EXAM II, PHYSICS 1403

November 2, 2009, Dr. Charles W. Myles

INSTRUCTIONS:Please readALLof these before doing anything else!!!

  1. PLEASE put your name on every sheet of paper you use and write on ONE SIDE of the paper only!! PLEASE DO NOT write on the exam sheets, there will not be room!
  2. PLEASE show all work, writing the essential steps in the problem solution. Write appropriate formulas first, then put in numbers. Partial credit will be LIBERAL, provided that essential work is

shown. Organized, logical, easy to follow work will receive more credit than disorganized work.

  1. The setup (PHYSICS) of a problem will count more heavily than the math of working it out.
  2. PLEASE write neatly. Before handing in solutions, PLEASE:a) number the pages & put the pages in numerical order, b) put the problem solutions in numerical order, & c) clearly mark your final answers. If I can’t read or find your answer, you can't expect me to give it the credit it deserves.

I HAVE145 EXAMSTO GRADE!!! PLEASE HELP ME GRADE THEM EFFICIENTLY BY FOLLOWING THESE SIMPLE INSTRUCTIONS!!! FAILURE TO FOLLOW THEM MAY RESULT IN A LOWER GRADE!!

A 8.5’’ x 11’’ sheet with anything on it & a calculator are allowed. Problem 1 (Conceptual) IS

REQUIRED! Answer any two (2) (this means all parts of the two!) of the remaining problems for a total of three (3) problemsrequired. Problem 1 is worth 34 points. Problems 2,3,4 are equally weighted at 33 points each.

1.REQUIREDCONCEPTUAL QUESTIONS!!! Answer briefly, in complete, grammatically\

correctEnglish sentences. I want answers which use mainly ENGLISHWORDS,NOTsymbols\

or equations! If you insist on using symbols, DEFINEall symbolsyou use! ZEROCREDIT will

be given for answers with ONLYsymbols!!!

a.StateNewton’s Universal Law of Gravitation.

b.State the Work-Energy Principle.

c.These questionsare about a box of mass m, under two differentconditions. In

both cases, itsits statically (not moving!) on a flathorizontal table.

1)Fig.1is the box’s free body diagram when the only forcesacting on itarethe

normal force FNfrom the table acting upwardit’s weight mgdownward. Is

the normal forceFN in this case equal &opposite to the weight?Which

Newton’s Law ofMotion did you use to answer this?

2)Fig. 2 shows the free body diagram when, in addition to the normalforce FN

weight mg, an additionaldownward force FP= 40 N acts on it whensomeone

pushes on the top. Is FNin this case equal oppositeto the weight?Which

Newton’s Law ofMotion didyou use to answer this?

d.See figure. A ball of mass m is twirled at the end of a string in a circle of constant

radius r & constant speed v. The ball’s free body diagrams at the top & at the

bottom of the circle are shown. Is the tension FTA that the string exerts on the ball

at the top of the circle (point A) less than, more than, or the same as the tensionFTB at

the bottom of the circle (point B)? WHY? Explain(in English!)youranswer using

Newton’s 2nd Law with centripetal acceleration.

e.I’ve stated that the Work-Energy Principle is one of Newton’sLaws of Motion,but

expressed in Work-Energy language rather than Forcelanguage. Fill in the blank:

The Work Energy Principle is Newton’s ______Law of Motion in Work-Energy

Language.

f.5 POINT BONUS!!!Near the end of the discussion of Newton’s Universal Law of Gravitation,

we talked about small objects orbiting larger ones, like artificial satellites around the Earth. As a

historical comment, I mentioned that Newton himself was the first to suggest putting objects in

Earth orbit. Briefly describe his idea for HOW to launch such objects into orbit. Be specific!(His

idea is NOTin the book! Newton didn’t know about rocket! Zero credit will be given for answers which talk about rockets).

g.5 POINT BONUS!!!Near the end of the gravitation discussion, we discussed the “effective

weightlessness” concept & the fact that reporters are VERYwrong when they say things like “the space shuttle has escaped the Earth’s gravity & is now in orbit.” In a few complete, grammatically correct sentences, EXPLAIN the reason that this statement is wrong.

