6 3 Newtons Third Law Newtons Third Law

6. 3 Newton’s Third Law Ø Newton’s Third Law (action-reaction) applies when a force is placed on any object, such as a basketball.

6. 3 The Third Law: Action/Reaction Ø Newton’s Third Law states that every action force creates a reaction force that is equal in strength and opposite in direction. Ø There can never be a single force, alone, without its actionreaction partner.

6. 3 Action and reaction Ø When sorting out action and reaction forces it is helpful to examine or draw diagrams. Here the action force is on the ________, and the reaction force is on the ________.

Solving Problems ØA woman with a weight of 500 newtons is sitting on a chair. ØDescribe one action -reaction pair of forces in this situation.

Solving Problems 1. Looking for: 2. Given 3. Relationships: 4. Solution Ø Ø Ø Fc = 500 N …pair of action-reaction forces …girl’s force. W = -500 N (down) Fw = -500 N Action-reaction forces are equal and opposite and act on different objects. Draw a free body diagram The downward force of 500 N exerted by the woman on the chair is an action. Therefore, the chair acting on the woman provides an upward force of 500 N and is the reaction.

6. 3 Collisions Ø Newton’s third law tells us that any time two objects hit each other, they exert equal and opposite forces on each other. Ø The effect of the force is not always the same.

6. 3 Momentum ØMomentum is the mass of a object times its velocity. ØThe units for momentum are kilogrammeter per second (kg·m/s).

6. 3 Momentum ØThe law of conservation of momentum states that as long as the interacting objects are not influenced by outside forces (like friction) the total amount of momentum is constant or does not change.

6. 3 Momentum ØThe result of a skateboarder throwing a 1 -kg ball at a speed of -20 m/sec is that he and the skateboard with a total mass of 40 kg move backward at a speed of +0. 5 m/sec (if We use positive and you ignore friction). negative numbers to show opposite directions.

6. 3 Collisions ØWhen a large truck hits a small car, the forces are equal. ØThe small car experiences a much greater change in velocity much more rapidly than the big truck. Which vehicle ends up with more damage?

Solving Problems ØIf an astronaut in space were to release a 2 kilogram wrench at a speed of 10 m/s, the astronaut would move backward at what speed? ØThe astronaut’s mass is 100 kilograms.

Solving Problems 1. Looking for: Ø … the velocity of the astronaut (backward) 2. Given Ø …velocity 1 = 10 m/s; mass 1= 2 kg; Ø. . . mass 2 = 100 kg; 3. Relationships: Ø m 1 v 1 = m 2 v 2 4. Solution Ø Draw a free body diagram.