12.1 forces · 12.1 forces . forces ... question #1 how is the motion of an object affected when a...
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12.1 Forces
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Forces
• A Force is a push or a pull on an object
Forces can cause a resting object to move
Forces can cause a moving object to accelerate by changing its speed or its direction
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Measuring Force
Force is measured by a spring scale (like at the supermarket)
Force is measured in Newtons (N)
o Named after Sir Isaac Newton
Force can be represented by an arrow on a diagram or graph as a vector.
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Spring Scales
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Force Math
Force arrows in the same direction add together Force arrows in the opposite direction subtract from
one another When the forces on an object are balanced, the net
force is zero and there is no change in the object’s motion
When the forces on an object are unbalanced, the object accelerates.
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Force Math
What happens in this situation situation?
The truck stays still
The truck moves to the right at 40 N
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Normal force When an object lies on a table or on the ground, the table
or ground must exert an upward force on it, otherwise gravity would accelerate it down.
This force is called the normal force (FN).
“Normal” means perpendicular (right angle). A normal force is always perpendicular to the contact surface.
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Normal force
weight
N
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If a flower pot is setting on an incline, the normal force is not vertical; it’s at a right angle to the incline.
Normal force
N
weight
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Tension Force A pulled rope has a force exerted on it.
This force is called tension and points along the rope.
Box Tension (T)
T
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Tension and normal forces are different.
o A pull on an object: Tension o A push from a surface: Normal Force o Either one or both may be present.
Normal Force
N
Weight
T
Tension and Normal Force
Tension Force
W
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Friction
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Friction The force that opposes motion of objects that are in contact with each other. Frictions depends on two things:
• Type of Surface or material • The force pushing the surfaces together
Examples:
Pulling a sled on snow vs. grass
Friction
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Friction
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Four Types of Friction:
1. Static 2. Sliding 3. Rolling 4. Fluid
Friction
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Static Friction Friction of objects that are not moving
Acts in the opposite direction of motion Static friction between your shoe and the floor keep it
from sliding when you take a step
Static Friction
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Sliding Friction
A force that opposes motion as it slides over a surface
Sliding friction is less than static friction, therefore object keeps moving
Sliding Friction
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Sliding Friction Examples
Helpful uses of Sliding Friction
1. Striking a match 2. Tires on road 3. Nail or screw in wood 4. Opening a jar lid with your hand 5. Brake pad on a disc brake 6. Gripping a bat or racquet 7. Bow on violin strings (plays music)
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Sliding Friction Examples
Harmful uses of Sliding Friction
1. Moving furniture on carpet 2. Pistons on cylinder walls 3. Sliding on gym floor (floor burn) 4. Hand on shovel handle (blisters) 5. Valves or slides sticking on trumpet/trombone 6. Drawers hard to open 7. Ring stuck on finger
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Rolling Friction
The friction force that acts on rolling objects
100 to 1,000 times less than sliding friction
So why do we use wheels to move heavy objects?
Rolling Friction
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Rolling Friction
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Fluid Friction
Opposes the movement of an object through a liquid or a gas
• Air Resistance is a type of fluid resistance
Fluid Friction
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Fluid Friction
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Helpful uses of fluid friction
1. Slowing down the space shuttle & parachute for landing 2. Burns more calories during water aerobics 3. Burns up meteors
Fluid Friction
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Harmful uses of fluid friction
1. Slowing down skis & boats in water 2. Oil flowing in a pipeline 3. Slowing down cars and planes
Fluid Friction
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Question #1 How is the motion of an object affected when a force acts on it?
A force can set an object at rest into motion, or it can accelerate a moving object by changing is speed or direction.
12.1 Assessment
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Question #2 List four types of friction
1. Static Friction 2. Sliding Friction 3. Rolling Friction 4. Fluid Friction
12.1 Assessment
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Question #3 You push a box and are unable to move it. What force opposes your push?
Static Friction
12.1 Assessment
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Question #4 If the net force on an object is zero:
a. Then object is stopped b. Then the object continues as it was c. The object accelerate d. The object explodes
The object continues as it was
12.1 Assessment