NOTE: WORK ANY TWO (2) (all parts!)OF PROBLEMS 2., 3., or 4.!!!!!

2.See figure. A box, mass m = 25 kg, is placed on a flat, horizontal surface. There is friction. The coefficient of kinetic friction between the box and the surface is μk = 0.2. The box is pulled a by a force FP = 70 N using a cord that makes an angle θ = 30º with the horizontal.

  1. Sketch the free body diagram for the box, properly labeling all forces. Calculate the horizontal and vertical components of the force FP.
  2. Calculate the weight of the box and the normal force FN between it and the surface. Is this normal force equal (& oppositely directed) to the weight? Why or why not? Justify your answer using Newton’s 2nd Law in the vertical direction.Calculate the frictional force Ffr that the box experiences as it moves to the right.
  3. Use Newton’s 2nd Law to find the acceleration experienced by the box. What forcescause this acceleration?
  4. Calculate the work done by the force FPafter the box has moved a distance d = 10 m across the surface. Calculate the work done by the friction force Ffrover that same distance. The work done by the weight mg and the work done by the normal force FN over distance d are both zero. WHY? Answer that question using English sentences, NOT equations!
  5. Calculate the net work done by all forces on the box after it has movedthe distance d = 10 m. If the box starts from rest, use the Work-Energy Principle(& not kinematic equations of Ch. 2) to

calculate the box’s speed after it has moved the distance d.

3.See figure. A car, mass m = 3,000 kg, rounds a curve on a flat road at a speed v = 20 m/s. The figure shows a top view. The radius of curvature of the curve is r = 75 m. There is obviously (static) friction between the road & the car tires, or the car would not stay on the curve.

a.Sketch the free body diagram for the car, properly labeling all forces. (Hint:This will show a front view of the car, as opposed to the top view of the figure!).Calculate the normal force between the road and the car tires. Is this normal force equal (& oppositely directed) to the weight? If so, why? If not why not? Justify your answer using Newton’s Laws.

b.Calculate the centripetal acceleration experienced by the car.

c.Calculate the “centripetal force” experienced by the car. What physicalphenomenon is the cause

of this centripetal force?

d.Calculate the force of (static) friction between the tires and the road.

e.If the given speed v = 20 m/s is known to be the maximum speed for which a carwill not skid on

this curve, compute the coefficient of static friction μs betweentires and the road.

4.See figure.Note: YOU MUST use scientific (power of 10) notation to solve this problem. PLEASEbe carefuldoing this! A satellite, mass m = 2600 kg, is in a circular orbit at constant speed around planet X, assumed to be a uniform sphere of constant density. (The planet’s resemblance to Earth is coincidental!) The radius of the orbit (measured from the planet’s center) is r = 7.5  107 m. The planet mass is M= 7.3 1024 kg. The gravitational constant is G = 6.67  10-11 N m2/kg2.

  1. The satellite’s orbit is circular, so it experiences a centripetal acceleration. Using

words (not equations, which will get zero credit!) tell me what the cause of this acceleration is.

(Hint: See part b!)

  1. Calculate the gravitational force of attraction between the satellite and the planet. What is the

“centripetal force” on the satellite? (Hint: Answers to ab should be consistent! You don’t need to

know the satellite’s speed to answer this!).

  1. Calculate the centripetal acceleration experienced by the satellite. What is it’s direction? (Hint: This will be very small! You don’t need to know the satellite’s speed to answer this!).
  2. Calculate the speed of the satellite in orbit. (Hints: Einstein told us that the largest speed possible foris the speed of light c = 3 × 108 m/s! If yourspeed is larger than c, or even a significant fraction of it, you’ve done something wrong! But, v should be large enough for the satellite to go the HUGE distance around the orbit in a reasonable time.FYI: Earth satellites can make 2 or more full orbits per day. Also, if you find a v as slow as that of ordinary objects moving on the Earth’s surface, such as, for example, 100 m/s, that’s much too slow & you’ve done something wrong!).
  3. Calculate the period of the satellite’s orbit. (Hint: This should be a reasonable period for a satellite around a planet similar in size to Earth. For a typical period, see the hint for part d.)