# Newtons Third Law of Motion Newtons Third Law

• Slides: 16

Newton’s Third Law of Motion

Newton’s Third Law of Motion Newton’s first law of motion is descriptive and his second law is mathematical In both cases, we consider the forces acting on only one object However, when your hand pushes on a desk in one direction, you feel a force of the desk pushing back on your hand in the opposite direction This brings us to the third law, which considers forces acting in pairs on two objects

Newton’s Third Law of Motion To illustrate Newton’s third law of motion, imagine a ball shot horizontally out of the tube of a toy car on wheels When the ball is pushed into the card, a spring becomes compressed Then, when the spring is released, the spring (and thus the cart) pushes forward on the ball We call this force, the action force

Newton’s Third Law of Motion At the same instant, the ball pushes backwards on the spring (and thus the cart) We call this the reaction force of the ball on the cart The action and reaction forces are equal in size but opposite in direction, and act on different objects

Newton’s Third Law of Motion Newton’s third law of motion deals with these action and reaction forces For this reason, it is sometimes called the actionreaction law

Newton’s Third Law of Motion (copy) If object A exerts a force on object B, then object B exerts a force equal in size, but opposite in direction For every action force, there is a reaction force equal in magnitude but opposite in direction

There is no situation where only a single force acts A single force is impossible……there is always a corresponding reaction force Why do they not cancel each other out? This is because they are acting on different bodies Eg. A skater bumps into a board The skater is exerting a force on the board (action force) The board is exerting a force on the skater (reaction force)

(copy) – add to note The action and reaction forces act on different objects ***since the action and reaction forces are acting on different objects, they will appear on separate FBDs

Practice/Examples 1. Describe the reaction force for each of the following forces A) the southward force of a field goal kicker’s toe on a football the northward force of the football on the kickers toe B) the. The backward force of a jogger’s shoe on shoe the ground forward force of the fground on the joggers C) the downward force of a book on a desk The upward force of the desk on the book

2. Two skates are standing on ice facing each other. Skater 1 pushes on skater 2 with a force of 70 N[E]. No friction acts on either skater. The mass of skater 1 is 50 kg and the mass of skater 2 is 70 kg. A) state the action and reaction forces B) state what will happen to each skater C) Draw the FBD of each skater D) calculate the acceleration of each skater

Newton’s Third Law - Applications Newton’s third law of motion has many interesting applications…

Newton’s Third Law -- Tides One of the most striking action-reaction pairs is fond in the Earth-moon system The moon attract the water at A more strongly that is attract the Earth at B because the water is closer to the moon than the Earth As the water is free to move, its level is raised on the side closest to the moon and a high tide is created

Similarly, the moon attracts the Earth at B more strongly than the water at C since this water is on the side of the Earth opposite the moon Due to inertial effects, the water at C in a sense is “left behind” because there is less force on it than on an equivalent mass of Earth Consequently, a high tide results at C

Practice Worksheet (questions #1 -4) Questions on page 141, #4, 5, 6, 7, 9 Time for a QUIZ (Forces and Newton’s Laws) try the practice on my website