rifle problem

I have been given the following problem

A rifle fires bullets downwards. Estimate the rate of the bullet fire needed to keep the rifle from fallong to ground, assuming it is kept pointing downward.

So I have assumed that

rifle=3kg
bullet-25g
velocity bullet=300m/s

i am not sure if these are accurate approximations

i have then put that m(rifle)g=m(b) x v(b) x rate

3x9.8=300x0.025x rate

rate =3.92 s-1

Can anybody help me with this

Thanks

Reply 1

DomAD

9

You need to find the force each bullet exerts on the rifle (using F= ma). You know the mass of each bullet, and you know what speed it accelerates to, but you need to find the actual acceleration. If you know the length of the rifle, use a SUVAT equation to find A.

Reply 2

Morbo

17

DomAD

You need to find the force each bullet exerts on the rifle (using F= ma). You know the mass of each bullet, and you know what speed it accelerates to, but you need to find the actual acceleration. If you know the length of the rifle, use a SUVAT equation to find A.

Not really.

You need to use the definition as force as the rate of change of momentum. The firing of the bullets downwards will provide an upwards force on the rifle (which will also lose mass as the rounds are fired, although this can be ignored in the first approximation).

Reply 3

smile!

OP

ok so im estimating the rifle to be 0.4 metres long. so my accerleration is 112,500 m/s^2? . so force on rifle is 2812.5 newtons. thats far to big as my the rifle force down is only 29.4

Reply 4

Topperfalkon

I think the OP has got it roughly right.

Reply 5

smile!

OP

i thought the rate of mometum = firce was what i used originally

Reply 6

DomAD

9

Morbo

Not really.

You need to use the definition as force as the rate of change of momentum. The firing of the bullets downwards will provide an upwards force on the rifle (which will also lose mass as the rounds are fired, although this can be ignored in the first approximation).

I get the same answer using either.

Reply 7

Topperfalkon

you got it right first time I think...

Reply 8

Morbo

17

smile!

ok so im estimating the rifle to be 0.4 metres long. so my accerleration is 112,500 m/s^2? . so force on rifle is 2812.5 newtons. thats far to big as my the rifle force down is only 29.4

F = \frac{\Delta(mv)}{\Delta t}

So if we ignore the fact that the rifle is losing mass as it fires the bullets, we require:

Mg = \frac{\Delta(mv)}{\Delta t} = mvR

where

R

is the rate of fire in

\mathrm{Hz}

.

R = \frac{Mg}{mv} \Rightarrow R = 3.9 \mathrm{Hz}

I don't understand why you're estimating the rifle length to find the force on it. The force on the rifle, from the above equation, is equal to its weight of