Newton’s 3rd Law

 Newton’s 3rd Law: Newton’s 3rd Law Newton’s 2nd Law is: A quantitative description of how forces affect motion. BUT: Where do forces come from? Answer this with EXPERIMENTS!! The results show that forces applied to an object are ALWAYS applied by another object  Newton’s 3rd Law:

Newton’s 3rd Law: “Whenever one object exerts a force F on a second object, the second object exerts an equal and opposite force -F on the first object.” “Law of Action-Reaction”: “Every action has an equal & opposite reaction”. Note that action-reaction forces act on DIFFERENT objects!

The Action-Reaction Principle Fon ball  Force on the ball Newton’s 3rd Law “When one object exerts a force on a second object, the second object exerts a force of the same magnitude & opposite direction on the first object”. This is often called The Action-Reaction Principle For example, see figure. There are 2 forces shown: Fon ball  Force on the ball Fon bat  Force on the bat Newton’s 3rd Law: Fon ball = - Fon bat

Newton’s 3rd Law: Consequences Newton’s 3rd Law tells us that FORCES COME IN PAIRS The 2 forces are always equal in magnitude & opposite in direction. Important!! The 2 forces act on different objects Figure: The person exerts force F1 on the refrigerator. The refrigerator exerts force F2 on the person. Newton’s 3rd Law: F2 = -F1

Another Statement of Newton’s 3rd Law “If two objects interact, the force F12 exerted by object 1 on object 2 is equal in magnitude & opposite in direction to the force F21 exerted by object 2 on object 1.” As in the figure

Example: Newton’s 3rd Law When a force is exerted on an object, that force is caused by another object. Newton’s 3rd Law: “Whenever one object exerts a force on a second object, the second exerts an equal force in the opposite direction on the first.” If your hand pushes against the edge of a desk (red force vector), the desk pushes back against your hand (purple force vector); the 2 colors tell us that this force acts on a DIFFERENT object.

Newton’s 3rd Law: Alternative Statements 1. Forces ALWAYS occur in pairs

Newton’s 3rd Law: Alternative Statements 1. Forces ALWAYS occur in pairs 2. A single isolated force CANNOT exist

Newton’s 3rd Law: Alternative Statements 1. Forces ALWAYS occur in pairs 2. A single isolated force CANNOT exist 3. The “action force” is equal in magnitude to the “reaction force” & opposite in direction.

Newton’s 3rd Law: Alternative Statements 1. Forces ALWAYS occur in pairs 2. A single isolated force CANNOT exist 3. The “action force” is equal in magnitude to the “reaction force” & opposite in direction. 4. One of the forces is the “action force”, the other is the “reaction force”

Newton’s 3rd Law: Alternative Statements 1. Forces ALWAYS occur in pairs 2. A single isolated force CANNOT exist 3. The “action force” is equal in magnitude to the “reaction force” & opposite in direction. 4. One of the forces is the “action force”, the other is the “reaction force” 5. It doesn’t matter which is considered the “action” & which the “reaction”

Newton’s 3rd Law: Alternative Statements 1. Forces ALWAYS occur in pairs 2. A single isolated force CANNOT exist 3. The “action force” is equal in magnitude to the “reaction force” & opposite in direction. 4. One of the forces is the “action force”, the other is the “reaction force” 5. It doesn’t matter which is considered the “action” & which the “reaction” 6. The action & reaction forces MUST ACT ON DIFFERENT OBJECTS & be of the same type.

Action-Reaction Pairs: Act on Different Objects The key to correct application of Newton’s 3rd Law is: THE FORCES ARE EXERTED ON DIFFERENT OBJECTS. Make sure that you don’t use them as if they were acting on the same object. Example An ice skater pushes against a railing. The railing pushes back & this force causes her to move away.

Rocket propulsion is explained using Newton’s Third Law. Hot gases from combustion spew out the tail of the rocket at high speeds. The reaction force is what propels the rocket. Note: The rocket doesn’t need anything to “push” against. Figure 4-10. Caption: Another example of Newton’s third law: the launch of a rocket. The rocket engine pushes the gases downward, and the gases exert an equal and opposite force upward on the rocket, accelerating it upward. (A rocket does not accelerate as a result of its propelling gases pushing against the ground.)