Newton’s Laws of Motion What are forces? How can diagrams be used to depict and analyze the forces...
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Transcript of Newton’s Laws of Motion What are forces? How can diagrams be used to depict and analyze the forces...
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 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
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.