Download 3.3 Notes - Trimble County Schools

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
 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
v
v
v
v
v
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