Newton’s Third Law

Demo: Dynamics Carts with Plunger, with and without mass. The Push Back (Demo) Demo: Dynamics Carts with Plunger, with and without mass.

The Push Back Whenever one body exerts a force on a second body, the second body pushes back with an equal and opposite force on the first body. “For every action there is an equal and opposite reaction.”

Reaction Force The reaction force is equal in magnitude and opposite in direction. FBDs Wall pushes on man Man Wall Man pushes on wall

Always in Pairs "action...reaction" means that forces always occur in pairs. Single, isolated forces never happen. Either force in an interaction can be the "action" force or the "reaction" force. "equal" means: Both forces are exactly the same size. Both forces exist at exactly the same time. "opposite" means that the two forces always act in opposite directions – exactly 180o apart.

IMPORTANT When two objects are involved, the action and reaction forces act on different objects. For example, Blocks X and Y are attached to each other by a light rope and can slide along a horizontal, frictionless surface. Block X has a mass of 10 kg and block Y a mass of 5.0 kg. An applied force of 36 N [right] acts on block X. XY Together FBD X Alone FBD Y Alone FBD

Consider the following situations:

Consider the following:

Now Predict Scenario Predicted Result Observed Result Action force Reaction force WHY? Blow up balloon and release   Balloon molecules push on air Air molecules push on balloon Air molecules in rm have greater mass than air molecules in balloon – therefore balloon moves Student steps off skateboard Person on skateboard Skateboard off person Person has more mass therefore skateboard has greater acceleration (x2) 2 students on skateboards- push each other – equal mass, unequal mass One student Other student The student with the greater mass will move less 2 students stand on skateboards – student throws ball to other student Student on ball Ball on student Ball goes further, less mass but both move

Other Scenarios Firefighters holding a hose Recoil of a gun Canon Fan cart Rocket Propellers on a plane Jet engine in space: jet engine on air particles, particles on jet engine Astronaut tethered to space station – tether breaks – drift indefinitely but you are holding onto a wrench – throw away and it will propel you back

Right or Wrong? If an object is moving, there must be some force making it move. If v = 0, then a and Fnet must be zero. An object must move in the direction of the net force. Wrong! It could be moving without accelerating. Wrong! Think of a projectile shot straight up at its peak. Wrong! It must accelerate that way but not necessarily move that way.

Right or Wrong? Heavy objects must fall faster than light ones. When a big object collides with a little one, the big one hits the little one harder than the little one hits the big one. If an object accelerates, its speed must change. Wrong! The rate is the same in a vacuum. Wrong! The 3rd Law says they hit it each other with the same force. Wrong! It could be turning at constant speed.

Example 1 Blocks X and Y are attached to each other by a light rope and can slide along a horizontal, frictionless surface. Block X has a mass of 10 kg and block Y a mass of 5.0 kg. An applied force of 36 N [right] acts on block X. (a) Calculate the action-reaction forces the blocks exert on each other. (b) Suppose the magnitudes of the force of friction on blocks X and Y are 8.0 N and 4.0 N respectively. Calculate the action-reaction forces the blocks exert on each other. XY Together FBD X Alone FBD Y Alone FBD

Example 2 A worker applied a small 30-N horizontal force to box A which also pushes onto box B in front. Box A is 25kg and box B is 15kg. They sit on a smooth surface. The two-box system accelerates forward at 0.75m/s2. What size of forces do the boxes exert on each other? How do their directions compare? Given: Required:

Example 2 Continued Consider box A alone: Newton’s second law applies. The mass times the acceleration of A must equal the applied force plus the force of box B pushing back on A. Consider box B alone: Newton’s second law still applies; however, the only force acting on box B is from box A. This demonstrates that the forces acting from box to box are equal in magnitude and opposite in direction, as we would expect from Newton’s third law. If the forces were not equal (unbalanced), on box would be accelerating away from the other box.

Practice Complete Practice Problems posted on our class blog.