TBR test 7 questions

[2010premed]

9. Kinetic friction is seen when two objects are in contact, and neither of them are rotating (but one of them is sliding) while static friction ivolved one object rotating and the other is stationaey, correct?
Q: What type of friction is generated between the brake plate (which is attached to the wheel) and brake pad (which is stationary) as the car decelerated from its initial velocity, assuming that the wheels do not lock up?
Answer: kinetic friction only

The explanation says that this is because “the brake plate is moving (rotating)”… Then wouldn’t this be an example of static friction?

11. At high constant speed, what produces the MOST drag?
A. Kinetic friction
B. Static friction
C. Rolling friction
D. Air resistance

The explanation says that the force due to air resistant is proportional to the square of the velocity with which the vehicle is traveling. Where did this equation come from?
Also, I didn’t know they were talking about a car… One I know that, I guess it’s easy to rule out kinetic friction since there is none. They say to rule out static friction because it’s te reason that the car is moving, but howcome this isn’t considered drag? And is force of rolling friction the same thing as static friction? Also, does air resistance produce the most drag for all moving objects or only for cars?

12. An antilock brake pad should be made of a material that:
A. maintains µs with increasing temperature.
</SPAN>B. increases µs with increasing temperature.
</SPAN>C. maintains µk with increasing temperature.
</SPAN>D. increases µk with increasing temperature.
</SPAN>
Why not increase? I know that in the passage it states that “Antilock disk brake: Braking efficiency remains constant over the period of time that the brake is applied” But if you increase µk you would increase friction, make the car slow down faster.

28. Assuming Theory 1 to be correct, the energy of a single photon could be determined from
Theory 1: e+ + e- = n + y (n = neutrino y = gamma ray)
A. &#8710;msystemc2
</SPAN>B. &#8710;msystemc2 + ½(&#8710;mve+2 + &#8710;mve-2 - &#8710;mv&#951;2)
</SPAN>C. &#8710;msystemc2 + ½(mve+2 + mve-2 - mv&#951;2)
</SPAN>D. &#8710;msystemc2 + ½(&#8710;pe+2 + &#8710;pe-2 + &#8710;p&#951;2)
</SPAN>I do not understand the explanation for this. What exactly is mc^2 for?

30. The product of an electron-positron annihilation, according to Theory 2, must consist of two photons instead of just one in order to conserve:
A. energy
B. Charge
C. momentum
D. mass
Theory 2: e+ + e- à 2y
I picked momentum just because I saw the collision of 2 thing, was my reasoning valid here

38. Why do liquids/solids expand upon heating? I always thought of PV=nRT, but this only applies to gases…

40. The MOST exothermic chemical reaction involves:
A: oxidation-reduction
Any other random facts I should know about this?

41. The MOST rapid temperature change would be observed with which system?
A. A 20.0-g pellet at 50°C added to 100 mL of water at 25°C.
</SPAN>B. A 20.0-g pellet at 70°C added to 100 mL of water at 25°C
</SPAN>
I know the answer intuitively, but what’s the concept here?
47. According to Table 1, what properties can be expected for a 1.00-meter rod with a diameter of 2 mm that is made of an aluminum-copper alloy known to be 85% aluminum by moles mixed with copper?
A. It has a density of 3.63 g/mL and a resistance of 8.15 x 10-3 &#937;.
</SPAN>B. It has a density of 3.63 g/mL and a resistance of 8.25 x 10-3 &#937;.

So the way I did it is I saw 3.63 is closer to 20 and I picked A. How are you supposed to see that the mole percent of copper varies uniformly with both density and resistance??


51. In a double-slit experiment using a single light source passing through two slits, what is observed at a point on a luminescing screen that is 3 1/6 &#955; from the upper slit and 4 2/3 &#955; from the lower slit, as shown in the diagram below?

how do u do this?
</SPAN>

---

Members don't see this ad.

 

[fizzgig]

kinetic friction is when 2 things slide past each other. static friction is when the two surfaces at the point of contact do not slide past each other. if a block slides down a surface, obviously the surfaces slide past each other. if a brake pad is applied, it is like pressing a sponge against the inside of a rotating drum (if they're drum brakes). the surfaces are sliding past each other. if a wheel is rolling, however, even though the wheel is moving, it is "rolling without slipping", meaning that at its point of contact on the ground, in the moment a point on the circle hits the ground and is lifted back up, it does NOT slip. the center of the wheel moves forward but the ground/wheel point velocity is zero. ooooh it has a name... who knew..

(can't insert a gif apparently. link here: http://en.wikipedia.org/wiki/File:CycloidAnim04.gif)

for drag i don't know if there was an equation or a chart to help you, but yeah the only rolling friction i'd see would be the wheels, which is static friction, and they can't both be right. drag is just typically associated with the loss from going through a fluid that's buffeting your frontal surface area... especially since they state high velocity...


when the brake is applied you're slowing the car by your kinetic friction between pad/drum. if the ks was HUGE then you'd stop the wheel very quickly, but you may not have dissipated the energy of that huge car going at a high velocity. if the tire stops turning but you're still going fast, the wheels lock, and the car just slides on the road like any block slides on a surface. you want to slow the car as quickly as possible WITHOUT forcing the wheels to stop rotating too early, or you lose control of the car.

can't really tell about your particle physics stuff. might need to see the rest of it..

sol/liq expand on heating sorta for the same reason gases do i think. they have more 'places to put' energy but eventually it goes into kinetic E and temp is going up. molecules moving with more energy bouncing off each other, even in a more confined config than gas, will still tend to take up more room given the pressure around them doesn't go up and hold that extra energy in...

40. good to know... was this a stand alone?

heat transfer by conduction has the form deltaT = (Q/t)R, where R is whatever else is left in that equation, the resistance to heat flow. like voltage, with some material resistance to heat flow, your actual 'current' will be proportional to the gradient you apply, here temperature diff instead of voltage.

47. the percent moles is right between values from 1 and 2. the density is too. no reason resistance shouldn't be. B seems closer to being right in the middle, maybe you don't know that stuff varies linearly but if one property does and you have no other info, best guess is the other does too.

prob need more info for the last one.