motion problems (1 Viewer)

[Eg155]

Can't wrap my head around this one:

An object traveling at 30m/s-1 experiences a steady retardation of 4m/s-2. How long will it take to travel 108m?

Please and thankyou!

(btw i did it and got 3.6 seconds i just think it's wrong... )

 

[qrpw]

You know u = 30, a = -4 and s = 108. So therefore use

s = ut + 1/2 at^2

Substitute your values in, solving the quadratic, you will get t = 6 and t = 9. So it will travel 108m in 6 seconds.

 

[Eg155]

You know u = 30, a = -4 and s = 108. So therefore use

s = ut + 1/2 at^2

Substitute your values in, solving the quadratic, you will get t = 6 and t = 9. So it will travel 108m in 6 seconds.

when u end up with t + t^2 on the RHS then wat do u do??

 

[qrpw]

Hmm... I assume you know how to factorise/solve quadratics? After subbing in you get

108 = 30t - 2t^2

t^2 - 15t + 54 = 0

Just factorise to get (t-6) (t-9) = 0

 

[rmilosavljevic]

Hi, I know this is such a late enquiry, but I was just wondering, when you're left with t = 6 & 9, how do you know which one to choose? Sorry, I'm a bit of a noob

 

[Fizzy_Cyst]

t = 6 refers to the first time it travels 108m -- it then continues travelling in the same direction and assuming constant retarding acceleration, it will eventually turn around and come back to the 108m displacement. Which it does after a further 3 seconds.

 

[Timske]

steady retardation of 4m/s-2

what de heck?