Originally Posted by

**topsquark** I know a professor from Alfred State College that gives problems like this. I just don't see the point in hitting students over the head with problems that, yes they ought to be able to do but that, have such a large number of steps before they can check to see if they have a reasonable answer.

And yes, I know the Europeans do it constantly, but I question *any* instructor who teaches the SI system of units and gives a *weight* in kg.

To the problem.

I'll give you the overview and you can fill in the missing steps. As with many problems where you know the final information, you need to work backward. Change in KE = work done by air resistance.

1) We need to know how fast the tank is initially moving so that it has a speed of 0 m/s at the very edge of the cliff. This is a simple 1 - D motion problem where the tank has a constant deceleration of 3 m/s^2.

2) We know how fast the tank needs to be moving after the collision with the projectile. The collision is perfectly inelastic (the two objects move as one at the end of the collision), and the tank is initially not moving. So find the speed of the projectile just before it hits the tank. Use Conservation of Momentum here (and at all other times) not Conservation of Energy. Energy is *not* conserved in this kind of collision, I don't care what your homework problem says about it.