Need help / tips (Topic is FORCE)

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Homework Help Overview

The discussion revolves around two physics problems related to force. The first problem involves a bullet penetrating a block of wood, requiring calculations of time to stop and the force exerted by the wood. The second problem concerns a parachutist and the forces acting on her during descent, including weight and net force calculations.

Discussion Character

  • Mixed

Approaches and Questions Raised

  • Participants explore the average speed of the bullet and its implications for calculating time. There are discussions about the formulas for time and acceleration, with attempts to clarify the calculations involved. In the second problem, questions arise regarding the necessity of a free body diagram and how to approach the forces acting on the parachutist.

Discussion Status

Some participants have provided hints and confirmations regarding the calculations for the first problem, while others are seeking clarification on their results. The second problem is still in the early stages of exploration, with participants discussing the relevance of free body diagrams and the approach to summing forces.

Contextual Notes

The original poster expresses uncertainty due to the newness of the topic and seeks guidance on both problems. There is an indication that homework constraints may influence the approach to the second problem.

Tastosis
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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!
 
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Hint: What's the average speed of the bullet during its travel through the block?
 
350 / 2? Which is 175 m/s?
 
Tastosis said:
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?
 
Tastosis said:
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)
 
Looks good to me.
 
  • #10
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 >.<
 
  • #11
Welcome to PF!

Hi Tastosis! Welcome to PF! :smile:

(it's usually best to start a new thread for each question :wink:)

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 :wink:

just add all the forces (being careful about whether each force is positive or negative), and then use F = ma …

what do you get? :smile:​
 

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