Download Ch 10.3 Forces - Aurora City Schools

Ch 10.3 Forces
• Force: action on an object to change
state of rest or motion (accelerate)
• (push or pull on object … “baby”answer)
• Objects change motion because of
forces
• 2 kinds:
– Contact forces are touching, like when you
push something (friction, applied force)
– Field forces do not touch, but act over an
area (field) like gravity or magnetism
Net forces
• Net force: total of all forces acting on
object
• if net force = 0
– forces are balanced
– object does not change rest/motion
• if net force is not 0
– forces are unbalanced
– object changes state of rest/motion
(accelerates)
Net forces
• Important distinction:
– Not true that something that has
balanced forces isn’t moving. It could be
moving at a constant velocity (speed and
direction)
– True that something with balanced forces
is not accelerating (changing speed,
direction or both)
• Balanced forces = constant motion
• Unbalanced forces = acceleration
(change in motion)
Balanced and Unbalanced
• So the reason things move differently,
in a scientific sense, is that it’s
being acted on by unbalanced
forces.
• Can something be moving when the
forces on it are balanced?
• Yes…but it won’t accelerate. It is
moving at a constant speed and
direction.
Balanced or unbalanced?
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Car moving in a straight line, 65 mi/hr
Car slowing down
Car parked
Car turning a corner
Car speeding up
Car stopped at a stoplight with engine
running
Balanced or unbalanced?
The Force of Friction
• Friction: acts against motion of an
object (opposite direction of
motion)
– static: when objects not in motion
(parked car on hill)
– kinetic: when things are moving (tires on
road around turn)
• Can be rolling or sliding
• Air resistance: form of friction with air
molecules
Friction and Motion
• Friction can be good or bad
• Bad: parts in engine rub together.
Decrease by:
– lubricant (oil)
– replace sliding with rolling (wheels, ball
bearings)
– smooth surfaces
• Good: keeps car on road. Increase by
adding
– more force
– rougher surface