Introduction to Forces Guided Notes
Name:____________________ Class:________
Forces throughout History
_________________________ (384-322 BC) believed that
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heavenly bodies (sun, moon, etc.) had circular motion
all earthly things had natural ‘resting places’ that they would return to
motion of earthly things was ‘violent motion’ that had external causes (being pushed or pulled)
For 2000 years after Aristotle, it was thought:
• Movement is against nature state
• No Force = no movement
In the 16th c. ________________________________ secretly developed his heliocentric theory – the theory that the
Earth and planets orbit the sun. Knowing it would be controversial, he published it only shortly before his death.
In 17th c. ____________________________ also theorized that force is NOT needed to keep an object in the motion
(straight-line, constant speed). Instead, he believed forces change motion.
Every object resists change to its state of motion/velocity. To change it, the force must act on it. We call this resistance
INERTIA.
Important Vocab
 Inertia ______________________________________________________________________________
• sort of laziness
(inerzia – laziness in Italian)
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Mass _____________________________________________________________________________
• more mass – harder to change velocity
____________________________________________________________________________
• depends only on the number and kind of atoms in it.
• doesn’t change due to gravity (same on moon as Earth)
Unit of Mass
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Weight ___________________________________________________________________________________
• depends on its mass and acceleration due to gravity:
W = mg
The acceleration due to gravity on Jupiter is more than two times
greater than that on Earth. How would the following quantities
change on Jupiter?
Show 1 = Larger on Jupiter, 2 = smaller on Jupiter, 3 = no change
Your weight
Your mass
Your inertia
Forces
• ___________________________________________________________________________
• Causes objects to accelerate
NOTE: ALL forces are _________________________________ between 2 objects
Forces are vector quantities, having both direction and magnitude.
Unit of force
1 N is the force that causes a 1-kg object to accelerate 1 m/s2.
The net force – resultant force
is the ________________________________________________________________________________________
It is the net force that changes an objects motion!
Check your understanding
Newton’s 1st Law
BIG IDEA 1: Zero NET force means no acceleration / no change in velocity. Balanced forces will not cause
acceleration.
BIG IDEA 2: An object is in equilibrium (i.e. it has zero net force) whenever it has constant velocity (including
constant velocity of 0).
How is Newton’s 1st Law related to Galileo’s idea of inertia?
Problems
o An object at rest has two forces acting on it. One force is 8N to the left.
What is the other force?
o
An object that experiences two forces moves to the right with a constant velocity of 3 m/s. One force is 1N
to the right. What is the other force?
o
Six force are acting on an object. What can you tell about the motion of that object? Is it at rest? Is it
moving? If it is moving, how?
Newton’s Second Law
The acceleration of an object produced by a net force on
that object is directly proportional to the net force applied,
and inversely proportional to the mass of the object.
Direction of the acceleration is in the direction of the net
force.
How does mass affect acceleration?
… greater mass
_________________________________________________
_________________________________________________
How does force affect acceleration?
… greater force
_________________________________________________
… zero force
_________________________________________________
Newton's Third Law
Whenever object A exerts a force on object B, object B exerts an equal in
magnitude and opposite in direction force on object A.
In every interaction, the forces always occur only in pairs, BUT _______________________________________________.
Common definition: To every action there is an equal and opposite reaction
Don’t use this! The problem with the common definition is that it isn’t clear that the forces happen to two different
objects.
Example: Forces between roller-skaters
What will happen when these two skaters push against each other?
Example: skater pushing against wall
What will happen when this skater pushes against a wall?
Types of Forces Guided Notes
Name:__________________________
Forces can be divided into two types:
1. __________________________________ occur because of physical contact between objects.
Examples: pushing open a door
pulling on a rope
2. ___________________________________ act at a distance through space. The presence of an object effects
the space around it, creating a region of space around the object called a field.
Example: gravitational field
magnetic field around a magnet
Contract Forces
⃗n
_______________________________ F
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Force that prevents an object from falling through the surface of another body
Always acts __________________________________________________
Always equals the forces applied to the surface (or surface will break!)
Frictional Forces Ffr
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Friction is a force that is created whenever two surfaces move or try to move across each other.
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Friction always opposes the motion or attempted motion of one surface across another surface.
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Friction is dependent on the texture/roughness of both surfaces. Friction is also dependent on the force which
presses the surfaces together.
Air resistance / Drag
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When an object moves through air or any other fluid, the fluid exerts a friction-like force on the moving object.
The force is called drag.
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Drag depends on the speed of the object, becoming larger as the speed increases. (UNLIKE FRICTION!)
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Drag also depends on the size and the shape of the object and the density and kind of fluid. (UNLIKE FRICTION!)
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B/c drag increases with speed, an object falling through the air reaches a terminal velocity– a maximum speed
at which the force of drag is equal and opposite to the force of friction, so there is no more acceleration.
Tension
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the force that the end of the rope exerts on whatever is attached to it.
Direction of the force is along the rope.
Spring Force
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Force due to the elasticity of a material
Depends on the elasticity of the spring
Direction is opposite displacement
How to solve force problems
1. Draw a free body diagram – label all the forces acting on one object.
2. Add up the forces
3. Apply Newton’s second law: F = ma.
We Do Problems
 Superman lifts a 35,000 kg truck off Jimmy Olsen at a constant velocity of 10 m/s. How much force did
Superman apply to the truck?
 Now Lois Lane is the one trapped under a semi. Superman applies a 450,000 N force to fling the truck off of
Lois. How fast does the truck accelerate?
You Do Problems
 A bookbag is hanging from your shoulder by one strap. Draw a free body diagram, making sure that your forces
are proportional.
 A fish accelerates as it swims horizontally through the water. It experiences an applied force (forward from its
fins), drag, buoyancy (upward force from the water), and gravity. Draw the free body diagram, making sure that
your forces are proportional.
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Does a net force exist? Yes or No.
For each problem, identify the value of the forces.
For each problem, determine the acceleration of the blocks. Each block has a mass of 10 kg.