2. Newton’s 3 rd Law (N3L) For every action there is an equal and opposite reaction - always involves more than one object

3. Newton’s 3 rd Law (N3L) - Examples 1. Hammer/Nail Action: Hammer pushes nail into wood. Reaction: Nail pushes back on hammer to stop it. Why do we assume that the nail does not apply a force to the hammer? 2. Object falling to Earth Action: Earth pulls object Reaction: Object pulls Earth Why don’t we notice the Earth being pulled toward the object?

4. Normal Force (F N ) The reaction force that a surface exerts on an object  acts at a right angle (normal) to the surface  the harder the object pushes on the surface the harder the surface pushes back (N3L)

5. Normal Force (F N ) Examples FNFN FaFa 90° F netX = F a – F N = 0 F N = F a FgFg FNFN F netY = F N – F g = 0 F N = F g

6. N1L, N2L, and N3L work together! According to N3L, Earth pulls an object toward it while the object pulls back on Earth. According to N1L, Earth does not appear to move because its inertia is so big. N2L can be used to show this result m Earth (a Earth ) = F = m object (a object )