PHY 211 Lecture 11

Matthew Rudolph

Syracuse University

February 20, 2020

Job opportunity

If you are looking for a job, the experimental high energy physics groupis looking for undergraduates to help with testing fabricated detectorpiecesIt’s not glamorous work, but it is paidWould schedule about 10 hours per weekLet me know if you would like to know more

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Plan for today

Focus on breaking down force problems, primarily involving circularmotionRemember that circular motion is just a special case of our generalforce problems where we know the magnitude and direction of theaccelerationAdd gravitation as another way to find the accelerationShould have time for some HW/recitation review

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Pre-lecture 1

If a roller coaster is going around the inside of a loop, at which point isthe normal force greatest?

A At the topB At the sideC At the bottomD All of these have the same normal force

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Pre-lecture 2

When a ball on a string is spinning around a vertical pole, the stringangles down towards the ground instead of being perfectly horizontal.Which way does the acceleration of the ball point?

A Along the stringB Horizontal towards the poleC Towards the groundD Horizontal away from the pole

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Plumb bob

A plumb bob is hanging inside a traincar. This train is travelling at aconstant speed of 90 km/h around acurve in the track with a radius of300 m. At what angle with respect tothe vertical will the plumb bob hang?

θ

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Greek waiter’s tray

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Problem 1

Assume that the length of the chain holding the trayis 1 m. If I hold the tray at 45◦ and spin around onceper second, what is the minimum coefficient of staticfriction needed to keep the glass on the tray?

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Question

Which way should I draw friction?A Along the tray, into the turnB Along the tray, out of the turnC Some other direction

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Gravitation

All mass in the universe exerts a pull on all other masses in theuniverseIt gets weaker the smaller the masses and the farther apart they are.

Magnitude of gravitational force

Fg =GMm

r2

Both masses are pulled by this force (Newton’s third law!)G = 6.673×10−11 Nm2/kg2 is a fundamental constant of nature (notderived)

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How massive is the Earth?

The Earth’s radius is 6371 km. We measuredhow much we accelerate in free-fall (g). Howmassive is the Earth?

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Question

Does it matter that the Earth is spinning?A YesB No

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Question

How much do you accelerate the Earth?

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Orbits

Many orbits are very close tocircularIn this case, gravitation is theforce that causes the centripetalacceleration

~v

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How massive is the Sun?

The Earth is 147.88×106 km from the sun, and travels in an almostcircular orbit. How massive is the sun?

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Homework review

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