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Forces and Motion
ForcesWhat is a Force? Anything that changes
the state of rest or motion
of an object
ie- any push or pull on an object when two objects meet/interact
Force causes changes in ___________ Therefore, it causes
ACCELERATION
Because force deals with VELOCITY- it has magnitude and direction Therefore force is a vector
Which of the following lines shows acceleration?
A B Both
Neith
er
9%
0%
32%
59%
1. A
2. B
3. Both
4. Neither
Force cont.A force can cause a
resting object to move, or it can accelerate a moving object by changing the object’s speed or direction.
Units = Newtons (N)
Forces can be COMBINED:Net Force: the combination of all of the forces
acting on the object
Balanced Forces: When the forces on an object are balanced, the net force is zero and there is no change in the object’s motion.
Example: Tug of war,
Pushing piano
Unbalanced Forces Results when net force acting on an
object is NOT equal to zero When an unbalanced force acts on an object,
the object accelerates in the direction of the net force
Fig 14, p.332
http://www.darvill.clara.net/enforcemot/friction.htm
*(moves in direction of net force)
Decide if the following objects have balanced or unbalanced net forces.
1. An airplane is flying at 150 km per hour for 30 minutes.
1. Balanced= no acceleration!
2. A book sitting on a table.1. Balanced= no acceleration!
3. A man sprinting to the finish line, accelerating at 2 m/s each second.
1. Unbalanced= acceleration!
Types of Forces-
Applied – contact force in the direction the object is moving (Fa)
Tension- caused by a rope, cable, ect., directed away from the object (Ft)
Friction- opposes the motion of objects, must have contact (Ff)
Normal- caused by a surface (Fn) Force of gravity- force at a distance, caused by
attraction between two objects (Fg)
Friction is a force: Friction is a force that opposes motion between two
surfaces that are in contact
There are two main types of friction:
Static- keeps things “static”/ “stationary”Ex- the force that is keeping this block from sliding
downhill
Kinetic- friction of movementthree types: Sliding,Rolling, Fluid
Complete the concept map for the 4 types of friction! Use your textbook (pages 332-334)
FRICTION
STATICSLIDING ROLLING FLUID
Thre friction between surfaces that are
stationary
force that exists when objects slide
past each other
force that exists when a round object
rolls over a flat surface (usually less than sliding friction)
force that exists when an object
moves through a fluid (air, water)
a book sitting on a table
-hockey puck on ice-child going down a
slide-a sled down hill
-a roller blade on a sidewalk
-bowling ball on bowling alley
-a car driving down the road
-swimmer swimming through pool
is defined as is defined as is defined as is defined as
& an example is & an example is & an example is & an example is
Can you think of a situation in which you would want to increase friction?
How can we decrease friction?Watch this demo and see…
http://www.scottso.net/examples_encfri.htm
Gravity is a force: natural phenomenon in which objects that have mass are
attracted to one another
Gravity is an attractive force pulls objects together
Earth’s gravity acts downward toward the center of the Earth.
So why don’t we get sucked into the center of the earth?
NORMAL FORCE
There is an upward force that balances gravity when you are standing on a surface called the NORMAL FORCE
Force of Gravity- (REVIEW)
Symbol Fg
Units: Newtons Related to WEIGHT
(Below will be covered in Newtons 2nd-3rd law notes)
Your weight (W) is the force of gravity acting on your mass
Equation: Fg = mg g = acceleration due to gravity, 9.8 m/s2
Falling Objects have two forces acting on them:
Gravity causes objects to accelerate downward air resistance acts in the direction opposite to
the motion & reduces acceleration.
Terminal Velocity:
when something falls with a constant velocity
(no net force = no acceleration= constant velocity)
Formal Definition: the constant velocity of a falling object when the force of air resistance is equal in magnitude & opposite in direction to the force of gravity
Terminal Velocity (cont) In other words…If something falls for a long
time,the upward force of air resistance becomes equal to downward force of gravity.
1) Example: Sky divers reach terminal velocity after a period of time(Fig 10, p.356)
Free Fall: the motion of a body when only the force of gravity is acting on the body
Free fall acceleration of an object is directed toward the center of Earth
Because free fall acceleration results from gravity, its symbol is g
Acceleration due to gravity on Earth = 9.8 m/s2
Formula for objects in free fall: d = ½ at2
A=“g”
In a vacuum, two objects would accelerate at the same rate because both are in free fall (Fig 8, p.354)
Question: What other force is not present in vacuum that would affect acceleration?
Answer = air resistance
II.) Newton’s First Law of MotionA) Historical Development
1) Aristotle (384 BC- 322 BC): Incorrectly proposed that force is required to keep an object moving
2) Galileo (1564 – 1642): Concluded that moving objects not subjected to friction or other force, would continue to move indefinitely; Disproved Aristotle
3) Newton (1643 – 1727): Defined mass and force; Introduced 3 Laws of Motion
C) Newton’s 1st Law of Motion 1) According to Newton’s 1st Law, the state of
motion of an object does not change as long as the net force is zero.
a) Basically saying that an object at rest will stay at rest unless a force acts on it
1) Example: Soccer ball will remain (at rest) on the grass unless a force is
acted on it
2) Sometimes called the “Law of Inertia”
a) Inertia: the tendency of an object to resist being moved or, if the object is moving, to resist a change in speed or direction until an outside force acts on the object
1) Car crash: You continue
forward because of inertia
“Science and the Consumer”
p.348
How is inertia related to mass?P 347 Mass is a measure of inertia.
Who would you rather be tackled by…a toddler or a defensive lineman?
What is easier to move? An empty garbage can or a garbage can full of lead? Why? The empty garbage can has less mass= less inertia=
less resistance to being accelerated.