jones physical science. friction what is friction? friction: the unbalanced force that acts...
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Friction
What is friction? Friction: the unbalanced force that acts
against an object’s direction of motion Why does friction occur?
The surface of any object is rough and sticks to other surfaces that are also rough
How does friction affect motion? It opposes any applied force How does a car keep moving?
Two types of Friction
Static friction The friction between stationary
(nonmoving) surfaces Kinetic friction
The friction between moving surfaces Can be either sliding friction or rolling
friction Sliding friction: when two objects slide
past each other Rolling friction: when a round object rolls
over a flat surface
Friction
Other than the friction caused by the tires coming in contact with the ground, what other friction is there to oppose the motion of the car?
Fluid friction Friction caused by an object moving
through a fluid, such as air What can car companies do to take
care of this potential problem?
Positive friction
Without friction, the tires of a car would not be able to push against the ground and move the car forward.
You are using friction right now to push ink or lead on your paper so much to leave a mark.
Without friction, you would slip and fall when you tried to walk.
Positive friction
How can positive friction be increased? Make surfaces
rougher Increase the
force pushing the surfaces together
Negative friction
How can negative friction be decreased? Use lubricants such
as motor oil, wax, and grease
Replace sliding friction with rolling friction Rolling friction is
usually less than sliding
Make the surfaces smoother
Newton’s Laws of Motion
Sir Isaac Newton established three laws that describe the relationship between motion and force.
Newton’s First Law
An object at rest remains at rest and an object in motion maintains its velocity unless it experiences an unbalanced force.
Inertia is the resistance in change of motion.
Inertia is related to mass An object with a small mass has less
inertia, and it is easier to change its motion
An object with a large mass has more inertia, and it requires more force to change its motion
Newton’s First Law
Explain the function of a car seat or a seat belt in terms of mass and inertia. Is the person in motion if the car is in
motion? YES If the car meets an unbalanced force,
what will happen to the person if there is no unbalanced force acting on the person as well as the car?
Newton’s Second Law
The unbalanced force acting on an object equals the object’s mass times its acceleration. Force = mass x acceleration (F=ma)
Page 349 Fig 3 Empty grocery car versus full grocery
cart Imagine pushing both carts with the
SAME force Which one will be moved farther? Why?
Gravity
All matter is affected by gravity
Gravitational force increases as mass increases
Gravitation force decreases as distance increases
Free-fallin’
When gravity is the only force acting on an object, it is said to be in free-fall.
The free-fall acceleration towards the center of Earth of ANY object is 9.8 m/s2
What is the difference between mass and weight? Mass – a measure of the amount of
matter in an object Weight – dependent on gravity; equal
to an object’s mass times free-fall acceleration Unit is Newtons (N) Weight = mass x 9.8 m/s2
Can weight influence the shape of living things? YES!
Large, land animals must have strong skeletons to support their mass against the force of gravity
Marine life does not require strong skeletons because it can drift due to the downward force of gravity being balanced by the upward force of the water.
What happens to objects moving through Earth’s atmosphere?
Both air resistance and gravity act on the object.
Velocity becomes constant when the force of air resistance is equal to the gravitational force on the object.
The object will stop accelerating and will reach its maximum velocity, also called terminal velocity.
Objects In Space…
Objects in space are in free fall (there is no air resistance)
Weight is a gravitational force that depends on the masses of objects and the distance between them Therefore, it is impossible to be
weightless anywhere in the universe.
Objects In Space…
Orbiting objects are also in free fall What causes orbiting?
Orbiting: when an object is traveling in a circular path around another object
Two motions are responsible for creating an orbit: Path of the object (Moon’s path, direction
of spaceship) Earth’s gravitational pull
When the forward motion combines with free fall, the object follows the curve of the Earth’s surface.
Projectile Motion
Projectile motion: the curved path an object follows when thrown, launched, or otherwise projected near the surface of the Earth
Two independent components: Horizontal motion Vertical motion The two do not affect each other, but
combine to form a curved path P 358 Figure 13
Projectile Motion
First component: Horizontal motion A force is exerted on an object to make it go
horizontal
Second component: Vertical motion Gravity pulls the object downward
Putting these together result in a curved path
Why do archers aim above the bull’s eye on a target?
Newton’s Third Law
For every action force, there is an equal and opposite reaction force.
Also called the “law of action and reaction”
Forces always occur in pairs Whenever a force is exerted, another
force occurs that is equal in size and opposite in direction.
Even when there is no motion Example:
Your weight pushing down on the chair while sitting: action force
The chair pushing back up with a force equal to your weight: reaction force
Force pairs do not act on the same object! P 361 Fig 16 Action force: the swimmer pushing
the water backward Reaction force: the water pushing
the swimmer forward
Momentum
Momentum: a property of all moving objects, which is equal to the product of the mass and the velocity of the object.
Momentum (p) = mass (m) x velocity (v)
Practice problems Page 363 #1-2
Momentum is dependent on mass Objects with less mass have less
momentum Objects with more mass have more
momentum
Momentum also has direction – An object’s momentum is in the same
direction as its velocity.
How can momentum of an object be changed? Apply a force to the object The object collides with another object Transferring the momentum from one
object to another
Changing momentum
Changing momentum
By applying force Happens when you force an object to
change its motion Extending the time needed to change
the momentum lessens the force For example, moving in the direction of
the motion after landing a pole-vault, high-jump, or gymnastics trick, extends the time of the momentum change, which results in the impact force decreasing.
Changing momentum
When two objects collide, momentum is conserved (total amount of momentum does not increase or decrease). Some momentum may be transferred
from one object to another, such as in the game of pool.
Fig 21 on Page 365 The rocket pushes gases
downward/backward toward the surface of the Earth
The gases push the rocket upward/forward toward the sky in an equal amount.