Forces and Newton’s 3 Laws of MotionForces and Newton’s 3 Laws of Motion Isaac Newton (1642-1727) Isaac Newton (1642-1727) Isaac Newton is without a doubt one of the most influential

Chapter 4

Forces and Newton’s3 Laws of Motion

Isaac Newton (1642-1727)

Isaac Newton (1642-1727)

Isaac Newton is without a doubt one of the most influential

men in history.

Just a few of his accomplishments:

•Built the first practical reflecting telescope

•Developed a theory of color including the idea that white

light is composed of all colors of the rainbow

•Studied the speed of sound

•Developed calculus from scratch!

•Defined the 3 Laws of Motion that govern all objects

•Studied the effects of gravity (story about the apple)

Force

A force is a push or a pull.

Force is not a thing in itself, but rather an interaction between two objects.

Force is a vector quantity… direction matters in the answer!

Common Forces

Common Forces

Newton’s First Law

“The Law of Inertia”

A body remains at rest or moves in a straight line at a constantspeed unless acted upon by a netforce. Objects do not accelerate unless a net force is

applied.Speed up, Slow down,

or change direction

Newton’s First Law

Inertia is a property of an object most closely related to it’s mass(not to be confused with momentum) that explains why objects with greater mass resist a change in motion more than those with a lesser mass.

Net Force

Net force is the vector sum of ALLforces acting on an object. If there is zero net force, then there is zero

acceleration (constant velocity), this is a special case called equilibrium.

If there is a net force, there will be an acceleration. That means that the object will be speeding up, slowing down, or changing direction.

Free Body Diagrams A Free Body Diagram is a simple drawing that

shows the magnitude and direction of all of

the force vectors acting on an object.

The length of the arrows in relation to each

other is VERY important.

Each arrow must point away from the “free

body” and be labeled appropriately.

The system, the object the force is applied to,

is drawn as a shaded circle.

Free Body Diagrams

Here is an example of a

FBD of a book at rest on

a table top.

Fg is acting downward

but is “balanced” by FN

acting upward.

Results is no net force

and zero acceleration.

Fg

FN

The

book is

drawn

as a ball

Free Body Diagrams Here is an example of a

FBD of a box being

pulled by a rope at a

constant speed on a flat

surface.

Fg and FN are still

opposite and equal.

FT and Ff are also

opposite and equal.

Fg

FN

FTFf

Object is in motion,

but not accelerating.

Free Body Diagrams Here is an example of a

FBD of a ball under free

fall conditions.

Fg is the only force acting

on this object.

The net force is down

and the object is

accelerating.

Fg

Object is in motion

and accelerating.

Review Questions: p. 89 #1-5Draw a FBD for the following situations:

1. A flowerpot falls freely from a windowsill. (Ignore any

forces due to air resistance.)

2. A sky diver falls downward through the air at constant

velocity. (The air exerts an upward force on the person.)

3. A cable pulls a crate at a constant speed across a

horizontal surface. The surface provides a force that

resists the crate’s motion.

4. A rope lifts a bucket at a constant speed. (Ignore air

resistance.)

5. A rope lowers a bucket at a constant speed. (Ignore air

resistance.)

Review Questions Answers

#1 #2

Fg

Fg

Ff

Review Questions Answers

#3 #4

Fg

FT

Fg

FN

FTFf

Review Questions Answers

#5

Fg

FT

Newtons’ Second Law

The acceleration of an object is directly

proportional to the net force acting on

the object, and inversely proportional to

the mass of the object.

a Fnet

m a m

Fnet

Newton’s Second law of Motion …mathematically

Net Force = (mass)(acceleration)

Fnet = ma

NEWTON'S 2nd LAW

Fa

or amF

aFm

F am

m

F a

m

m

m

ma

1

F a

F a

F aM

M

M

Newton’s 2nd Law Practice

Two horses are pulling a 100-kg cart in the same direction, applying a force of 50 N each. What is the acceleration of the cart?

A. 2 m/s2

B. 1 m/s2

C. 0.5 m/s2

D. 0 m/s2

Answer B

Reason: If we consider positive direction to be the direction of pull

then, according to Newton’s second law,

netnet= ,Since 50 N 50 N 100 N,

Fa F

m

2100 N= =1m/s

100 kga

Newton’s Third Law

Action-Reaction LawTwo forces that make up an interaction pair of forces are equalin magnitude, but opposite in direction and act on differentobjects.

Newton’s Third Law

For every action, there is

always an equal (magnitude)

and opposite (direction)

reaction.

By “action” or “reaction”, we

mean a force.

Action/reaction forces do not

act on the same object.

Action: rocket pushes on gasesReaction: gases push on rocket

Action: tire pushes on road

Reaction: road pushes on tire

Identify at least six pairs of action-reaction

force pairs in the following diagram:

WeightLocation Mass

Earth

Moon

Orbiting

Earth

18.4 kg

18.4 kg

18.4 kg

180 N

30 N

0 N

1/6 of

Earth’s

Mass

Mass is the amount of matter in an

object (not to be confused with weight)

Also considered a measure of the

inertia of an object

measured in SI unit of kilograms

(kg)… if mass is given in grams you

must convert!

Weight

Weight is the downward force

upon an object due to

acceleration caused by gravity

weight = mass acceleration due to gravity

Fg = mg

measured in Newtons (N)

The weight of a 10 kg brick is...

A) 98 N

B) 10 kg

C) 9.8 kg

D) 10 N

E) 98 kg

Mass and Weight

On the Moon, the force of gravity is only 1/6 as strong as on the Earth. (approx. 1.63m/s2)

While orbiting, you are practically weightless but your mass remains unchanged.

Your mass does not depend on where you are. e.g. Earth, Moon, or space

Falling with Air Resistance

Air resistance (drag

force) increases with

speed and increased

cross-sectional area

and can be affected

by the size and shape

of an object.

Terminal VelocityAcceleration = g

Acceleration < g

Acceleration << g

Acceleration = 0

v = 0but motion is about to

begin

v increasing

downward

v still increasing

downwardjust not as rapidly as

before

terminal

velocity

Net Force

ZERO

Terminal Velocity

Terminal velocity occurs when the drag force of air

resistance becomes large enough to balance the force of

gravity.

At this instant in time, there is no net force — the object

stops accelerating (see D below); terminal velocity has been

reached.