MOTION AND FORCES(CH 10 AND 11)

JonesPhysical Science

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?

Newton’s Second Law

Practice problems Page 351 #1-3 in your notes

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.