Forces and Motion

ForcesWhat is a Force?

-A push or pull on an object

-UNITS: NEWTONS (N)

What is a Newton?

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

Force causes changes in ___________ Therefore, it causes __________________

Is it a vector? 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, etc.,

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 downhill

Kinetic- friction of movementNOTICE: three types (on chart):

Sliding, Rolling, Fluid

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

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

The force of Gravity:

objects that have mass are attracted to one another

Gravity is an attractive force pulls objects together

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= 10.0 m/s2

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

Concept REWIND

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

NORMAL FORCE

Can you think of other forces that can oppose gravity?

Tension Force and Friction Forces Air resistance can

also be called “drag”

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 When the forces on an object are balanced, (the

net force is zero) and there is no change in the object’s motion.

Does this mean the object has NO VELOCITY? NO!!!

Can be at rest OR moving at a constant velocity Ex’s? Write one down!

Unbalanced Forces Fnet is not =0 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 (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!

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

A) Historical Development

1) 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

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

the grass unless a force acts on it

What about an object in motion? An object in motion will also stay in motion

unless a force acts on it. Why does a baseball stop after you’ve thrown it? Forces are acting on it:

Friction force (air resistance) Force of gravity

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

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.

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

Units: F= N

M= g

A= m/s2 = N/kg

See Math Skills p.350

Acceleration = Net Force a = F F= maa 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 = 10 m/s2

W=mg a=F/m

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 = ACTION

2) Wall exerting a force on hand = REACTION

Equal & opposite

Action and Reaction Forces:

Action-Reaction forces do not cancel each other out

Example: 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)