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Model of parachute that doesn't include gravity

  • Thread starter hholzer
  • Start date
  • #1
37
0
I'll post this question here because it isn't a homework question.
I've already solved this problem that I have taken from a physics
book. My question pertains to why would gravity not be accounted
for? (The reason I think no gravity is account for is because the
solution provided by the book does not include gravity as a force
a part of the net force.).

The problem is as follows:

"When a parachute opens, the air exerts a large drag force on it.
This upward force is initially greater than the weight of the sky diver
and, thus, slows him down. Suppose the weight of the sky diver is
906 N and the drag force has a magnitude of 1105 N. The mass of
the sky diver is 92.4 kg. What are the magnitude and direction of his
acceleration?"

This is a clear problem of sum(F_i) = ma -- where 'a' is a vector quantity
and each F_i corresponds to a force that makes up the net force.

Why would gravity(acting in a downward direction) not be included
in the net force? I don't see anything in the problem which might
indicate that it should be ignored.
 

Answers and Replies

  • #2
4,239
1
Your right. The question is clearly rubbish.
 
  • #3
epenguin
Homework Helper
Gold Member
3,717
754
Isn't 'the weight of the sky diver' the gravitational force?
 
  • #4
37
0
It isn't clear to me why you would consider them the same.

Could you elaborate on this?
 
  • #5
berkeman
Mentor
57,330
7,327
I'll post this question here because it isn't a homework question.
I've already solved this problem that I have taken from a physics
book. My question pertains to why would gravity not be accounted
for? (The reason I think no gravity is account for is because the
solution provided by the book does not include gravity as a force
a part of the net force.).

The problem is as follows:

"When a parachute opens, the air exerts a large drag force on it.
This upward force is initially greater than the weight of the sky diver
and, thus, slows him down. Suppose the weight of the sky diver is
906 N and the drag force has a magnitude of 1105 N. The mass of
the sky diver is 92.4 kg. What are the magnitude and direction of his
acceleration?"

This is a clear problem of sum(F_i) = ma -- where 'a' is a vector quantity
and each F_i corresponds to a force that makes up the net force.

Why would gravity(acting in a downward direction) not be included
in the net force? I don't see anything in the problem which might
indicate that it should be ignored.
(thread moved to HH/Intro Physics. Even if the problem is for self-study, it belongs here. Dem's the Rules)

On your question -- you are given the weight of the skydiver. What force causes that weight?
 
  • #6
D H
Staff Emeritus
Science Advisor
Insights Author
15,393
683
It isn't clear to me why you would consider them the same.
Two reasons:
1. You are given that the "weight of the sky diver is 906 N". What else is this other than the gravitational force on the skydiver?

2. That weight is a synonym for gravitational force is the most widespread meaning of the word "weight", particularly so in most introductory physics texts and in most aerodynamics texts (regardless of level).
 

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