3/2/2018 1 FORCES Learning Targets 1. I can define the net force of an object as the sum of all of the forces acting on it. 2. I can create and use force diagrams / free body diagrams to compare the forces on an object. 3. I can compare an object’s direction and speed when its net force is zero and when its net force is greater than zero. 4. I can analyze and compare forces that oppose motion of an object.
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3/2/2018
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FORCES
Learning Targets1. I can define the net force of an object as the sum of all of the
forces acting on it.2. I can create and use force diagrams / free body diagrams to
compare the forces on an object.3. I can compare an object’s direction and speed when its net force
is zero and when its net force is greater than zero.4. I can analyze and compare forces that oppose motion of an
object.
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Vocabulary within this set of notes1. Force:
2. Gravity:
3. Friction:
4. Air Resistance:
5. Drag:
6. Newton (N):
7. Net Force:
8. Normal Force:
9. Free Body Diagram (Force Diagram):
Types of
Forces
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What is a force?
A push or a pull‐
“Contact Forces”‐ If your objects are touching
Push Pull
What is a force? • There are other types of forces called “Field forces” where your objects do not need to touch!
– Gravity
– Electromagnetism (Magnetic and electric fields)
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Types of forces!GRAVITY:• The force that
ATTRACTS all masses towards each other
• Objects with MOREmass have more gravity
• WEIGHT is a measure of how much gravity’s pull
• Mass is a measure of how much matter is in an object.
WEIGHT AND MASS ARE 2 DIFFERENT THINGS!!!!!
Types of Forces!
FRICTION
• Acts in the OPPOSITE direction of motion
• Friction causes objects to SLOW DOWN and eventually STOP.
• Rough surfaces produce MOREfriction than smooth surfaces.
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Types of forces!• Air Resistance• Force of AIR working
against an object
• Also called DRAG
• “Friction with the air!”• What direction would air
resistance be for this skydiver?
• Why do they use parachutes?
How do we measure force?
The metric unit used to measure force is the Newton (N)
Forces have both a MAGNITUDE (size) and a
DIRECTION. (It is a vector measurement, Oh
Yeah!)
Example: The sheep pushed the man 5 N down
hill.
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Direction of the force
The direction of a force is the direction of the push or pull.
Be sure to include a direction in all your answers!
In what direction is the push or pull below?
Combining Forces
When more than one force acts on an object, the forces combine.
• The combination of all the forces on an object is called the net force. This means that the forces are unbalanced.
• If an object is experiencing a net force, it will acceleratein the direction of that net force!– Accelerate: This means that the object will speed up, slow down, or direction!
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How do forces in the same direction combine?
ADD the forces together to form net force.
What is the net force below? 10 N to the right
How do forces in opposite direction combine?
SUBTRACT: larger force minus the smaller force
What is the net force below?
In what direction will the object move?
Larger force ‐ smaller force6 N ‐ 3 N
= 3 N
To the left
3 N
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Balanced ForcesWhat do balanced forces look like?
When forces are balanced, there is no change in the object’s speed.If the forces are balanced, Net Force = 0
So, the object will NOT accelerate!1. An object standing still will not move2. An object moving at a certain speed will keep that same speed!
Unbalanced Forces
• When forces are unbalanced, there will be a change in the object’s speed or direction!
• The object will accelerate in the direction of the net force. (VERY IMPORTANT)
What do unbalanced forces look like?
Book slides across table
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Balanced or Unbalanced? 1. You hold a basketball motionless over your
head.
2. You are skydiving in a squirrel‐suit and you notice your speed is increasing a you jump from a cliff!
3. You take a car for a spin, literally, and drive in circles.
What is the normal force? • The normal force counteracts gravity and prevents objects from being pulled to the center of the Earth. If a book sits on a table, or you stand on the sidewalk, the normal force pushes upward, balancing gravity
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What is the applied force?
• Any push or pull directly on the object is called an “applied force”.
Force Diagrams
• Free body Diagrams (force diagrams) Help show the forces on an object.
15 N down
35 N up
1. What is the net force acting on the object?
2. Are the forces balanced or unbalanced?
3. Which way will the ball move?
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Example 1
• Net force? • 4N‐3N
• =1N
• Direction? – To the Right
Example 2
• Net force? • 4N+3N = 7N• 8N – 7N• =1N
• Direction? – To the Left
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Example 3
• Net force? • 3N + 3N
• =6N
• Direction? – To the Left
One final word about free body diagrams / force diagrams
• Sometimes they will not write the magnitude of the force, but will just use arrows. The longer the arrow, the stronger the force.
• If two forces are meant to be the same, a single line will be drawn through them.
This plane is not accelerating, but is flying at a constant speed. All forces are balanced.
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