# Thread: Kinematics Aplication problem - variable acceleration.

1. ## Kinematics Aplication problem - variable acceleration.

Hi I need help with this problem.

A bullet is fired into a sandbag. The acceleration of the bullet within sandbag is
-40√v, where v is velocity in ft/sec. When will the bullet stop if it enters the sandbag at 950 ft/sec.

Than you.

Hi I need help with this problem.

A bullet is fired into a sandbag. The acceleration of the bullet within sandbag is
-40√v, where v is velocity in ft/sec. When will the bullet stop if it enters the sandbag at 950 ft/sec.

Than you.
Start by setting up an appropriate differential equation. Can you do that?

3. Originally Posted by mr fantastic
Start by setting up an appropriate differential equation. Can you do that?
I am trying to work with m(dv/dt)=F-kv

dv/dt=950-40v^1/2

Now I can solve this by separation of variables.
Does this make sense.
Thanks

A bullet is fired into a sandbag. The acceleration of the bullet within sandbag is
-40√v, where v is velocity in ft/sec.
When will the bullet stop if it enters the sandbag at 950 ft/sec.
*** $\displaystyle \frac{dv}{dt} = -40\sqrt{v}$

separate variables and solve for velocity as a function of time

5. Originally Posted by skeeter
*** $\displaystyle \frac{dv}{dt} = -40\sqrt{v}$

separate variables and solve for velocity as a function of time
Thank you, I got it now.