I need help with ratios

[chiddler]

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I get them right a lot of the time, but I fumble through them and often spend too much time reasoning and thinking. And a lot of the time doesn't really cut it D:

I am looking for a systematic technique that will help me solve questions such as the following EK question:

The centripetal force acting on an object is doubled. The radius of the object's motion is also doubled. The speed of the object:

A. Increases by a factor of 4.
**B. Increases by a factor of 2.
C. Decreases by 2
D. obviously wrong.

 

[MedPR]

Oops 🙂

 

[Pisiform]

F=mv^2/r

so here you are comparing velocity with Force & radius ... so make velocity the subject

mv^2 = Fc r

Now forget abt everything else except v, Fc and r (others are constant)

v^2 = Fc * r

v^2 = (*2) (*2) --- you are doubling Fc and r both

v^2 = 4

v = sqrt 4 = 2 = which means actually (*2) increasing by a factor of 2

 

[chiddler]

i like!

i don't solve them like that but that's way more efficient. will practice, thanks.