question

[fredrick]

When a bullet is fired from a rigidly held rifle, the force exerted by the rifle on the bullet is equal and opposite  to the force  exerted by the bullet on the rifle.

a)explain why the bullet accelerates while the rigidy held rifle does not

b)In most cases when a rifle is fired, the shooter's shoulder moves back as the rifle 'recoils'. If a 4.0 kg rifle fires a 20 g bullet with an initial speed on 300m/s, what is the initial recoil speed of the rifle?

there are no answers to this question( among many others) in the back of the text book?

help

[Mao]

a) the two forces @ work are:
rifle on bullet -> large force small mass -> fast acceleration
bullet on rifle -> equal amount of force, but it is acting on a larger mass -> lower acceleration. In this case it is "rigidly held", we can then assume it has some kind of support and the force is transferred...

b) conservation of momentum
assuming the positive direction is the direction of the bullet

[dcc]

The speed will be negative because the direction of the recoil is opposite to the direction of the bullet. (which we consider to be the positive direction)

For example, due to conservation of momentum, the momentum after firing should equal 0





Using positive 1.5 would yield a result of 12 N

[Mao]

The speed will be negative because the direction of the recoil is opposite to the direction of the bullet. (which we consider to be the positive direction)

For example, due to conservation of momentum, the momentum after firing should equal 0





Using positive 1.5 would yield a result of 12 N

yeah... i'm lazy

but then that's the thing that seperates 40+ from 30s... *makes major mental note* thnx dcc

[fredrick]

Thanks!
And one more:
A doodgem car of mass 200kg is driven due south into a rigid barrier at an initial speed of 5.0m/s. the car rebounds at a speed of 2.0m/s. it is in contact for 0.20 secs. calculate:
a) the average acceleration of the car during its interaction with the barrier.
b) the average net force applied to the car during its interaction with the barrier

[cara.mel]

a)
a = v/t
=(2--5)/.2
=35ms^-2 north

b)
F = ma
= 200*35 = 7000N

Might be wrong because I have a headache. =/

[fredrick]

yeh i got 35m/s^2 too but the back says 3.5m/s^2?

thanks for clearing that up

[dcc]

The speed will be negative because the direction of the recoil is opposite to the direction of the bullet. (which we consider to be the positive direction)

For example, due to conservation of momentum, the momentum after firing should equal 0





Using positive 1.5 would yield a result of 12 N

yeah... i'm lazy

but then that's the thing that seperates 40+ from 30s... *makes major mental note* thnx dcc

lol its just like if you have 0 = x + y, x = -y, not x = y

[phagist_]

yeah... i'm lazy

but then that's the thing that seperates 40+ from 30s... *makes major mental note* thnx dcc

I think most of the time they ask for 'speed' which is a scalar quantity thus direction doesn't matter.
Nevertheless learn the difference between velocity and speed.

[sxcalexc]

Hmm question - about the way questions are worded. It says the 'speed' is 5ms but then it gives the direction ie. due south. So would we count this as a vector - and thus count acceleration as a vector when questions are worded like this, ALWAYS?

[Mao]

Hmm question - about the way questions are worded. It says the 'speed' is 5ms but then it gives the direction ie. due south. So would we count this as a vector - and thus count acceleration as a vector when questions are worded like this, ALWAYS?

pretty much.... =S

[cara.mel]

Afterthought that's been touched on:
These questions will always test you on vectors. Objects tend to enjoy either travelling due north or due south. In a proper answer (vs a lazy answer) you first say something like 'let north direction be positive' and ALWAYS chuck the direction in your final answer.
When they don't give you directions like that, eg the rifle question, still call one way positive and the opposite way negative.
Yes it gets boring and yes it is obvious but if you get in the habit now you won't accidently forget about it.

When you get up to projectile motion you'll be doing a similar thing as well (stating at the start up == positive direction)

Edit: So if you're really bored at school, make your directions the opposite way to what is 'normal' during tests etc. They can't mark you wrong

[sxcalexc]

Afterthought that's been touched on:
These questions will always test you on vectors. Objects tend to enjoy either travelling due north or due south. In a proper answer (vs a lazy answer) you first say something like 'let north direction be positive' and ALWAYS chuck the direction in your final answer.
When they don't give you directions like that, eg the rifle question, still call one way positive and the opposite way negative.
Yes it gets boring and yes it is obvious but if you get in the habit now you won't accidently forget about it.

When you get up to projectile motion you'll be doing a similar thing as well (stating at the start up == positive direction)

Edit: So if you're really bored at school, make your directions the opposite way to what is 'normal' during tests etc. They can't mark you wrong

Haha thanks, but I don't think i'll take the risk to try that little exercise at the bottom