This document is posted to help you gain knowledge. Please leave a comment to let me know what you think about it! Share it to your friends and learn new things together.
• Consider the following objects moving in circles at constant speeds`:• A ball tied to a string being swung in a circle • The moon as it travels around Earth• A child riding rapidly on a playground merry-go-round• A car traveling around a circular ramp on the highway
• For each example above, answer the following:• What is keeping the object in the circular path?• Are the objects accelerating?
• An object moving in a circle is accelerating.• So, there must be a force.• The force is always pointed towards the center!• This “center- seeking” force is called a
centripetal force centripetal force (LEARN THIS!!!)• The feeling that you are being pulled outward is
your INERTIA and is called centrifugal (center fleeing) and is a FALSE FORCE!!!
• Imagine yourself as a passenger in a car turning quickly to the left, and assume you are free to move without the constraint of a seat belt.– How does it “feel” to you during the turn? – How would you describe the forces acting on you during this
turn?
• There is not a force “away from the center” or “throwing you toward the door.”– Sometimes called “centrifugal force”
• Instead, your inertia causes you to continue in a straight line until the door, which is turning left, hits you.
• A 35.0 kg child travels in a circular path with a radius of 2.50 m as she spins around on a playground merry-go-round. She makes one complete revolution every 2.25 s.– What is her speed or tangential velocity? (Hint: Find
the circumference to get the distance traveled.)– What is her centripetal acceleration?– What centripetal force is required?
Imagine an object hanging from a spring scale. The scale measures the force acting on the object. • What is the source of this force? What is pulling or
pushing the object downward?• Could this force be diminished? If so, how?• Would the force change in any way if the object was
placed in a vacuum?• Would the force change in any way if Earth stopped
Newton’s Thought Experiment• What happens if you fire a
cannonball horizontally at greater and greater speeds?
• Conclusion: If the speed is just right, the cannonball will go into orbit like the moon, because it falls at the same rate as Earth’s surface curves.
• Therefore, Earth’s gravitational pull extends to the moon.
• If gravity is universal and exists between all masses, why isn’t this force easily observed in everyday life? For example, why don’t we feel a force pulling us toward large buildings?– The value for G is so small that, unless at least one of
the masses is very large, the force of gravity is negligible.
• Newton’s law of universal gravitation is used to explain the tides. – Since the water directly below the moon is closer than
Earth as a whole, it accelerates more rapidly toward the moon than Earth, and the water rises.
– Similarly, Earth accelerates more rapidly toward the moon than the water on the far side. Earth moves away from the water, leaving a bulge there as well.
– As Earth rotates, each location on Earth passes through the two bulges each day.
• Electric forces and gravitational forces are both field forces. Two charged particles would feel the effects of both fields. Imagine two electrons attracting each other due to the gravitational force and repelling each other due to the electrostatic force. • Which force is greater?
• Is one slightly greater or much greater than the other, or are they about the same?
• The force between two charged particles depends on the amount of charge and on the distance between them.– Force has a direct relationship with both charges.– Force has an inverse square relationship with distance.
• The electron and proton in a hydrogen atom are separated, on the average, a distance of about 5.3 10-11 m. Find the magnitude of both the gravitational force and the electric force acting between them.– Answer: Fe = 8.2 10-8 N, Fg = 3.6 10-47 N
• The electric force is more than 1039 times greater than the gravitational force. – Atoms and molecules are held together by electric
• A balloon is rubbed against a small piece of wool and receives a charge of -0.60 C while the wool receives an equal positive charge. Assume the charges are located at a single point on each object and they are 3.0 cm apart. What is the force between the balloon and wool?