ForceForce

What is a What is a ForceForce??

Force can be defined as a push or Force can be defined as a push or a pulla pull… …

or anything else that has the ability or anything else that has the ability to to change motionchange motion..

Units for Units for ForceForce

kg x m/s2

kilogram x m/sec2

Newton is the scientific unit!!!

N=kg x m/s2

To Understand To Understand ForceForce……

You must understand the difference between

AND

2 Kg Mass

What is What is MassMass??

1 KgMass

2 KgMass

1.5 KgMass

The amount of “stuff” in an object

MassMass is measured in kilograms.kilograms.

How is How is Mass Mass Different Than Different Than WeightWeight??

Weight is a measurement of Weight is a measurement of forceforce!!!!Weight is not measured in kg, it is Weight is not measured in kg, it is

measured in pounds (lbs) or measured in pounds (lbs) or Newtons. Newtons.

Gravity has an effect on the weight, Gravity has an effect on the weight, but not the mass!! but not the mass!!

1 KgMass

2.2 Pounds

On the sun 1.0 kg would weigh 59.5 lbs. On the sun 1.0 kg would weigh 59.5 lbs.

On Mars 1.0 kg would weigh 0.8 lbs. On Mars 1.0 kg would weigh 0.8 lbs.

On the moon 1.0 kg would weigh 0.3 lbsOn the moon 1.0 kg would weigh 0.3 lbs. .

For example:For example: Mass is the Same:1 kgMass is the Same:1 kgBut the WEIGHT…But the WEIGHT…

7

Newton’s 3 laws of motionNewton’s 3 laws of motion

Newton’s First Law: Balanced Forces

An object will maintain a constant state of motion (balanced).

This means an object at rest tends to stay at rest and

an object in motion tends to stay in motion.

Forces that are balanced can be:

Not in motion In Motion

Stay in motion: body not

restrained by a seat belt in car accident

Stay at rest:

a book on a table

IPC BOOK

Table pushes up on book

Gravity pulls down on book

Newton’s First Law: Unbalanced Forces

The book slides and then stops because of a resistant force called friction. Friction is force that opposes motion.

In space there is no resistance to cause friction, so a bowling ball would stay in motion ...unless another object got in the way…

An unbalanced force is a force that changes the motion.

Of course “frictional” force happens on earth...

but, if you weren’t wearing a safety belt

you would continue your motion…

Do not let this happen to you; buckle up!

InertiaInertia and and MassMass InertiaInertia

an object’s resistance to any change in an object’s resistance to any change in motionmotion

The amount of lazinessThe amount of lazinessObjects with greater mass have ______ inertiaObjects with greater mass have ______ inertia

GreaterGreater

Force and acceleration are related.

Newton’s Second Law: Force = Mass X Acceleration

An object will have greater An object will have greater acceleration if a greater force is acceleration if a greater force is applied to itapplied to itWhich will accelerate faster—a car Which will accelerate faster—a car with one person pushing it or the same with one person pushing it or the same car with eight people pushing it?car with eight people pushing it?

The mass of an object also affects The mass of an object also affects acceleration. acceleration.

Smaller mass=faster accelerationSmaller mass=faster accelerationWhich will accelerate faster—a car Which will accelerate faster—a car

with two people pushing it or the with two people pushing it or the same two people pushing a fully same two people pushing a fully loaded gravel truck?loaded gravel truck?

Newton’s Second Law: Force = Mass X Acceleration

How do we calculate force?How do we calculate force?Force involves mass and movement so Force involves mass and movement so

therefore we calculate force bytherefore we calculate force byMass x accelerationMass x acceleration

V

m

F

a

“How much force is needed to accelerate a 1400-kg car 2 m/s2? ”

Consider the problem…

F =m =a =

Formula Plug-in Answer

Units, units, units!

Step 1 Read the problem. Draw a picture.

1400-kgcar

2 m/s2

How much force?

F =m =a =

Formula Plug-in Answer

Units, units, units!

1400 kg2 m/s2

Step 2 Write down what you know, What are you trying to find?

1400-kgcar

2 m/s2

“How much force is needed to accelerate a 1400-kg car 2 m/s2? ”

Formula Plug-in Answer

Units, units, units!

Step 3 Set up the formula.Solve.

F =m =a =

1400 kg2 m/s2

m x a

“How much force is needed to accelerate a 1400-kg car 2 m/s2? ”

F = m x a

Formula Plug-in Answer

Units, units, units!

Step 4 Plug-in the numbers.Solve.

2800kg x m/s2

F =m =a =

1400 kg2 m/s2

“How much force is needed to accelerate a 1400-kg car 2 m/s2? ”

m x a 1400 kg x 2 m/s2 2800 kg x m/s2

Quick ReviewQuick Review

Newton’s first law: Objects at rest stay at Newton’s first law: Objects at rest stay at rest and objects in motion stay in motion rest and objects in motion stay in motion with the same velocity unless acted on by with the same velocity unless acted on by a net forcea net force

Newton’s second law: F=maNewton’s second law: F=maSo….objects will speed up, change So….objects will speed up, change

direction or stop only if acted on by a net direction or stop only if acted on by a net forceforce

Questions:Questions:

Why does my car slow down when I take Why does my car slow down when I take my foot off the gas?my foot off the gas?

Why is hard for me to move a desk with a Why is hard for me to move a desk with a student sitting in it?student sitting in it?

Friction

FrictionFriction

Friction is a force that opposes Friction is a force that opposes motion between two surfaces that motion between two surfaces that are touchingare touching

It also changes the energy of It also changes the energy of motion into heat energy---try motion into heat energy---try rubbing your hands together and rubbing your hands together and feel the heatfeel the heat

Types of Types of FrictionFrictionStatic FrictionStatic Friction

The friction between two surfaces The friction between two surfaces that are in contact but not movingthat are in contact but not moving

Sliding FrictionSliding FrictionThe friction that opposes the motion The friction that opposes the motion

of two surfaces that are in contact of two surfaces that are in contact and sliding past each otherand sliding past each other

Rolling FrictionRolling FrictionThe friction between an object rolling The friction between an object rolling

and the surface it is rolling onand the surface it is rolling on

Some things that affect motion!Some things that affect motion! Air resistanceAir resistance

The opposition to something moving through The opposition to something moving through the airthe air

Depends on shape, size, and speedDepends on shape, size, and speedA good portion of the gas that cars burn is to A good portion of the gas that cars burn is to

overcome air resistanceovercome air resistance Terminal VelocityTerminal Velocity

When an object’s weight matches the air When an object’s weight matches the air resistance—it is the highest velocity that a resistance—it is the highest velocity that a falling object will reachfalling object will reach

QuestionQuestion

What are the three What are the three variables that Newton’s variables that Newton’s Second Law of motion Second Law of motion

connects?connects?

Newton’s Third Law: Action / Reaction

For every action there is an equal and opposite reaction.

Look at the picture below:

What is the action?

What is the reaction?

As the gases push downward out of the rocket,

The rocket is pushed upward by gases.

Gravity is the force that ALL objects in the UNIVERSE exert on each other...

Newton said that gravitational force depends on 2 things2 things:

The DISTANCEDISTANCE of the objects The MASSMASS of the objects

LESS gravitational force compared to...

MORE gravitational force here.

The blue objectblue object has more gravitational force than the green objectgreen object because the blue objectblue object has more MASSMASS.