![Newton’s 3rd Law
• For every action, there is an equal and opposite reaction.
“NewtonsLawOfUniversalGravitation.svg;” by Dna-Dennis derivative work: Onio72 (talk)) [CC BY 3.0 Unported]](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)
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Ch04 Newton's 3rd Law; Action-Reaction Pair Examples
- 1. Chapter 4 NEWTON’S 3rd LAW OF MOTION Action – Reaction Pairs Examples Figures from OpenStax College, ancillaries to College Physics. OpenStax College. 21 June 2012. http://cnx.org/content/col11406/latest/, licensed under a CC 4.0 International License, unless otherwise specified.
- 2. Newton’s 3rd Law • For every action, there is an equal and opposite reaction. “NewtonsLawOfUniversalGravitation.svg;” by Dna-Dennis derivative work: Onio72 (talk)) [CC BY 3.0 Unported]
- 3. Example 1 – swimmer pushing on a wall
- 4. Example 2 – Person pushing a cart
- 5. Check Your Understanding 1 While driving down the road, a firefly strikes the windshield of a bus and makes a quite obvious mess in front of the face of the driver. The firefly hit the bus and the bus hits the firefly. Which of the two forces is greater: the force on the firefly or the force on the bus? http://www.physicsclassroom.com/Class/newtlaws/u2l4a.cfm
- 6. Check Your Understanding 1 - Answer The forces are equal! For every action, there is an equal reaction. The fact that the firefly splatters only means that with its smaller mass, it is less able to withstand the larger acceleration resulting from the interaction. http://www.physicsclassroom.com/Class/newtlaws/u2l4a.cfm
- 7. Check Your Understanding 2 A rifle recoils when it is fired as a result of action-reaction force pairs. A gunpowder explosion creates hot gases that expand outward allowing the rifle to push forward on the bullet. The bullet pushes backwards upon the rifle. The acceleration of the recoiling rifle when compared with that of the bullet is: a. greater. b. smaller. c. the same. http://www.physicsclassroom.com/Class/newtlaws/u2l4a.cfm
- 8. Check Your Understanding 2 - Answer Correct Answer: B The force on the rifle equals the force on the bullet. Yet, acceleration depends on both force and mass. 𝐹 = 𝑚 𝑎 The bullet has a greater acceleration due to the fact that it has a smaller mass. http://www.physicsclassroom.com/Class/newtlaws/u2l4a.cfm
Editor's Notes
- The force on m1 due to m2 (F21 in the picture) is equal in magnitude to the force on m2 due to m1 (F12 in the picture) but points in the opposite direction.
- The picture shows a swimmer pushing (exerting a force Ffeet on wall on) the wall. She accelerates in the direction opposite to that of her push. This opposition occurs because, in accordance with Newton’s third law of motion, the wall exerts a force Fwall on feet on her, equal in magnitude but in the direction opposite to the one she exerts on it. The line around the swimmer indicates the system of interest. Note that Ffeet on wall does not act on this system (the swimmer) and, thus, does not cancel Fwall on feet . Thus the free-body diagram shows only Fwall on feet , w , the gravitational force, and BF, the buoyant force of the water supporting the swimmer’s weight. The vertical forces w and BF cancel each other out since there is no vertical motion.
- The figure in example 2 shows a person pushing a cart with equipment, and the various forces acting on the cart. The lengths of the arrows are proportional to the magnitudes of the forces (except for f , since it is too small to draw to scale). Two systems are identified by the two lines around the system of interest. System 1 is appropriate if one wants to calculate the acceleration of the entire group of objects. Only Ffloor and f are external forces acting on System 1 along the line of motion. All other forces either cancel or act on the outside world. System 2 is chosen so that Fprof will be an external force and enter into Newton’s second law. In the right hand side of the picture, note the corresponding free-body diagrams.