Transcript Newton`s Second Law of Motion

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 motion
at 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?
CONSTANT
NEVER CHANGES
Depends on elevation
Varies by planet
Measured with
balance
scale (springs)
LINK:
Weight = Mass x Acceleration due to Gravity
On Earth, accel. due to gravity = -9.8 m/s2
Free Body Diagrams

Standard representation of the

relative magnitude and direction
of 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.
Fnet
1
a  Fnet
a
a
m
m
If standard units (kg, m/s 2 , and N) are used :
Fnet
a
or,
Fnet  m  a
m
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?
Fnorm = 98 N
Draw a free body diagram first!
Fnet = m·a = (10 kg)·(5 m/s2) = 50 N
Fapp = 75 N
F
=
?
fric
Fnet = Fapp – Ffric
50 N = 75 N – Ffric
Ffric = 75 N – 50 N = 25 N
Fgrav = 98 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 actionreaction 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 actionreaction 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.