Chapter 3Chapter 3

Force, Mass and AccelerationForce, Mass and Acceleration

Newton’s LawsNewton’s Laws

Sir Isaac NewtonSir Isaac Newton1642-17271642-1727

English Mathematician and PhysicistEnglish Mathematician and Physicist Formulated the three laws of motion and Formulated the three laws of motion and

law of universal gravitationlaw of universal gravitation Invented calculus Invented calculus All before the age of 30All before the age of 30

Newton’s discoveries helped scientists Newton’s discoveries helped scientists understand ideas such as: what causes understand ideas such as: what causes tides, how do the planets move and why tides, how do the planets move and why do objects of different masses fall at the do objects of different masses fall at the same rate?same rate?

A NewtonA Newton

Newton came up Newton came up with a formula to with a formula to help scientists help scientists understand force.understand force.

Force is measured in Force is measured in Kg.m/sKg.m/s22 this is called this is called a Newton (named a Newton (named after guess who? – after guess who? – Newton)Newton)

There are 4.48 N in 1 There are 4.48 N in 1 lblb

What is Force?What is Force?

Force is a push, pull or any action Force is a push, pull or any action that has the ability to change motionthat has the ability to change motion• Will a force always cause motion?Will a force always cause motion?

There are two units that are There are two units that are commonly used to measure force, commonly used to measure force, Newtons and Pounds. Scientists Newtons and Pounds. Scientists prefer Newtons(N)prefer Newtons(N)

Newton’s First LawNewton’s First Law Law of InertiaLaw of Inertia

““An object at rest will stay at rest until An object at rest will stay at rest until acted upon by an unbalanced force, an acted upon by an unbalanced force, an object in motion will continue with object in motion will continue with constant speed and direction until acted constant speed and direction until acted upon by an unbalanced force”upon by an unbalanced force”

Basically – An object at rest remains at Basically – An object at rest remains at rest and an object in motion remains in rest and an object in motion remains in motion unless acted on by an unbalanced motion unless acted on by an unbalanced force.force.

Newton’s 1Newton’s 1stst Law Law

InertiaInertia

Objects ability to resist changing its Objects ability to resist changing its state of motion (Keep on doing what state of motion (Keep on doing what it is doing)it is doing)

Inertia depends on mass, so an Inertia depends on mass, so an object with more mass would have a object with more mass would have a more inertiamore inertia

InertiaInertia

Which is harder to move, a bowling Which is harder to move, a bowling ball or a beach ball?ball or a beach ball?

Newton’s 2Newton’s 2ndnd Law Law

Relates force to an object’s mass and Relates force to an object’s mass and acceleration acceleration

It says that force causes It says that force causes acceleration, mass resists acceleration, mass resists acceleration and the acceleration is a acceleration and the acceleration is a ratio of force over massratio of force over mass

Common sense: If something has Common sense: If something has more mass, it would take more force more mass, it would take more force to cause the object to accelerateto cause the object to accelerate

Newton’s 2Newton’s 2ndnd Law Law

F = maF = ma

Using Newton’s 2Using Newton’s 2ndnd Law Law

F = maF = ma

F

M A

An object’s Force equals its mass times its acceleration

22ndnd Law Law

A car engine produces a force and A car engine produces a force and you can calculate the acceleration of you can calculate the acceleration of the car when you know this force and the car when you know this force and the mass of the carthe mass of the car

If you want to increase the If you want to increase the acceleration of the car you must acceleration of the car you must decrease the cars mass or increase decrease the cars mass or increase the force of the enginethe force of the engine

Balanced vs. Unbalanced ForcesBalanced vs. Unbalanced Forces Net Force – the sum of all forces Net Force – the sum of all forces

acting on an objectacting on an object

The motion of an object depends on The motion of an object depends on the total of all forces acting on it the total of all forces acting on it

If the forces are balanced(net force If the forces are balanced(net force =0), then the object is in equilibrium =0), then the object is in equilibrium and there is no chance of movement and there is no chance of movement occurring because the net force is occurring because the net force is zerozero

Balanced ForcesBalanced Forces

If the forces are balanced(net force If the forces are balanced(net force =0), then the object is in equilibrium =0), then the object is in equilibrium and there is no chance of movement and there is no chance of movement occurring because the net force is occurring because the net force is zerozero

Unbalanced ForcesUnbalanced Forces

If there is a net force, then the forces If there is a net force, then the forces are unbalanced, and movement are unbalanced, and movement occursoccurs

