question on lecture tests for examkrackers physics

aa4

Member
10+ Year Member
those of you are studying with examkrackers, need a bit of help.

physics lecture test 5, question 103: can someone explain this to me, the explanation they give aren't cutting it. they seem to get numbers out of no where!

also, same test, question 108. wouldn't answer choice A also be a correct answer. looking at the equation, a higher density will just cancel out with eachother.

thanks!

WilliamsF1

10+ Year Member
103) 1st line of the passage says the specific gravity of the unknown is 5.0. That means it's 5 times as dense as water. The 2kg mass displaces 0.5kg of the unknown. It's 4 times as dense as the unknown liquid. To find the specific gravity of the object you multiply the two. The object is 4 times as dense as the unknown, which is 5 times as dense as water. 4x5=20

108) Density affects the magnitude of each pressure (pressure above and pressure below the ball). First let's make it clear that the velocity of the air speeds are not the same in opposite directions. This would make 0 pressure difference so the equation would be useless since density remains the same. Let's say that the effective airspeed above the ball is 6 and below is 5.

Example 1: Density = 1.

delta P = (1/2)(1)(6^2)-(1/2)(1)(5^2) = 18 - 12.5 = 5.5

Example 2: Density = 10

delta P = (1/2)(10)(6^2)-(1/2)(10)(5^2) = 180 - 125 = 55

Density changed the difference in pressure, which is "directly proportional to the lift Force." So it changed the lift Force. The question asks which will NOT increase lift force. So will lowering the density increase lift force? No, if you lower density (go from density = 10 to 1), the force is REDUCED.