Velocity/acceleration/time to stop

  • Thread starter Thread starter lauriecherie
  • Start date Start date
AI Thread Summary
The discussion revolves around calculating the time required to reduce a car's speed from 132 km/h to below 90 km/h using brakes that decelerate at 4.9 m/s². Initial calculations led to an incorrect estimate of 8.57 seconds due to a failure to convert speeds from km/h to m/s. After receiving guidance, the correct calculation yielded a time of approximately 2.38 seconds. The importance of unit conversion in physics problems is emphasized. Accurate calculations are crucial for determining stopping times effectively.
lauriecherie
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Homework Statement



The brakes on your car are capable of slowing down your car at a rate of 4.9 m/s^2.
If you are going 132 km/h and suddenly see a state trooper, what is the minimum time in which you can get your car under the 90 km/h speed limit?

Homework Equations



v(t)= at + initial velocity

(final velocity^2) - (initial velocity^2) = 2*a*delta X (change in position)

The Attempt at a Solution



I came out with 8.57 seconds. Is there some converting I maybe missed?
 
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Hi Laurie, welcome to PF!
I don't know how you got 8.57. I got between 2 and 3s. Show your calc and we'll find your error.
Did you remember to convert the speeds to m/s ?
 
Delphi51 said:
Hi Laurie, welcome to PF!
I don't know how you got 8.57. I got between 2 and 3s. Show your calc and we'll find your error.
Did you remember to convert the speeds to m/s ?


No, I didn't convert. Whoops. So let me work this out again. Thanks for the advice.
 
2.380952388 seconds. :!) Thanks soooo much for your help!
 
Most welcome!
 
Thread 'Variable mass system : water sprayed into a moving container'

Thread 'Variable mass system : water sprayed into a moving container'

Starting with the mass considerations #m(t)# is mass of water #M_{c}# mass of container and #M(t)# mass of total system $$M(t) = M_{C} + m(t)$$ $$\Rightarrow \frac{dM(t)}{dt} = \frac{dm(t)}{dt}$$ $$P_i = Mv + u \, dm$$ $$P_f = (M + dm)(v + dv)$$ $$\Delta P = M \, dv + (v - u) \, dm$$ $$F = \frac{dP}{dt} = M \frac{dv}{dt} + (v - u) \frac{dm}{dt}$$ $$F = u \frac{dm}{dt} = \rho A u^2$$ from conservation of momentum , the cannon recoils with the same force which it applies. $$\quad \frac{dm}{dt}...
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