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Physics I Honors
Specific Forces
Centripetal Force
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Physics I Honors
Warm up
• Planet Zonk is twice as far from the sun as Earth. What is its period?
• Herman the astronaut ( mass 80kg) is floating in orbit about earth and feels a gravitational attraction of 360 newtons. What is the value of gravity at his location?
• What is his orbital radius as a multiple of earth’s radius.
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Physics I Honors
Objectives
· Explain why a body moving with uniform circular motion must be regarded as undergoing acceleration and state the direction of the acceleration.
· Predict the path of an object which has a force acting on it perpendicular to the direction of its velocity.
· Indicate the direction of the instantaneous velocity, centripetal acceleration, and centripetal force for an object moving at constant velocity in a circle.
· Calculate centripetal acceleration and force.
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Physics I Honors
Imagine…Imagine…• Think about the moon on its journey around
the earth…• How about an amusement park ride that
spins you around in a constant circle…• How do you describe this kind of circular
motion?• Is the velocity constant or is it accelerating?• How does the moon stay up there in the
sky?
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Physics I Honors
What Is Uniform Circular Motion?
• Uniform Circular Motion is motion in a circle with: – Constant Radius R
– Constant Speed v = |vv|
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Physics I Honors
What if the String Breaks?
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Physics I Honors
Centripetal Force
• Provides acceleration toward the center which causes an object to continually change velocity because it is changing direction while the speed remains constant.
Fc = mac
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Physics I Honors
What happens when you turn.
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Physics I Honors
Without Centripetal Force
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Physics I Honors
With Centripetal Force
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Physics I Honors
Force is Perpendicular to the Velocity
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Physics I Honors
Check your understanding
An object is moving in a clockwise direction around a circle at constant speed. Use your understanding of the concepts of velocity, acceleration, and force to answer the next five questions. Use the diagram shown at the right.
A
B
C
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Physics I Honors
Vector 1
1. Which vector below represents the direction of the force vector when the object is located at point A on the circle?
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Physics I Honors
Vector 2
• 2. Which vector below represents the direction of the force vector when the object is located at point C on the circle?
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Physics I Honors
Vector 3
• Which vector below represents the direction of the velocity vector when the object is located at point B on the circle?
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Physics I Honors
Vector 4
• 4. Which vector below represents the direction of the velocity vector when the object is located at point C on the circle?
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Physics I Honors
Vector 5
• 5. Which vector below represents the direction of the acceleration vector when the object is located at point B on the circle?
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Physics I Honors
Motion in a circle
• Velocity is a vector; it has direction• An object travelling in a circle at constant speed is
continuously changing direction• Therefore, it is accelerating• Thus, requires a force
r
2v
a
rmmF
2v
a
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Physics I Honors
Centripetal Acceleration
• This acceleration can be calculated
ac = v2 / r
a is the centripetal acceleration
v is the speed
r is the radius of the circle
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Physics I Honors
Circular Speed
• As speed is distance traveled divided by the time it takes to travel the distance
v = 2r/Tv is the speed
r is the radius of the circle
T is the period of revolution
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Physics I Honors
Practice 1
A runner moving at a speed of 8.8 m/s rounds a bend with a radius of 25 m.
What is the centripetal acceleration of the runner?
3.1 m/s2
What provided this acceleration to the runner?
friction between his shoes and the track
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Physics I Honors
Practice 2
Racing on a flat track, a car going 32 m/s rounds a curve 56 m in radius.
What is the car’s centripetal acceleration?18 m/s2
What would be the minimum coefficient of static friction between tires and road that would be needed for the car to round the curve without skidding?
1.8
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Physics I Honors
Homework
• A/c # 6 and 7
• Problems 13 to 20