rasen58

A satellite moves in a low circular orbit about a spherical earth that has no atmosphere (it barely skims the surface of the earth). Approximately how many minutes will it take the satellite to make one revolution around the earth? I think you would use Kepler's third law here, but there are no numbers and the answer choices are 80,82,84, and 86 minutes. How do you do this?

Oh wow, I didn't know that. Thanks, that helps.

A car accelerates uniformly from rest, covering a distance of 125 m in 5 s. What is the car's acceleration in m/s^2? I thought the velocity would just be d/t = 125/5 = 25 m/s And then I thought the acceleration would be v/t = 25/5 = 5 m/s^2 But apparently the answer is 10 m/s^2 Why?

Is it possible to turn on the tap so that lots of water flows down and then put a piezoelectric crystal under it to generate power?

I saw that before, but I don't understand how that works for this problem. The answer is 8, but if you first use the equation by setting T = 10,000 and then setting T = (30,000-10,000), wouldn't you get a factor increase of 16, and not 8? Edit: Oh never mind, I read the answer wrong. It's 81, so I guess you'd have to use T=30,000 and 3^4 = 81, which makes sense.

If the temperature of a star increases from 10,000K to 30,000K, by what factor will the rate of energy radiated per second increase? What formula am I supposed to use to determine this?

I see. Thanks.

Ft = delta p So a greater momentum will have a greater force. But I thought the impact force would occur before any change in momentum, so I thought that since they have the same initial momentum, the impact forces would be equal.

Two balls of the same mass are thrown towards a wall and collide with it moving with a speed of 5 m/s. Ball A hits the wall and rebounds with a speed of 4 m/s. Ball B hits the wall and stops. Assume that the collisions times are the same for each ball. Compared to ball B, ball A has ____ velocity change, ____ momentum change, and ____ impact force. So I figured out that A has a *greater* velocity change because it changes from +5 to -4 whereas B only changes from +5 to 0. And I figured out that the momentum change for A is also *greater* because of the same reason above because momentum = mass x change in velocity. And I kept getting the last blank wrong, but apparently the answer is that A has a greater impact force. I thought they'd have the same impact force since both balls have the same mass and hit the ball at the same speed. Why does A have a greater impact force?

Oh I see, so it doesn't matter what the train is doing since the ball leaves the hand at the same velocity and gravity is constant anyway. But for my other question, does the ball land behind the person when the train is accelerating?

A ball is thrown upward in the air by a passenger on a train that is moving with constant velocity. a. Describe the path of the ball as seen by the passenger. Describe the path as seen by a stationary observer outside the train. b. How would these observations change if the train were accelerating along the track? So I now understand that to the passenger in the train moving at constant velocity, the ball would appear to go up and down in a straight line. And I sort of see that to the observer outside, the ball would appear to go forward in the air in a downward arc. But for part b, it seems the answer is that the ball would appear to move in a parabolic arc to both people. But I was wondering, is there any difference in the arcs that they both see (like in the length of the arc or something else. Also, if the train is accelerating and the ball is thrown straight up, does that mean that the ball lands behind the person who threw it and would thus be a backwards arc?

Oh, I see now! I didn't realize that it couldn't hit the ceiling, so there must be an angle. I was able to calculate the answer now. Thanks!

Problem: A physics teacher is designing a ballistics event for a science competition. The ceiling is 3.00 m high, and the maximum velocity of the projectile will be 20.0 m/s. a) What is the maximum initial velocity in the vertical direction that the projectile could have? So I think that the max velocity of a projectile will always be when it is launched or right before it hits the ground, so wouldn't the answer be 20.0 m/s? If you point the cannon or whatever is launching the projectile vertically upwards with no tilt, there would be 0 horizontal velocity, so wouldn't all the velocity be in the vertical direction? But the answer is apparently 7.68 m/s, which I don't understand.

Oh wow, that makes sense, thank you. Swansont, yes I have derived them multiple times before when I used to forget it.

