- #1

lozzajp

- 14

- 0

Ok this will be a long post, sorry in advance. I am an online student of a university and the help I am getting from them is ridiculous, I am also using a book called quicksmart introductory physics, and for the most part it is quite rubbish.

1st Problem: (not homework questions, I am trying to figure out the example and how it works)

http://postimage.org/image/wfk1kp6jx/ [Broken] scan of the page

If you can see that scan of the page, great. It is about two masses on a pulley/string and one mass is on an incline. I can work out the acceleration of the seperate masses (g for straight down, g x Sinθ for the incline) and that F=ma.

I am having trouble understanding how to get the resulting acceleration of the masses (I seem to have no trouble when it is a matter of two objects straight down on a string/pulley with different masses). As well as I can not figure out the Tension either.

I really need a simple explanation of how to do this process, as the book just goes from easy enough physics to all this overdrive in this example.

2nd Problem:

Short story, 2 mass on a pulley both directed at the ground. resulting in a 20kg mass accelerating towards the ground at 5.88m/s/s and has 2m to fall.

The acceleration part i worked out easy enough, but the time to calculate it to move those 2m i cant get it.

i used s=ut+0.5a(t^2)

rearranged it to get t^2 = 2.944/2

t = 1.21s

however this is apparantly wrong and the book answers says a formula t^2 = 2s/a

t = 0.82s

where did this come from?

the resulting velocity i got from a = v/t so i mistakingly went v = a/t which ironically gave me the right answer for v (but i had t wrong).

v = at provides the right answer if t is right.

So why doesnt my first formula work for t?

Problem 3:

car 1 is travelling at a constant 30m/s and is 50m ahead of a stationary car 2.

car 2 uniformly begins accelerating at 4m/s/s. when will the car catch up?

The book gives me no information at all how to work this out.

The solution gives 50 +30t = 0 + 1/2 x 4t^2

I can see this is a modified s = ut + 0.5at^2 but I have no idea how it works or how to get to it?

Thanks if i can get just a little help understanding this!!

1st Problem: (not homework questions, I am trying to figure out the example and how it works)

http://postimage.org/image/wfk1kp6jx/ [Broken] scan of the page

If you can see that scan of the page, great. It is about two masses on a pulley/string and one mass is on an incline. I can work out the acceleration of the seperate masses (g for straight down, g x Sinθ for the incline) and that F=ma.

I am having trouble understanding how to get the resulting acceleration of the masses (I seem to have no trouble when it is a matter of two objects straight down on a string/pulley with different masses). As well as I can not figure out the Tension either.

I really need a simple explanation of how to do this process, as the book just goes from easy enough physics to all this overdrive in this example.

2nd Problem:

Short story, 2 mass on a pulley both directed at the ground. resulting in a 20kg mass accelerating towards the ground at 5.88m/s/s and has 2m to fall.

The acceleration part i worked out easy enough, but the time to calculate it to move those 2m i cant get it.

i used s=ut+0.5a(t^2)

rearranged it to get t^2 = 2.944/2

t = 1.21s

however this is apparantly wrong and the book answers says a formula t^2 = 2s/a

t = 0.82s

where did this come from?

the resulting velocity i got from a = v/t so i mistakingly went v = a/t which ironically gave me the right answer for v (but i had t wrong).

v = at provides the right answer if t is right.

So why doesnt my first formula work for t?

Problem 3:

car 1 is travelling at a constant 30m/s and is 50m ahead of a stationary car 2.

car 2 uniformly begins accelerating at 4m/s/s. when will the car catch up?

The book gives me no information at all how to work this out.

The solution gives 50 +30t = 0 + 1/2 x 4t^2

I can see this is a modified s = ut + 0.5at^2 but I have no idea how it works or how to get to it?

Thanks if i can get just a little help understanding this!!

Last edited by a moderator: