You are Here: Home >< Physics

# Momentum - A2 Physics watch

1. I think the book is wrong (yet again) but can't be sure so thought i'd better check...

A basic air riflr fires slugs os mass 0.5 g at a velocity 160 m/s. The air rifle of mass 0.8kg is free to move when a slug is fired.
Using conservation of momentum the air rifle recoils at 0.1 m/s.

It then asks to calculate the mean force the person experiences to prevent the rifle recoilling more than 0.5 mm.

The momentum of the air rifle is 0.1 x 0.8 = 0.08 kg m/s

As the velocity of the air rifle is 0.1 m/s and it cant move more than 0.5 mm this is a time of (0.5 x 10^-3) / (0.1) = 5 x 10^-3 s

so the force needed is (0.08) / (5 x 10^-3) = 16 N

However the answer in the book is only 8 N so someone must be wrong...
2. (Original post by Chris2505)
I think the book is wrong (yet again) but can't be sure so thought i'd better check...

A basic air riflr fires slugs os mass 0.5 g at a velocity 160 m/s. The air rifle of mass 0.8kg is free to move when a slug is fired.

Using conservation of momentum the air rifle recoils at 0.1 m/s.

It then asks to calculate the mean force the person experiences to prevent the rifle recoilling more than 0.5 mm.

The momentum of the air rifle is 0.1 x 0.8 = 0.08 kg m/s

As the velocity of the air rifle is 0.1 m/s and it cant move more than 0.5 mm this is a time of (0.5 x 10^-3) / (0.1) = 5 x 10^-3 s

so the force needed is (0.08) / (5 x 10^-3) = 16 N

However the answer in the book is only 8 N so someone must be wrong...
I get 8N as well.

You can't use speed = distance x time for a non-constant velocity situation. Use . This should give you the answer.

(Often, this is the case when you get double the answer given in the solutions - it's a hint that you've omitted the possibility of non-constant velocity in your calculations. )
3. (Original post by james.h)
I get 8N as well.

You can't use speed = distance x time for a non-constant velocity situation. Use . This should give you the answer.

(Often, this is the case when you get double the answer given in the solutions - it's a hint that you've omitted the possibility of non-constant velocity in your calculations. )
ahhh of course!! though s = [(u + v) / 2] x t is probably better to use to calculate time.
4. Use force times distance is work done.
Work done is the loss in the KE of the rifle. Use 1/2 m v^2 for it.
This equals average force times distance (0.5mm)
5. (Original post by Chris2505)
ahhh of course!! though s = [(u + v) / 2] x t is probably better to use to calculate time.
Yes, sorry, I was thinking F = ma (constant mass), but finding the time also works.

Stonebridge's suggestion along the lines of energy considerations appears a simpler route, though.
6. (Original post by james.h)
Yes, sorry, I was thinking F = ma (constant mass), but finding the time also works.

Stonebridge's suggestion along the lines of energy considerations appears a simpler route, though.
Yes quite a bt Thanks
7. very good question!!!!!

### Related university courses

TSR Support Team

We have a brilliant team of more than 60 Support Team members looking after discussions on The Student Room, helping to make it a fun, safe and useful place to hang out.

This forum is supported by:
Updated: January 23, 2010
Today on TSR

### University open days

Wed, 25 Jul '18
2. University of Buckingham
Wed, 25 Jul '18
3. Bournemouth University
Wed, 1 Aug '18
Poll

The Student Room, Get Revising and Marked by Teachers are trading names of The Student Room Group Ltd.

Register Number: 04666380 (England and Wales), VAT No. 806 8067 22 Registered Office: International House, Queens Road, Brighton, BN1 3XE