# Need help / tips (Topic is FORCE)

We just got into this topic and my teacher gave me an "introductory assignment"

## Homework Statement

1.) A 0.22 rifle bullet, traveling at 350 m/s, strikes a block of soft wood, which it penetrates to a depth of 0.130 m. The block of wood is clamped in place and doesn't move. The mass of the bullet is 1.80 g. Assume a constant retarding force. A.) How much time is required for the bullet to stop? B.) What force, in Newton's, does the wood exert on the bullet?

## Homework Equations

(I'm not sure)
V = Vo + at
V^2 = Vo^2 + 2a (X - Xo)

## The Attempt at a Solution

As of now, I have no clue because this is new to me. Help would be really appreciated. Thanks!

Doc Al
Mentor
Hint: What's the average speed of the bullet during its travel through the block?

350 / 2? Which is 175 m/s?

Doc Al
Mentor
350 / 2? Which is 175 m/s?
Exactly! Use that to solve for the time.

Do I use this formula: time = distance / velocity ?

If so, how do I get the distance? Is it the 0.130 m?

So using the above formula, I got 0.000743 seconds. Can someone clarify this?

Also, any tips on how to get B?

Doc Al
Mentor
So using the above formula, I got 0.000743 seconds. Can someone clarify this?
Looks good.

Also, any tips on how to get B?
Start by finding the acceleration.

Acceleration is 471063.26 m/s^2 ?

I used 350 m/s / 0.000743 seconds

If that's correct, the force would be... 847.91 N ? (I converted 1.8 g to kilograms)

Doc Al
Mentor
Looks good to me.

OK. Thanks. I'll clarify the answer I got to my teacher tomorrow.

I have a second problem. >.<
A parachutist relies on air resistance (mainly on her parachute) to decrease her downward velocity. She and her parachute have a mass of 55.0 kg and air resistance exerts a total upward force of 620 N on her and her parachute.
A.) What is the weight of the parachutist? B.) What is the net force? Is it upward or downward? C.) What is the acceleration (magnitude and direction) of the parachutist?

I need help / tips on how to go about this problem. I even don't know what the Free Body Diagram looks like >.<

tiny-tim
Homework Helper
Welcome to PF!

Hi Tastosis! Welcome to PF! (it's usually best to start a new thread for each question )

you don't really need a free body diagram for this problem (unless it's part of the question, of course) …

they're useful for 2D and 3D problems, but they don't really help for a 1D problem like this just add all the forces (being careful about whether each force is positive or negative), and then use F = ma …

what do you get? 