Forces and Motion

Forces and Motion

ForcesWhat is a Force?-A push or pull on an

object-UNITS: NEWTONS (N)

What is a Newton?

1 Newton of force will cause a 1 kg object to experience 1 m/s2 of acceleration.

Force causes changes in ___________ Therefore, it causes __________________ Because force deals with VELOCITY- it has

magnitude and direction Therefore force is a vector

VELOCITY!

ACCELERATION!

Force cont.A force can cause… a resting object to move, or … A moving object to accelerate How?

By changing the object’s speed or direction.

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) Normal- caused by a surface (Fn) Friction- opposes the motion of objects, must

have contact (Ff) Force of gravity- force at a distance, caused

by attraction between two objects (Fg)

Applied Force Examples

Tension Force Examples

Normal Force Examples

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- between stationary objects (“static”=

“stationary”)Ex- the force that is keeping this block from sliding

downhillKinetic- friction of movement

NOTICE: three types (on chart): Sliding,Rolling,Fluid

Complete the concept map for the 4 types of friction! Use your textbook (pages 332-334)

FRICTION

STATIC SLIDING 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?

The force of Gravity: objects that have mass are attracted

to one another Gravity is an attractive force pulls objects

together Earth’s gravity acts toward the center of the

Earth.

WEIGHT: equal to Force of GRAVITY (Fg) Not the same as MASS!!!

DO NOT CONFUSE THE TWO “GRAVITIES!” FORCE of Gravity= FORCE in NEWTONS

(an object’s mass x the acceleration due to gravity) ACCELERATION due to Gravity= 9.8 m/s2

So, W = mg and F = ma

Concept REWIND

So why don’t we get sucked into the center of the earth?

NORMAL FORCE That upward force that balances gravity when

you are standing on a surface

Can you think of other forces that can oppose gravity?

Tension and Friction Tension force can

also oppose gravity

Air resistance is a type of frictional force: (also called “Drag”)

AIR RESISTANCE:

acts in the direction opposite to the motion & reduces acceleration.

Forces can be COMBINED:Net Force: Fnet : the combination of all of the forces

acting on the object Use Free Body Diagrams to show the net force:

Remember the Units? NEWTONS

Free body diagrams:

Book on desk

Free fall with air resistance:(no friction)

Balanced Forces: Fnet = 0 (the net force is zero) and there is no change in the object’s

motion. Can be at rest OR moving at a constant

velocity

Example: Tug of war, pushing a piano, car movingat a constant velocity

Unbalanced Forces Fnet is NOT equal to zero The object accelerates in the direction of the

net force (can cause it to speed up OR slow down)

Fig 14, p.332

http://www.darvill.clara.net/enforcemot/friction.htm

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!

A) Historical Development1) Aristotle (384 BC- 322 BC): Incorrectly proposed that force is required to keep an object moving

Newton (1643 – 1727): Defined mass and force; Introduced 3 Laws of Motion

Newton’s Laws of Motion

Newton’s 1st Law of Motion The motion of an object does not change if its

net force is zero. Basically -an object at rest will stay at rest or

an object in motion will stay in motion unless an unbalanced force acts on it.

Example: Soccer ball will remain (at rest) on the grass unless a force acts on it

1st Law

Inertia is the tendency of an object to resist changes in its velocity: whether in motion or motionless.

These pumpkins will not move unless acted on by an unbalanced force.

1st Law Once airborne,

unless acted on by an unbalanced force (gravity and air – fluid friction), it would never stop!

1st Law

Unless acted upon by an unbalanced force, this golf ball would sit on the tee forever.

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 .

1) Car crash: You continue forward because of inertia

“Science and the Consumer”

p.348

Newtons’s 1st Law and You

Don’t let this be you Adonis! Wear your seat belt!Because of inertia, objects (including you) resist changes in their motion. When the car going 80 mi/hour is stopped by the brick wall, your body keeps moving at 80 mi/hour.

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.

2nd Law

2nd Law

The net force of an object is equal to the product of its mass and acceleration, or F=ma.

2nd Law

When mass is in kilograms and acceleration is in m/s/s, the unit of force is in newtons (N).

One newton is equal to the force required to accelerate one kilogram of mass at one meter/second/second.

1 N = 1 kg x 1 m/s2

“the acceleration of an object is equal to the net force acting on it divided by the object’s mass.”

Units: F= NM= g

A= m/s2 = N/kg

See Math Skills p.350

Acceleration = Net Force a = F F=ma Mass m

Newton’s 2nd Law of Motion

Mass: the amount of matter in an objectWeight: measure of the gravitational force

Units for weight = N Basically Newton’s 2nd Law

W is Forceg is Acceleration

Weight = Mass x Acceleration due to gravity

W = mg g = 9.8 m/s2

W=mg F = ma

Weight is a result of F=ma

Rewind

Do you remember Inertia? How is this related to mass?

How is that different than weight?

Mass is a measure of the inertia of an object; weight is a measure of the force of gravity acting on an object.

Your weight on Earth is affected

by Earth’s gravity

You would weigh less on the

Moon b/c gravity on Moon is less

You would weigh almost nothing in interstellar

space

Newton’s Third Law of Motion & MomentumNewton’s Third Law

“when one object exerts a force on a 2nd object, the 2nd object exerts an equal and opposite force on the first object.”

Two forces are called action and reaction forces

Example: Pushing your hand against a wall

1) Hand pressing on wall = ACTION2) Wall exerting a force on hand =

REACTION

Equal & opposite

Action and Reaction Forces:

Action-Reaction forces do not cancel each other outExample: Swimming action-reaction forces do not act on the same object.

Action= pushing backward on the water, Reaction = water pushing forward on the

swimmer causes the swimmer to move because that

force is overcoming the friction on the swimmer.

Action-reaction forces can produce motion, but not all do- it depends on what other forces are acting

on the objects.

Pushing a wall = no motion(because the wall has other forces acting on it…gravity, etc)

Swimming = motion (because force of water is greater than friction on the swimmer)

Rewind State in simple terms Newton’s Three Laws.

(in a way you can remember which is which)

1st Law of Inertia (An object at rest will stay at rest…)

2nd- F=MA 3rd Action-Reaction (explains conservation of momentum)

Extension- do not teach

in 2014

Newton’s Laws explain Projectile Motion: Projectile motion= the curved path that an object

follows when thrown, launched, or otherwise projected near the surface of the Earth [Fig 13, p.358]

What FORCES are affecting the acceleration? (A=F/M) Force of the “throw”, Air resistance and gravity

Water Balloon Toss- http://library.thinkquest.org/2779/

How does this affect the acceleration? The combination of an initial forward velocity and

the downward force of gravity causes the ball to follow a curved path.

Animation: http://www.physicsclassroom.com/mmedia/vectors/hlp.cfm

Another Law

Law of conservation of Momentum

Momentum: the product of the mass & velocity of an object

An object has a large momentum if the product of its mass and velocity is large.

SI Units: kg x m/s

Momentum=mass x velocity p=mv

Law of Conservation of Momentum:

momentum cannot be created or destroyed, so the total momentum is conserved [Fig 20, p.364]

In a closed system, the loss of momentum of one object equals the gain in momentum of another object—momentum is conserved.

Usually that’s not the whole story There’s one more concept to explain the

acceleration of an object

Impulse

Impulse is related to momentum and forceDefinition: Change in Momentum over time

SI Units: N(s)

Impulse=Force x Time I=Ft

The fact that impulse depends on both force and time means that there is more than one way to apply a large impulse to an object