What does F = ma say? F = ma basically means that Something very small (low mass) that’s changing speed very quickly (high acceleration), like a bullet, can still exert a _______ force. Something very low mass changing speed very slowly will exert a very ________ force. Something very massive (high mass) that’s changing speed slowly (low acceleration), like a glacier, can still exert (push with) a ____________ force.

Friction A force that opposes motion between surfaces in contact. It is due to microscopic imperfections in the surface. Solid surfaces only make contact in a few specific points. Each point is under enormous pressure and sometimes actually bonds to the other surface (see diagram) as electrons from one atom are pushed closer to nucleii of atoms from the other object.

The surface is “smooth” but on microscopic scale it appears rough.

Friction (contd.) Friction keeps your car on the road, allows the tires to grip. Friction wastes energy as kinetic energy is lost as heat. Friction force depends only on three things: Surfaces moving or not. Force pushing surfaces together Roughness of surfaces.

Newton’s Third Law If object A pushes or pulls on object B, Object B pushes or pulls on object A with the SAME force in the OPPOSITE DIRECTION.

What does this mean? For every force acting on an object, there is an equal force acting in the opposite direction. Right now, gravity is pulling you down in your seat, but Newton’s Third Law says your seat is pushing up against you with equal force. This is why you are not moving. There is a balanced force acting on you– gravity pulling down, your seat pushing up.

Think about it... What happens if you are standing on a skateboard or a slippery floor and push against a wall? You slide in the opposite direction (away from the wall), because you pushed on the wall but the wall pushed back on you with equal and opposite force. In this case the forces were not balanced. Can you explain why? Why does it hurt so much when you stub your toe? When your toe exerts a force on a rock, the rock exerts an equal force back on your toe. The harder you hit your toe against it, the more force the rock exerts back on your toe (and the more your toe hurts).

Examples (Think about what’s going on and fill in the blanks) A jet engine pushes _____ backward. The ______ pushes the ______ _______. A rocket engine pushes ______ out and backwards, the _______ pushes the ________ __________. A fly hits a windshield with 0.1N of force and pushes it _______, the windshield pushes on the ________ with ___ N and pushes the fly ________. The bat hits the baseball forward with 1200N of force, the ____hits the bat ______ with _____ N of force.

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