forces and motions. forces what is a force? anything that changes the state of rest or motion of an...
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Forces and Motions
ForcesWhat is a Force? Anything that changes
the state of rest or motion
of an object It’s what causes
ACCELERATION
has magnitude and direction Therefore force is a vector
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.
Warning: Doesn’t always mean
the object is at rest;
Example: Tug of war,
Pushing piano, car
traveling at constant
velocity
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)
Force
A push or pull on an object Units: Newton (N) = 1 kg x m/s2
Net Force (Fnet) : sum of all of the forces that are acting on an object
Balanced Forces: Fnet = 0 Object is at rest or moving with constant velocity
Unbalanced Forces: Fnet is not equal to 0, Object is accelerating or decelerating
Types of Forces Applied – contact force in the direction the
object is moving Tension- caused by a rope, cable, etc.,
directed away from the object Friction- opposes the motion of objects,
must have contact Normal- caused by a surface Force of gravity- force at a distance, caused
by attraction between two objects
Two Types of Friction Static
Friction of an object at rest Greater than kinetic friction (harder to get an
object moving than to keep it moving) Friction is greatest when surfaces are rough
ex. the force that is keeping this block from
sliding downhill Kinetic
Friction of a moving object
Types of Kinetic Friction
Sliding Two surfaces moving past each other
Rolling Movement of a circular object on a surface Ex. Cart on wheels
Fluid Friction Movement of an object through a liquid or gas Ex. Air resistance
FRICTION
STATICSLIDING ROLLING FLUID
The 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.
There is an upward force that balances gravity AIR RESISTANCE
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.
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 gt2
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
Drawing Free Body Diagrams
Identify the types of forces acting on the object Use a dot or a box to represent the object Use arrows to represent the direction and size
of the force -change the size of the arrow if more force is
being applied in one direction versus the other Label the arrows with the type of force
Free Body Diagrams
How will the object move in the diagram to the right?
Answer: Object may be at rest and not move or it could be moving at a constant velocity since all the forces are balanced and the net force is zero
Free body diagram practice
Free Body diagram practice
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.