PHYS 211	Test #2	October 25, 2002

NAME:____________________________________________________________

General Instruction: You may do any four of the following six questions (25% each). You may do the remaining two for 5% bonus points each. Please circle the bonus question numbers. Show your work clearly and concisely.

-I-

A)	According to ancient Greek astronomers, heavenly bodies such as the moon and the planets follow circular orbits. They believed circular orbits were natural and a force was not required to maintain such a motion. Is this consistent with Newton's Laws? Explain why or why not?

B)	Using Newton's first law, explain what happens to motorists who do not wear seat belts when their cars collides with a stationary object.

C)	According to an exaggerated story, a man sneezed so hard that he fell backwards from his tall stool bar. Which Newton's Laws, if any, does this illustrate?

D)	An engineer suggests that a rocket is propelled due to the reaction force its exhaust experiences in air. He concludes, therefore, in vacuum the reaction force will not be available and it will be impossible to propel a rocket. Is this true or false? Explain.

-II-

A.	Block B in the figure at left has a 2 kg mass. If k = 0.5 for the
table top and the block, calculate the maximum mass of block A for
which the system will be in static equilibrium.

B.	An airplane of mass m if flying around a horizontal circle of unknown
radius R at a speed of 500 km/hr with its wings tilted by 400 to the
horizontal. The airplane is supported by the "aerodynamic lift" Flift that
is perpendicular to the wing surface. (Hint: Flift has a vertical component
that supports the weight of the airplane and horizontal component that
causes centripetal acceleration). Calculate R in meters.

- III -

In the figure at left, the mass of A = 50 kg, k = 0.4, and = 370.

A.	If mass A is to slide up at constant speed,

What is the mass of the hanging block?

B.	Calculate the tension of the string connecting the masses.

C.	If the string breaks, calculate the accelerations of the masses A and B.

-IV-

A physics student wants to pull a box of mass M = 100kg on a level floor by using the minimum possible force that makes an angle with the horizontal. The coefficient of kinetic friction between the box and the floor is k

A.	Derive an expression for opt so that the applied force is minimum? Show your work. (Hint: First obtain an expression for F as a function and then use the condition for minimum from calculus.)

B.	If k = 0.577, what is the applied force when = opt is used?

C.	What is the work done by the student if he pulls the object with the minimum force for a distance of 10 meters

-V-

(A) An object experiences a displacement = (4i + 5j −6k)m due to a force F = (5i − 2j −2.5k)N. What is the work done? What is the angle ( ) between the displacement and the force?

(B)	An object moves from xi = 0.1m to xf = 1m under the influence of a force F = (−10x2)N/m2 . Calculate the work done on the object.

(C)	A 2 meter tall student raises a 50 kg back pack from the floor to the top of his head in 4.9 seconds. What is the work done? What is the student's power in Kw?

-VI-

A)	When a nucleus captures a stray neutron, (The neutron's mass is 1.67 × 10-27 kg.) it must bring the neutron to a stop within the diameter of the nucleus by means of the strong force. That force, which "glues" the nucleus together, is approximately zero outside the nucleus. Suppose that a stray neutron with an initial speed of 2.6 × 107 m/s is just barely captured by a nucleus with diameter d = 1.2 × 10-14 m. Assuming that the strong force on the neutron is constant, find the magnitude of that force using the two methods suggested below.

1.	Find the deceleration using the relation and then use Newton's 2nd Law to determine the force.

2.	Use the work-energy theorem which states change in KE = work done

B)	A physics student has a 100 kg mass. What is his weight:

(Just fill in you answers for in the Blanks)

1.	In a stationary elevator?	___________N

2.	In an elevator that is accelerating up at a = 5.2 m/s2?	____________N

3.	In an elevator that is decelerating at a rate a = -5.2 m/s2?	____________N

4.	In an elevator that is falling freely?	____________N