Question

A 2000 kg car is rolling at 2.0 m/s. You would like to stop the car by firing a 12 kg blob of sticky clay at it. How fast should you fire the clay?

i keep getting 25.8 but apparently that isn't right so i'm at a loss

Answer 1

This is an example of Conservation of Momentum. This means total momentum in the system is conserved. Put into an equation this is
p1initial + p2initial = p1final + p2final
Let's say that the car is obj 1
Minitial = 2000 kg
vinitial = 2.0 m/s
Mfinal = 2000 kg
Vfinal = 0 m/s(because it is stopped by the clay)
Obj 2 is the clay
Minitial = 12 kg
vinitial = 0 m/s
Mfinal = 12kg
Vfinal = ? m/s
so... knowing that p = m* v, and substituting the known values
2000kg * 2 m/s + 0 = 0 + 12kg * vfinal of clay
v final of clay = (2000kg*2m/s)/12kg

v = 333.3 m/s