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Chapter 6: Lesson 3 - Centripetal Force
Chapter 6: Lesson 3 - Centripetal Force
Centripetal Force
Centripetal Force
When an object moves in a circle there has to be some force that is causing it to deviate from its straight line path. This force is called a centripetal force. A centripetal force is some physical force pushing or pulling the object towards the center of the circle. Think of the word centripetal as center seeking. The force goes towards the center of the circle.
The centripetal force is not a new force, it just describes the direction of the net force acting on an object. Physicists are lazy... they don't want to say, "the net force of an object traveling in a circle." They would rather just say, "centripetal force" and people will know they mean the same thing.
Demonstration!
Demonstration!
When an object moves in a circle there is a force acting upon it to cause it to deviate it from its normal straight line path.
In this demonstration, watch which way the ball moves when I spin it in a circle.
In the demonstration above, the ball moves inward and along the circular path. The circular path in this demonstration is the circle that I moved the jar in.
The same thing would happen if we used a candle flame. If you turn around in a circle, (not too fast that you blow out the candle,) then the flame will move in towards the center of the circle.
Examples of Centripetal Force
Examples of Centripetal Force
Below are four examples of centripetal force. Be sure to watch my two demonstration videos!
Car Making A Turn
Car Making A Turn
As a car makes a turn, the force of friction acting upon the turned wheels of the car provides the centripetal force required for circular motion.
Moon Orbiting the Earth
Moon Orbiting the Earth
As the moon orbits the Earth, the force of gravity acting upon the moon provides the centripetal force required for circular motion.
Rotation of the Earth
Rotation of the Earth
What is the shape of the Earth? Why is it that way? Watch the video to find out!
Spinning a Bucket of Water
Spinning a Bucket of Water
As a bucket of water is tied to a string and spun in a circle, the force of tension acting upon the bucket provides the centripetal force required for circular motion.
Will the water fall on my head when I spin it around? Watch the video to find out!
Centrifugal Effect
Centrifugal Effect
The centripetal force points towards the center of the circle, but when you spin something in a circle, you feel a force pulling away from the center. This is actually the object's tendency to move in a straight line, which is Newton's First Law of Motion. As we have mentioned before, physicists want an easy way to express something. Saying that an object is pulling away from the center of a circle due to inertia takes too long. Saying the centrifugal (pronounced cen-tri-fi-gul) effect is much easier.
You will sometimes see the word force next to centrifugal. That is technically incorrect. The centrifugal effect is not a force. For a force to occur, you have to have an interaction between two objects. When I was spinning the bucket around in a circle, I was not interacting with the water, the water just wanted to go in a straight line, but the bottom of the bucket was preventing that from happening. I was providing the centripetal force on the bucket and the bucket was providing the support force to keep the water going in the circle. The water just wanted to go in a straight line, due to Newton's First law of Motion. We could also say that there was a centrifugal effect on the water.
When you see the word centrifugal, specifically the 'f' in centrifugal, think fleeing, or away from the center. You can also think false force. The centrifugal effect is not a force, it is just an effect of inertia.
Inertia and the right hand turn
Inertia and the right hand turn
When making a left hand turn in your car, your tendency is to be thrown to the right. This is because of inertia rather than a force.
Your car is turning to the left, but you want to continue in a straight line. It isn't until your seat belt or the door of the car pulls on you to get you to travel with the car that you feel that force pulling you back to the direction that the car is traveling in.
When you push against the door of the car, the car pushes back on you with the same force.
Rotational Inertia
Rotational Inertia
Rotational inertia is the tendency to resist a change in the direction of its axis of rotation. The faster an object spins, the greater the amount of force needed to change its position.
This concept is called the gyroscope effect and is the cause of a gyroscope's stability. The faster an object spins the harder it is to change its state of motion.
Mrs. Markey helps with another demonstration!
Mrs. Markey helps with another demonstration!
Is it easier to ride a fast moving bicycle or a slow moving bicycle? Mrs. Markey tries to move a bike tire when it is stationary and when it is moving. When do you think it will be harder to change the tire's state of motion?
Rotational Inertia and Gyroscopes
Rotational Inertia and Gyroscopes
A gyroscope is a device used for measuring or maintaining orientation and angular velocity. It is a spinning wheel or disc in which the axis of rotation (spin axis) is free to assume any orientation by itself.
The faster it spins, the harder it is to change its state of motion. Watch my video to see a quick demonstration.
Below are three great videos that explain gyroscopic precession, rotational inertia and why something appears to weigh less when it is spinning in a circle. Be sure to watch the videos in order so they make sense!
Gyroscopic Precession Explained
Gyroscopic Precession Explained
Watch this video first to understand gyroscopic procession.
Anti-Gravity Wheel?
Anti-Gravity Wheel?
It appears to be easier to lift a fly-wheel when it is spinning rather than when it is stationary. Make a prediction at the end of the video... will his weight increase, decrease, or stay the same when lifting the spinning fly-wheel?
Anti-Gravity Wheel Explained
Anti-Gravity Wheel Explained
Check your prediction in this last video and see the explanation of why!
You have finished Lesson 3!
You have finished Lesson 3!
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