View Full Version : Physics for the soul! (help please)

2010-01-24, 05:30 PM
The van travels over the hill described by

y= -.0015(x^2) + 15

If it has a constant speed of 75 ft/s , determine the x and y components of the van's velocity when x= 50 ft.

then determine the x and y components of the vans acceleration at x=50 ft.

A little help with the first part can help me get the second part.

i just took the derivative as it is to get V(y)= -.0015/2*x where x = 50

then i put it in terms of x and did the same thing but got the wrong answer. Anyone know why? :smallconfused:

note: this is a parabola.

an a different problem that i got half of:

A particle travels along the circular path x^2 + y^2 = r^2

If the y component of the particle's velocity is V(y)=2*r*cos(2t), determine the x and y components of its acceleration at any instant.

I found the right x acceleration after some math, but wouldnt the y acceleration just be the derivative, which is:

-4*r*sin(2t) since r is constant for a circle. I did this and it said it is wrong. Note it is online hw.

Thanks for any help and more problems will come if i have trouble.

2010-01-24, 05:53 PM
Well, I can tell you that on the first part you've improperly taken the derivative, since the derivative of x2 is 2x, not 1/2x. But if this is for the mechanical engineering class you posted for earlier, then you're probably looking for a DE solution.

Secondly, your solution for the velocity components in the first part should result in a vector with magnitude 75 ft/s, since we're told that this is constant. Only the direction is changing.

What did you get for the x-acceleration on the second question?

2010-01-24, 05:55 PM
4rcos(2t) which is correct and im surprised people still remember. :smalltongue:

im thinking it has to do with x and x prime, meaning the velocity. in relation to the first one of course. i skipped it for now and moved on so i dont get stuck.

2010-01-24, 08:50 PM
for the van problem,

V(x)= -74.2

no idea how they got it <.<

can someone explain?