Newton’s Laws of Motion

What are forces?

How can diagrams be used to depict and analyze the forces acting on an object?

What are the effects of net force and mass on the acceleration of an object?

Forces: An Overview

Force: A push or a pull on an object Result of an interaction between two objects

Units: newtons (N) Vector quantity

Multiple forces can act on an object at once Sum of multiple forces produces a net force

Types of Forces

Gravity (Fgrav) attractive force between any two masses Fgrav between an object and the Earth is called

weight

Friction (Ffric) resistive force that acts when an object moves or

attempts to move across a surface

Types of Forces

Air Resistance (Fair) type of friction which occurs when something

moves through the air

Tension (Ften) force transmitted through a rope, string, or wire

when it is pulled tight at both ends tension is the same everywhere throughout the

string

Types of Forces

Normal Force (Fnorm) support force exerted on an object which is in

contact with another stable object or surface always exerted perpendicular to the surface.

Applied Force (Fapp) exerted directly by a person or object

Newton’s First Law of Motion An object at rest will …

remain at rest,

an object in motion will …remain in motionat a constant velocity,

unless …the object is acted on by unbalanced forces.

Newton’s First Law of Motion If an object is at rest and all the forces are

balanced… the object will remain at rest. No change in velocity. No acceleration.

Newton’s First Law of Motion If an object is at rest and the forces are NOT

balanced… the object will begin to move. Velocity will change. It will accelerate.

Newton’s First Law of Motion If an object is in motion and all the forces are

balanced… the object will remain moving at the same velocity. No change in velocity. No acceleration.

Newton’s First Law of Motion If an object is in motion and the forces are

NOT balanced… the object will begin to speed up or slow down. Velocity will change. It will accelerate.

Newton’s First Law of Motion SUMMARY:

Forces Balanced No acceleration Forces Unbalanced Acceleration

Newton’s First Law of Motion Also known as the Law of Inertia Inertia:

tendency of an object to maintain its state of motion

measure of an object’s mass More mass

more inertia greater tendency to remain at rest or in motion

Mass vs. Weight

MASS WEIGHT

Definition amount of matter in an object

FORCE of GRAVITY

Units kg N

Variation? CONSTANTNEVER CHANGES

Depends on elevationVaries by planet

Measured with balance scale (springs)

LINK: Weight = Mass x Acceleration due to GravityOn Earth, accel. due to gravity = -9.8 m/s2

Free Body Diagrams

Standard representation of therelative magnitude and directionof all forces acting upon an object.

Object represented by a box, forces by arrows Size of the arrow reflects magnitude of force Direction of the arrow shows force direction Each arrow is labeled to indicate the force type Arrows are always drawn outward from the box Must depict all the forces which act on the object

Check out www.physicsclassroom.com for a bunch of great examples!

Net Force Vector sum of all forces

acting on the object Vector addition considers

different directions Same direction: add Opposite direction: subtract What about at right angles? What about at obtuse or

acute angles?

Check out www.physicsclassroom.com for a bunch of great examples!

Net Force The net force is known for each situation below. However, the magnitudes of a few of the individual

forces are not known. Analyze each situation individually and determine

the magnitude of the unknown forces.

Newton’s Second Law of Motion The acceleration of a body is directly

proportional to the net force acting on the body and inversely proportional to the mass of the body.

amFm

Fa

m

Fa

maFa

netnet

netnet

or,

:used are N) and ,m/s (kg, units standard If

1

2

Consider this: 1 N = 1 kg·m/s2 Does that make sense?

Newton’s Second Law of Motion If the net force increases,

acceleration increases, as long as mass remains constant.

If the mass increases, acceleration decreases, as long as net force remains constant.

Newton’s First & Second Laws of Motion

Newton’s Second Law of Motion Basic Problem Solving:

What is the net force acting on a 5 kg crate that is accelerating at 3 m/s2?

Fnet = m·a

Fnet = (5 kg)·(3 m/s2)

Fnet = 15 kg·m/s2

Fnet = 15 N

Newton’s Second Law of Motion Advanced Problem Solving:

A 75 N applied force causes a 10 kg crate to accelerate to the right at 5 m/s2. What is the force of friction acting on the crate?

Draw a free body diagram first! Fnet = m·a = (10 kg)·(5 m/s2) = 50 N

Fnet = Fapp – Ffric

50 N = 75 N – Ffric

Ffric = 75 N – 50 N = 25 NFgrav = 98 N

Fnorm = 98 N

Ffric = ?Fapp = 75 N

Newton’s Third Law of Motion For every action, there is an equal and

opposite reaction. What does this mean?

Action: force that object A exerts on object B Reaction: force that object B exerts on object A Equal in magnitude & opposite in direction These forces make an Action-Reaction Force Pair These forces occur simultaneously

Painful Example You slam your fist into the wall.

Your hand exerts a LARGE force on the wall to the left

Painful Example (continued) You slam your fist into the wall.

The wall exerts a LARGE force on your hand to the right

Painful Example (continued) See Action/Reaction PAIRS AT THE SAME TIME

How can we tell what the action-reaction pair is? The action:

YOUR HAND pushes THE WALL to the LEFT

(Replacing “hits” with “pushes”) The REaction:

THE WALL pushes YOUR HAND to the RIGHT

If you switch the words around, you have the action-reaction pair! Reverse the direction, too.

Newton’s Third Law of Motion Do the forces in an Action-Reaction Force

Pair “cancel out”? No…

because they act on different objects only one force from the pair acts on a particular

object to determine if the forces on an object are

balanced, you need to examine all the forces from different action-reaction force pairs.