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Physics 101 Exam 2
12 November 2004 Prof L. Weinstein

There are 16 problems. Please give a short explanation for all multiple choice questions. Show your work for all numerical answers.

Earth's mass $M_e = 6\cdot 10^{24}$ kg Earth's radius $R_e = 6.4\cdot 10^6$ m
Moon's mass $M_m = 7\cdot 10^{22}$ kg Moon's radius $R_m = 1.7\cdot 10^6$ m
Sun's mass $M_S = 2*10^{30}$ kg Sun's radius $R_S = 7*10^8$ m
Earth-Moon distance $d_{E-m} = 3.8\cdot 10^8$ m Earth-Sun distance $d_{E-S} = 1.5\cdot10^{11}$ m
G = $6.67\cdot10^{-11}$ N$\cdot$m$^2$/kg$^2$  

  1. You throw a 2-kg block upward at an angle of 45$^o$ and a speed of 6 m/s toward your friend. What is its kinetic energy as it leaves your hand?







  2. You throw a 3-kg block upward at an angle of 37$^o$ with a kinetic energy of 24 J toward your friend. What is its kinetic energy immediately (ie: a split second) before your friend catches it? (Assume that your friend catches it at the same height that you threw it from. Ignore air resistance.)

    = 0.5in \epsffile{throw2.eps}







  3. Aircraft carriers use catapults to rapidly accelerate airplanes to high enough speeds that they can remain airborne. (Airplanes crash if they fly too slowly.) Assume the catapult can do a fixed amount of work on each airplane. If the catapult can give a plane with mass $m$ an initial speed $v$, how much speed can the catapult give a lighter plane with mass ${1\over2}m$?

    1. ${1\over2}v$
    2. between ${1\over2}v$ and $v$
    3. $v$
    4. between $v$ and $2v$
    5. $2v$
    6. need more information






  4. You compete in the pumpkin dropping competition and build a 1-m tall pumpkin catcher that successfully catches (without breaking) a 4-kg pumpkin dropped from a 10-story building. If a new competition is held where the pumpkin is dropped from a 20-story building, how tall should your new pumpkin catcher be in order to successfully catch the pumpkin? (Assume that the new pumpkin catcher is built just like the old one, except for its size. Ignore air resistance.)

    1. 1/4 m
    2. 1/2 m
    3. 1 m
    4. 2 m
    5. 4 m
    6. need more information






  5. When you drive your car one mile at a speed of 30 mph, the tires of your car make a certain number of rotations. Next you test drive a new vehicle with tires twice the diameter of your car's tires. How many rotations do these tires make in one mile?

    1. 1/4 as many
    2. half as many
    3. the same
    4. double
    5. quadruple
    6. need more information






  6. Norfolk is at a latitude of about 40 degrees N. Our linear speed due to the rotation of the Earth about its axis is about 350 m/s. If you move to the equator your linear speed due to the rotation of the Earth about its axis will

    1. decrease
    2. stay the same
    3. increase
    4. need more information






  7. A favorite playground item is a large horizontal tire held up by chains. The tire is free to swing and also to rotate. Let's just consider the rotational motion. Ignore friction. The pictures show the swing as viewed from above. Two children of about equal mass get on the swing and sit up straight (the left-hand picture). Their parents start the swing rotating at some rotational speed. If the children now lean way out (the right-hand picture), the rotational speed of the swing will

    = 1.5in \epsffile{tireswing.eps}

    1. decrease
    2. stay the same
    3. increase
    4. need more information

  8. In order to be in Low Earth Orbit (ie: about 200 km from the surface of the Earth or about 6600 km from the center of the Earth), you need to have a tangential speed of 8 km/s. If the Earth expands so that it has the same mass but twice the radius, what speed would you need to be in Low Earth Orbit 200 km above the (expanded) surface?

    1. less than 8 km/s
    2. 8 km/s
    3. more than 8 km/s
    4. need more information






  9. Calculate the gravitational force between the Earth and the Sun (see the numbers at top of exam).











  10. You drive your car on the curved highway exit ramp at 20 mph. In order for your car to follow the curve and stay on the road, you need 2,000 N of frictional force.

    a) draw an arrow showing the direction of the frictional force when the car is at point $A$.

    b) How much force will you need to follow the curve and stay on the road if you drive on the same curve at 60 mph?

    = 1.5in \epsffile{ramp.eps}




  11. If you moved to another planet with twice the mass of the Earth but the same radius, the acceleration due to gravity at the surface would be

    1. four times smaller than Earth's (ie: 2.5 m/s$^2$)
    2. two times smaller than Earth's (ie: 5 m/s$^2$)
    3. the same as Earth's (ie: 10 m/s$^2$)
    4. two times larger than Earth's (ie: 20 m/s$^2$)
    5. four times larger than Earth's (ie: 40 m/s$^2$)
    6. need more information






  12. (no explanation needed) When you accelerate your standard (ie: non-hybrid) gasoline-powered car from 0 to 60 mph, you are converting chemical energy in the gasoline to kinetic energy (and other forms of energy). When you use your brakes to stop, what form of energy is the kinetic energy converted to?

    1. chemical
    2. potential
    3. thermal (ie: heat)
    4. other
    5. need more information

  13. You throw a 7-kg rock with an initial speed of $v=25$ m/s such that its horizontal speed is 15 m/s and its vertical speed is 20 m/s. How much time is the rock in the air? Ignore air resistance. Assume the rock lands at the same height that you threw it from.

    = 1.5in \epsffile{throw.eps}









  14. Olympics divers need to rotate their bodies a certain number of times for specific dives. At the beginning of the dive, a diver's body is typically fully extended and she is rotating at a certain rate. When the diver 'tucks' (ie: draws her legs up to her chest and brings her arms in), will she rotate faster or slower than when her body is extended?

    = 1.1in \epsffile{diver.eps}

    1. faster (more rotations per second)
    2. same rotational speed
    3. slower (fewer rotations per second)
    4. need more information

  15. You jump out of an airplane and are in free fall. While you are falling, you take out a cup and a bottle of water. You tilt the bottle in order to pour the water from the bottle into the cup. Ignore air resistance. What happens?

    1. the water pours normally
    2. the water pours faster than normal
    3. the water pours slower than normal
    4. the water does not pour (it stays in the bottle)
    5. need more information






  16. In one of the Star Trek movies, Captain Kirk falls off of a high cliff. If he hit the ground, he would die. Fortunately, Mr. Spock swoops in horizontally with his aircar and saves Kirk by catching him about 6 inches from the ground. Would this work in real life?

    = 1.1in \epsffile{kirk.eps}

    1. yes
    2. no
    3. need more information







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2004-11-16