3.3 Motion & Force

Objectives Explain how forces and motion are related. Compare and contrast static friction and sliding friction. Describe the effects of air resistance on falling objects.

Force Push or pull that one object exerts on another Vector with size (strength of force) & direction SI Units = Newtons (N) Can cause the motion of an object to change Example: hitting a tennis ball shooting pool/billiards

Balanced Forces Forces equal in size but opposite in direction doesn’t change velocity

Net Force When 2 or more forces act on an object at the same time Sum of all forces acting on an object

Unbalanced Forces When forces combine to produce a net force that is not zero Forces that aren’t equal  The object moves Changes the velocity of an object

Friction The force between two objects, in contact, that opposes the motion of either object Unbalanced force Why does a ball stop rolling? Why do we have to keep applying gas to the engines in our cars to keep moving? Better yet, why do we put oil in a car’s engine? Why do runners wear running shoes? Why do parachutes work?

Friction Friction depends on the surfaces of the objects in contact Smooth things tend to have less frictional force  they are more slippery Ice, oil on concrete, my old shoes, my road bike tires Rough things tend to have more frictional force  less slippery Asphalt, my new running shoes, my mountain bike tires

Frictional Force Increases when the force pushing surfaces together increases Static Friction = frictional force that prevents 2 surfaces from sliding past each other

Rolling friction = similar to sliding friction Sliding friction = force that acts in opposite direction to the motion of a surface sliding past another surface Rolling friction = similar to sliding friction why it is easier to move something on wheels

Friction Air Resistance is a form of friction Caused by the interaction between an object and the air molecules it comes into contact with The bigger the object the more air resistance The faster the object the more air resistance

Air Resistance Causes objects to fall at different acceleration and speed Acts in opposite direction to velocity Air resistance not mass causes objects to fall at different rates

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Terminal Velocity When an object falls with constant velocity Upward air resistance becomes large enough to balance downward force of gravity http://www.iop.org/activity/education/Teaching_Resources/Teaching%20Advanced%20Physics/Mechanics/Images%20200/img_mid_4140.gif