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Chapter 5 Newton’s Third Law of Motion For every “action” there is a “reaction” 22-May-17 Physics 1 (Garcia) SJSU Newton’s Third Law of Motion Whenever an object exerts First Object (Hammer) a force on a second object, the second object Second Object (Nail) exerts a force of equal magnitude in the opposite direction on the first object. 22-May-17 Physics 1 (Garcia) SJSU Check Yourself A 2 ton car, going 60 m.p.h. hits a 5 ton truck, going 20 m.p.h.. The force of impact is greatest on which vehicle, the car or the truck? The change in velocity (the acceleration) is greatest for which vehicle? By what principle of physics? 22-May-17 Physics 1 (Garcia) SJSU Check Yourself Arnold Strongman and Suzie Small pull on opposite ends of a rope in a tug of war. The greater force exerted on the rope is by 1. Arnold. 2. Suzie. 3. Neither. The force is the same. 22-May-17 Physics 1 (Garcia) SJSU Action and Reaction Common expression of 3rd Law is, To every action there’s an equal and opposite reaction. What’s an “action”? How can reaction be “equal” and “opposite”? 22-May-17 Physics 1 (Garcia) SJSU Action-Reaction Pairs Here are some examples of action-reaction pairs. Think of examples of an object exerting a force on a second object. Practice identifying action-reaction pairs. 22-May-17 ???? ???? ???? ???? Physics 1 (Garcia) SJSU Walking, Running & Jumping What forces accelerate us into motion when we walk, run, or jump? 22-May-17 Physics 1 (Garcia) SJSU Forces when Walking or Jumping For a person walking, running, or jumping, the three main forces on the person are: • Gravity (Downward) • Support of the floor (Upward) • Frictional force of the floor (Horizontal) Only these forces can accelerate the person. Gravity is constant but the force exerted by the floor can increase in reaction to the person exerting a force on the floor. 22-May-17 Physics 1 (Garcia) SJSU Walking Forward When weight is on back foot it acts by pushing back on the floor. Reaction is the friction of the floor, which pushes your body forward 22-May-17 If there were no friction then dancer would fall Reaction straight down and Action could not walk forward Physics 1 (Garcia) SJSU Jumping Jumping is done by pushing downward on the ground (action) so the ground pushes upward on you (reaction). How high you jump depends on the force and on the distance over which you apply that force. 22-May-17 Physics 1 (Garcia) SJSU Can only push while in contact with the ground so squatting helps by increasing distance. IMPORTANT!!! Action force & reaction force NEVER cancel because they act on different objects! Repeat this to yourself over and over again 22-May-17 Physics 1 (Garcia) SJSU Check Yourself Miss A pushes the car (action); car pushes back on her (reaction). Do these forces cancel? Miss A Action Reaction Force on Miss A is to the left; how can she move forward (to the right)? What if floor had zero friction? 22-May-17 Reaction Action Physics 1 (Garcia) SJSU ActionReaction Pairs Check Yourself Miss B also pushes the car; can she move the car by herself? ActionReaction Pairs In terms of Newton’s laws, why is this not possible? What other force does Miss B exert on the car besides her hands? 22-May-17 Physics 1 (Garcia) SJSU Miss B Action Reaction Action Reaction Adding Forces When two forces or more forces act in different directions, finding the net force is more complicated. Have to consider the angle for each force. 22-May-17 Physics 1 (Garcia) SJSU Vector Addition Forces are vectors, with magnitude & direction. Net Force A+B (25 Newtons) Force B (10 N) Object 22-May-17 Force A (20 Newtons) Physics 1 (Garcia) SJSU Parallelogram rule Check Yourself Nellie Newton hangs motionless by one hand from a clothesline as shown—which is on the verge of breaking. Which side of the line is most likely to break? Two upward forces must add together to balance Nellie’s weight. 22-May-17 1. Left side 2. Right side 3. 50/50 chance of either side breaking Physics 1 (Garcia) SJSU Lab: Force Table Practice addition of forces as vectors in the Physics 1L lab using “force tables.” Hang weights and adjust angles until forces balance. 22-May-17 Physics 1 (Garcia) SJSU