Force pairs Forces always come in pairs equal and opposite action / reaction pairs. Force pairs are acting ON TWO DIFFERENT OBJECTS Action: Bug hits windshield Reaction: Windshield hits bug
Balanced Forces vs. Force pairs Normal Force Weight The table is pushing up on the tissue box. The Earth is pulling down on the tissue box. ACTION Balanced Forces (Both forces acting on the tissue box) ACTION REACTION REACTION The tissue box is pushing down on the table. The tissue box is pulling up on the Earth.
How does a fish swim? A fish uses its fins to push water backwards. But a push on the water will only accelerate the water backwards. According to Newton’s 3 rd law, the water reacts by pushing the fish forwards. Thus the fish is propelled through the water!
Which of the following forces is considered a force pair? 1.Action: Bat pushes baseball to the right Reaction: Baseball pushes bat to the left 2.Action: Bat pushes baseball to the right Reaction: Air resistance pushes baseball to the left
Action / Reaction Which of the following forces is considered a force pair? Action – bat pushes baseball to the right Reaction – baseball pushes bat to the left Action – bat pushes baseball to the right Reaction – air resistance pushes baseball to the left Force pairs involve two objects!
While driving down the road, an unfortunate bug strikes the windshield of a bus. Quite obviously, this is a case of Newton's third law of motion. The bug hit the windshield and the windshield hit the bug. Which of the two forces is greater: the force on the bug or the force on the bus? 1.Force on the bug 2.Force on the bus 3.They are equal 4.Not enough information
Trick Question! Each force is the same size. For every action, there is an equal... (equal!). The fact that the bug splatters only means that with its smaller mass, it is less able to withstand the larger acceleration resulting from the interaction.
Compare BUG F = m · a Bus Windshield F = m · a
Compare m = 0.01 kg a = 200 m/s 2 m = 1000 kg a = 0.002 m/s 2 F = 2 N F = 2 N F = ma Forces are equal
A gun recoils when it is fired. The recoil is the result of action-reaction force pairs. As the gases from the gunpowder explosion expand, the gun pushes the bullet forwards and the bullet pushes the gun backwards. The acceleration of the recoiling gun is... 1.greater than the acceleration of the bullet. 2.smaller than the acceleration of the bullet. 3.the same size as the acceleration of the bullet.
Answer The force on the gun equals the force on the bullet. Yet, acceleration depends on both force and mass. The bullet has a greater acceleration due to the fact that it has a smaller mass. Remember: acceleration and mass are inversely proportional.