How to calculate the angle using work, displacement and 2 forces?

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Homework Help Overview

The problem involves calculating an angle based on given work, displacement, and forces. The original poster presents a scenario where work, weight, displacement, and an applied force are provided, but there is confusion regarding the correct approach to find the angle.

Discussion Character

  • Mixed

Approaches and Questions Raised

  • Participants discuss the need for clarification regarding the problem setup and the specific angle being sought. There are attempts to differentiate between two problems, one where the angle is given and another where it needs to be calculated. Some participants suggest reevaluating the approach to calculating work done by different forces separately.

Discussion Status

The discussion is ongoing, with participants providing guidance on how to approach the problem correctly. There is recognition of the need to separate the calculations for different forces and to consider the direction of the applied force relative to the displacement.

Contextual Notes

There is confusion stemming from mixing up two different problems, which has led to incorrect assumptions about the angle and the forces involved. Participants are addressing the implications of weight and its contribution to work in this context.

fixedglare
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Homework Statement



Given:
Work = 12000 J
Weight: 800 N
displacement: 200 m
Force: 120 N
angle = ?

Homework Equations


W = Fd

W = Fd cos θ

cos -1 (W/Fd) = θ


The Attempt at a Solution



First I added the 2 forces together.

800 + 120 = 920 N

calculated work; 200 m * 920 N = 184000 J

then used the second formula;

cos -1 (12000 J /184000 J) = 86.3°

but the answer in my book says the answer is supposed to be 60°...

help me please.
 
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You're going to have to clarify the situation. What is going on in the problem? A block sliding, it looks like? Also, what do mean by the angle? The angle between what?
 
jackarms said:
You're going to have to clarify the situation. What is going on in the problem? A block sliding, it looks like? Also, what do mean by the angle? The angle between what?

The question says;

a person uses a cord to pull a boat with a mass of 1000 kg by 50.0 m along the board walk. The cord makes a 45 degrees angle with the horizontal force. If a force of 40 N is applied to the boat, how much work is realized?
 
I think you have the wrong problem -- in that one you're given the angle, but in this one you're trying to find it.
 
jackarms said:
I think you have the wrong problem -- in that one you're given the angle, but in this one you're trying to find it.

You're right, I got them mixed up.

The real one says,

12000 J of work is required to pull a wagon that weighs 800 N, in a distance of 200 m. If the applied force over the cord is 120 N, what is the angle?

Sorry for that.
 
No problem.

You have the same problem as before -- adding the forces to find the work, when they need to be done separately. So first find W_{weight} and then W_{app}.
 
@Jacko: Please stop trying to lift boats and wagons. They stay level. No Wweight.
 
The point was to figure out that the work done by weight is zero.
 
Fixo: You're doing the right thing. W is work in your second relevant formula, not Weight. Don't let the weight or Jacko distract you. What is W/Fd ? right, no need for a calculator!

The first formula doesn't apply: F isn't in the direction of d. That's where the cos comes in: to calculate the component of F that is in the direction of d.
 
  • #10
jackarms said:
The point was to figure out that the work done by weight is zero.

I would have never figured that out. But with this new found information I will try to do it again.
 

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