Torque about P (Cross Product?)

Click For Summary
SUMMARY

The discussion focuses on calculating torque using the cross product of vectors, specifically in the context of a physics homework problem. The correct formula for torque is established as Torque = r X F, where r is the moment arm and F is the force vector. The user initially attempted to calculate torque using scalar multiplication, yielding an incorrect result of 389.71 ft-lb instead of the correct 32.5 ft-lb. The discussion emphasizes the importance of defining vectors correctly to perform the cross product accurately.

PREREQUISITES
  • Understanding of torque and its formula: Torque = r X F
  • Knowledge of vector representation in physics
  • Familiarity with trigonometric functions, particularly sine and cosine
  • Basic skills in performing vector cross products
NEXT STEPS
  • Study vector representation and operations in physics
  • Learn how to calculate torque using the cross product in various scenarios
  • Explore the implications of moment arms in torque calculations
  • Review trigonometric identities and their applications in physics problems
USEFUL FOR

Students studying physics, particularly those focusing on mechanics, as well as educators and tutors looking for clear explanations of torque calculations and vector operations.

Reefy
Messages
62
Reaction score
1

Homework Statement



2mnn6l2.png



Homework Equations



The book says that

Torque = the length of the moment arm X the magnitude of the vertical component of F
= r X F

The Attempt at a Solution



My problem is that I don't have any vectors to do a cross product. Instead I tried to just multiply what I know:

50lbs x 9in x sin60° ≈ 389.71ft-lb

which is far from the correct answer, 32.5ft-lb. The problem seemed simple enough so am I making some small mistake or going about this completely wrong?
 
Physics news on Phys.org
Just because you aren't given any explicit vectors doesn't mean you can't formulate your own.

Looking at the problem, suppose that we set up an x-axis along the wrench.
In this case, vector r is (9,0) inches, with the emphasis on inches

The force vector F is going to be (50 cos (-60), 50 sin (-60)) lbs.

You can then evaluate r x F in the usual manner

Remember: in x lbs is not equal to ft-lbs
 
Ah, you're right! Thank you, that makes perfect sense.
 

Similar threads

  • · Replies 2 ·
Replies
2
Views
8K
  • · Replies 1 ·
Replies
1
Views
2K
  • · Replies 7 ·
Replies
7
Views
2K
  • · Replies 3 ·
Replies
3
Views
5K
  • · Replies 5 ·
Replies
5
Views
2K
Replies
14
Views
2K
Replies
1
Views
2K
Replies
21
Views
4K
Replies
2
Views
3K
  • · Replies 2 ·
Replies
2
Views
2K