A 2.0 kg otter starts from rest at the top of an incline 85 cm long and slides down to the bottom in 0.50 seconds. What net external force acts on the otter along the incline? So I was planning on using F=ma, and since we have the mass, we need the acceleration to find F. a = change in velocity/time. velocity = change in displacement/time = 85/.5 = 170 cm/s =1.7 m/s a = 1.7/.5 = 3.4 m/s2 Then F=(2)(3.4) = 6.8 N. But that is wrong because it should be 14 N instead. Apparently if you use delta x = v0t + .5at2, you can solve for a that way and you get a = 2(delta x)/(t2) = (2)(.85)/(.52) which gives you 6.8 m/s2 And then if you use F=ma, you get (2)(6.8)=13.6=14 N. Why did my first way not work?

Oh wow, thanks, That was detailed.

Yes, and that is what I already did. Thanks.

There is no acceleration perpendicular to the plane right?

studiot That makes a lot of sense now. So if it's not in equilibrium, then the left side of the x axis wouldn't equal the right side right? So I don't see how to make the equation if you can't determine the other side. The only thing I see is the F=ma and we have m of 75 and a of 3.6, so multiplying those would give you 270 N. But I think I already have that. swansont Oh I think I see it now! Because it isn't in equilibrium like studiot said, the resultant force is equal to 270 N, which is the final force. So that means that because Fgx is going down at 311 N, but the Fk is pulling up at some number x, and the final resulting down force is 270, you can figure out x by doing Fa=Fgx+Fk, and that is 270 = 311 + Fk, so that means Fk is -41, but the negative makes sense because it's in the opposite direction, so it has a force of 41 N to the right. I get it now! Thanks! Fa as the result makes sense. Thanks a lot.

studiot: The force Fa comes from gravity pulling it down. And that relates to the second law which relates a force, mass, and acceleration. swansont: Well I know that F=ma, and since we have a already (3.60 m/s2), I decided to calculate the force in the x direction only. So it would be Fx=max. And the force in the x direction is Fk or Fa+Fgx, so I used the second one. So that becomes (Fa+Fgx)=(m)(a) -> (Fa+311)=(75)(3.60) -> Fa+311=270 -> Fa=-41 N

That's also what I was thinking, so I thought of doing Fa - Fgx but then I'd get a negative number, which I thought wouldn't make sense. But I just realized that it could actually make sense because it's just the opposite sign?

A 75 kg box slides down a 25.0° ramp with an acceleration of 3.60 m/s2. Find uk between the box and ramp. So I drew a free body diagram and angled it and then figured out almost all the values I need. Fa is the force that the box already has as it's sliding down. Fn is the normal force. Fg is the force of gravity. Fk is the force of kinetic friction. The only one left is FK but it seems like I'm doing it wrong. I thought it would be equal to Fgx + Fa but the answer says that I should be doing 311 - 270 and that is equal to FK. Can anyone explain why that is? I thought it would just be the opposite of the other side of the x axis. Or did I draw my diagram wrong?

Elements A and X combine to form the compound A3X2. If the electron-dot symbol of the neutral atom A shows two electrons, the number of electrons in this symbol for atom X must be 2 4 5 6 My initial answer would have been 3 because if each A has 2, and there are three of them, there would be 6 total for A, so you need 6 total for X, so each X would need 3, but that would not an option. So I just picked 5 because 3+5=8. 5 is the right answer, but why?

Oh, so is it just convention to say that the NO3- is reduced when it's only the N part of it being reduced?

Cu(s) + 2NO3-(aq) + 4H+(aq) -> Cu2+(aq) + 2NO2(g) + 2H2O(l) Which of the following takes place during the reaction above? A) Cu(s) is oxidized B)Cu(s) is reduced C)H+(aq) is oxidized D)H+(aq) is reduced E)NO3-(aq) is oxidized The answer is A, and I got it right, but the solution also said that NO3- is reduced, but I don't see how that's possible.Wouldn't it be oxidized if it's going from NO3- to NO2?