Weight vs. MassWeight vs. Mass

Mass is the amount of matter in an object, Mass is the amount of matter in an object, usually measured in kilogramsusually measured in kilograms

Weight is the force of gravity acting on a Weight is the force of gravity acting on a certain mass, measured in Newtons or certain mass, measured in Newtons or poundspounds

Weight changes with gravityWeight changes with gravity

Mass is the same everywhereMass is the same everywhere

Gravity, Mass and WeightGravity, Mass and Weight An object weighs An object weighs

less on the moon less on the moon but has the same but has the same mass. It weighs mass. It weighs less because there less because there is less gravity.is less gravity.

You would weigh You would weigh more on Jupiter more on Jupiter than Earth because than Earth because Jupiter has more Jupiter has more gravity than Earthgravity than Earth

GravityGravity

A force that pulls every mass toward A force that pulls every mass toward another massanother mass

Earth is the biggest mass for us so Earth is the biggest mass for us so other masses are pulled toward other masses are pulled toward earth’s centerearth’s center

On earth, every kilogram of mass On earth, every kilogram of mass weighs 9.8 Newtonsweighs 9.8 Newtons

Weight is a forceWeight is a force

You can figure out the weight by You can figure out the weight by multiplying the mass times the multiplying the mass times the acceleration of gravityacceleration of gravity

F= m X a F= m X a Weight is a force (pulled to earth’s Weight is a force (pulled to earth’s

center)center) The acceleration due to gravity on The acceleration due to gravity on

Earth is 9.8 m/sEarth is 9.8 m/s22

Gravity, Cont.Gravity, Cont.

On Earth, every object will fall at the same On Earth, every object will fall at the same rate (not counting air friction)rate (not counting air friction)

The Acceleration of gravity is 9.8 m/sThe Acceleration of gravity is 9.8 m/s22 meaning that every second, a falling meaning that every second, a falling object accelerates 9.8 m/sobject accelerates 9.8 m/s

In other words, every second something is In other words, every second something is falling it is moving 9.8 m/s fasterfalling it is moving 9.8 m/s faster

Gravity with no resistanceGravity with no resistance

If you drop a If you drop a bowling ball and a bowling ball and a match box car at match box car at the same time, the same time, they will hit the they will hit the ground at the same ground at the same timetime

Newton’s Third LawNewton’s Third Law

““For every action, there is an equal and For every action, there is an equal and opposite reaction”opposite reaction”

There must be a PAIR of objects There must be a PAIR of objects

The action and reaction act on separate The action and reaction act on separate objectsobjects

Newton’s Third LawNewton’s Third Law

If you press down on the table, you If you press down on the table, you are exerting a force on it. Is the are exerting a force on it. Is the table exerting a force on your hand?table exerting a force on your hand?

Your force on the table is action Your force on the table is action force, the table’s force on you is force, the table’s force on you is reaction forcereaction force

33rdrd Law Law Skateboard – foot Skateboard – foot

on ground is action on ground is action – earth against – earth against skateboard is skateboard is reactionreaction

Throw ball on a Throw ball on a skateboardskateboard

Recoil – backward Recoil – backward acceleration from acceleration from reaction forcereaction force

Law of Universal GravitationLaw of Universal Gravitation

The force of attraction (gravity) The force of attraction (gravity) between 2 objects depends on the between 2 objects depends on the mass of the object and the distance mass of the object and the distance between thembetween them

More noticable between large objects More noticable between large objects (plants and stars)(plants and stars)

Force = Force = G x mass 1 x mass 2G x mass 1 x mass 2

dd22

G = 6.67 x 10-11G = 6.67 x 10-11

FrictionFriction

Forces that work against the motion Forces that work against the motion of the objectof the object

Force of friction is always in the Force of friction is always in the opposite direction of the motionopposite direction of the motion

Must consider when finding net forceMust consider when finding net force Will cause motion to stop eventually Will cause motion to stop eventually

unless force is continually suppliedunless force is continually supplied

Air FrictionAir Friction

Caused by air moving around objectsCaused by air moving around objects

Sliding FrictionSliding Friction

Two objects sliding against one Two objects sliding against one anotheranother

Rolling FrictionRolling Friction

One object rolling over another One object rolling over another objectobject

Viscous FrictionViscous Friction

Object moving in water or fluidObject moving in water or fluid