the water pushes the swimmer forward. Swimming slow Swimming fast

Newton’s 3 rd Law Friction was the main factor in the past couple of examples. What happens when friction isn’t there. Think about walking on ice. There is much less friction, so when you try to push back on the ice to move forward, the ice pushes you forward equally. To move forward on skates, you increase By having your skates not straight

Newton’s 3 rd Law Identifying the action and reaction is not always obvious. A 500 kg boulder falls off the cliff and heads towards the ground. Is Earth pulling the boulder with a force? A) YesB) No

Newton’s 3 rd Law How much force is the Earth pulling on the 500 kg rock? A) 500 NB) 5,000 NC) 50,000 Force = Mass x acceleration According to Newton, if Earth pulls on the rock with 5,000 N of force, there must be an equal and opposite force. Earth is pulling the rock at 5,000 N The Rock pulls the Earth at 5,000 N

Newton’s 3 rd Law Confused yet? To understand how the rock pulls the Earth, think about what causes the Earth to pull the rock. GRAVITY If Earth pulls the rock with a gravitational force, the rock pulls the earth with its own gravitational force (granted it is small in comparison to the earth). So if Earth pulls the rock at 5,000 N, how can rock pull the Earth at 5,000 N???

Newton’s 3 rd Law Stay with me here! Both the rock and the Earth pull of each other with 5,000 N. Referring to Newton’s 2 nd Law: F=ma The rock: 5,000 N = 500 kg x 10 m/s² The Earth: 5,000 N = 5,970,000,000,000,000,000,000,000 kg x acceleration F 5,000 N Rearranged: a = m = 5.97 x kg = 1.19 x m/s 2 This is the gravitational acceleration the rock is pulling on the earth.

Newton’s 3 rd Law The difference in masses between objects determines how much the force accelerates the objects. A force creates an equal and opposite force Forces makes the things move (Accelerate) Things move (accelerate) according to their mass

Newton’s 3 rd Law We know that the Earth pulls the moon, does the moon pull on the Earth? A)YesB) No Which pull is stronger? A) Earth B) Moon C) Same They pull with the same force on each other, but because the masses are different, the accelerations towards each other are different.

Newton’s 3 rd Law The rocket pushes the gas The ______ pushes the rocket A)Atmosphere B)Gas C)Force How can gas push a rocket?

Newton’s 3 rd Law The forces are the same The mass of the rocket and gas are different The acceleration of both are different This is Molecular Man!

Newton’s 3 rd Law If forces are equal and opposite, why don’t they cancel? You have to look at each system