A bullet of mass 0.045kg is fired from a gun of mass 9kg, the bullet moving with an initial velocity of 200m/s. Find the initial backward velocity of the gun.

Answer 1

In this case, we will use the conservation of momentum.

The forward momentum of the bullet equals the backward momentum of the gun.

m = mass of bullet
M = mass of gun
v = velocity of bullet
V = velocity of gun

=> MV = mv
=> V*(9) = (0.045)*(200)
=> V*(9) = 9
=> V = 1 m/s

Answer 2

The only issue I have with this is the principle of conservation of momentum is that final momentum is equal to initial momentum
initial=0
final= sum bullet and gun
0=MV+mv
MV=-mv
and this leads to
V=-1m/s, and of course, the velocity sign means the opposite direction of the bullet. Momentum is a vector quantity.

Answer 3

Right, agreed.
Though to prevent the need for the negative sign I specified 'forward' and 'backward' velocities, so that I could omit the direction from the calculations.

Answer 4

I know what you did, but your statement is not true: he forward momentum of the bullet equals the backward momentum of the gun.

The magnitude of is equal to . When teaching, it is usually to let the math work out the answer from the math, not from a mental construct.
Otherwise, good. Over many years, I have learned many pitfalls for students to fall into, and how to avoid them.

Answer 5

Alright, thanks for pointing it out!

Answer 6

student

Answer 7

I dont understand this oo