class 091028

Class 091028

STUDENT QUESTION

Could you tell me how to solve for the 'dPE, and could you give it to me in equation form if possible, or just tell me which qa gives the info.

INSTRUCTOR RESPONSE

The answer to your question depends on the situation. I'm giving you a general answer here; after working to understand this, if you have additional questions for a specific situation, I'll be glad to help you see how to apply these ideas. Just be sure to include a copy of this document, as well as the specific situation you want to address and your best thinking about that situation.

Any time you think about potential energy, you should immediately remind yourself of the definition of potential energy:

`dPE for a system is equal and opposite to the work done by the conservative force on the system for the interval in question.

This definition involves work, so you should also remind yourself of the definition of work:

Work is the product of force and displacement in the direction of the force.

The definition of work involves force and displacement.

So if you know the conservative forces and the initial and final position of the system, you can find `dPE:

Identify the displacement `ds for the interval and multiply it by the conservative force.

Be careful about the directions of the force and the displacement. The displacement you use is either in the same direction as the force (in which case the force and displacement have the same sign and the work is positive), or opposite the direction of the force (in which case the force and displacement have opposite signs and the work is negative).`

You can also determine `dPE if you know the work done by nonconservative forces and the change in KE:

The work-energy theorem tells us that `dW_NC_ON = `dPE + `dKE.
It follows immediately that `dPE = `dW_NC_ON - `dKE.

`q001. ** One-step-at-a-time questions:

If the velocity of a 5-kg object changes from +3 m/s to -2 m/s, what is the change in its velocity, and what is the change in its momentum?

A 12-kg object experiences a velocity change of +2 m/s during a time interval of .01 second. What average force was exerted on it during that interval?

What is the resultant of two displacement vectors, the first having magnitude 8 meters directed at angle 53 degrees, the second with x component -3 meters and y component -4 meters?
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Two objects collide, one of mass 5 kg and the other of mass 12 kg. The momentum of the 5 kg object changes by -40 kg m/s. What is the momentum change of the 12 kg object, and what is the change in its velocity?

What is the direction of a displacement vector whose x and y components are +5 m and - 4 m? What are the components of a force vector of magnitude 80 N in the same direction?

A rubber band chain exerts force 4 N when its length is 50 cm, and 12 N when its length is 80 cm. As the chain is extended from the first length to the second, what average force is exerted by the rubber band (assuming that the force vs. length is linear)? How much work is done by the chain during this interval?

A 200 kg mass is subjected to a net force of 1000 Newtons for half a second. What is its acceleration? What therefore is its change in velocity? What therefore is the change in its momentum? What is the impulse of the force on this interval?

`q002. Give your data for the experiment in which you attempted to release the pendulum the point that would maximize its horizontal range. Include all data that would be necessary to replicate your trials.

Attempt to organize your data in such a way that it is easy to read at a glance.

Include a description how you conducted the experiment.

`q003. How much force would it take to accelerate an Atwood machine consisting of two 50 kg masses at 9.8 m/s^2?

What is the total weight of these two masses?
If the addition of a single dollar coin to one side of the system results in an acceleration of 5 cm/s^2, then what is the mass of the coin? What is the weight of the coin?
How many coins would it therefore take to match the mass of the system?
(Assume that the friction in the pulley is negligible).

`q004. Three rubber bands are attached to a small ring at the origin of an x-y coordinate system. The opposite ends of the rubber bands are respectively at the points (8 cm, 6 cm), (-10 cm, 0) and (0, -12 cm).

· For each rubber band, determine the angle of the displacement vector from the origin to its other end.

Using a graph of force vs. length for each rubber band, we find that they exert forces of 5.0 N, 3.8 N and 3.2 N, respectively.

· Find the x and y components of each each force.

· Use your results to find the x and y components of the net force, and

· find the magnitude and direction of the net force.

`q005. A ball starts rolling along the x axis at point x = x0, with velocity v0. Its acceleration is a. This continues through time interval `dt.

By how much does the velocity of the ball change?
What is the ball's displacement during the interval?
What therefore is its final position along the x axis?

Homework:

Your label for this assignment:

ic_class_091028

Copy and paste this label into the form.

Report your results from today's class using the Submit Work Form. Answer the questions posed above.

Note that you should be preparing to take Test 1 sometime next week.

You have been assigned the first six problems, and should now learn the rest of the problems in Set 5 of the Introductory Problem Set. A link that should work is at http://vhmthphy.vhcc.edu/ph1introsets/default.htm .

You should also read Chapter 6 in your text, which can serve you in the future as a good organized reference. By Monday you should have read Chapters 2, 4 and 6. You should make note of some of the differences in notation (e.g., my use of `dW_NC_ON where the book uses just W_NC).

Work through and submit q_A_18, q_A_19 and q_A_20

URL's of qa's 10-20:

http://vhcc2.vhcc.edu/dsmith/genInfo/qa_query_etc/ph1/ph1_qa_10.htm
http://vhcc2.vhcc.edu/dsmith/genInfo/qa_query_etc/ph1/ph1_qa_11.htm
http://vhcc2.vhcc.edu/dsmith/genInfo/qa_query_etc/ph1/ph1_qa_12.htm
http://vhcc2.vhcc.edu/dsmith/genInfo/qa_query_etc/ph1/ph1_qa_13.htm
http://vhcc2.vhcc.edu/dsmith/genInfo/qa_query_etc/ph1/ph1_qa_14.htm
http://vhcc2.vhcc.edu/dsmith/genInfo/qa_query_etc/ph1/ph1_qa_15.htm
http://vhcc2.vhcc.edu/dsmith/genInfo/qa_query_etc/ph1/ph1_qa_16.htm
http://vhcc2.vhcc.edu/dsmith/genInfo/qa_query_etc/ph1/ph1_qa_17.htm
http://vhcc2.vhcc.edu/dsmith/genInfo/qa_query_etc/ph1/ph1_qa_18.htm
http://vhcc2.vhcc.edu/dsmith/genInfo/qa_query_etc/ph1/ph1_qa_19.htm http://vhcc2.vhcc.edu/dsmith/genInfo/qa_query_etc/ph1/ph1_qa_20.htm