# Homework Help: Tensile force calculation

1. Nov 9, 2015

### jbrandon

This question is confusing me, trying to find an equation for "tensile force" has left me confused since ive found no equation for it just for tension, tensile stress and tensile strain, just wanting to make sure ive not missed anything or done it incorrectly

1. The problem statement, all variables and given/known data

At a building site, an iron girder of mass 420 kg is suspended from a crane
by a steel cable. Assume that the cable has a circular cross-section of
diameter 25 mm.

Determine the tensile force in newtons on the cable in kN to 2 decimal
places. (Take the acceleration due to gravity as 9.8 m s−2 and ignore the
mass of the cable).

2. Relevant equations
i calculated it as tension assuming tensile force means tension

T = mg x ma

3. The attempt at a solution
(420kg x 9.8ms2) x (420kg x 9.8ms2) = 8232 kg ms2

which i then converted to newtons = 8232N (8.23Kn)

2. Nov 9, 2015

### Staff: Mentor

This is not correct. The tension in the cable is just equal to the weight of the girder.

3. Nov 9, 2015

### jbrandon

what about the gravitational force ? also is tension the same as tensile force or are they 2 separate measurements ?

4. Nov 9, 2015

### Staff: Mentor

The gravitational force is the same thing as the weight.

Chet

5. Nov 9, 2015

### jbrandon

so basically the tensile force is 420N or 0.42Kn, are tension and tensile force 2 different measurements or the same ?

6. Nov 9, 2015

### SteamKing

Staff Emeritus
It's not clear where you got this equation. BTW, it's meaningless.
(420 × 9.8) × (420 × 9.8) = (420 × 9.8)2 ≠ 8232

Also, what are the units here? Hint: they're not kg⋅ms2

Remember, g = 9.8 ms-2, which is not the same as 9.8 ms2. Things like - signs and × signs are important.

7. Nov 9, 2015

### Staff: Mentor

They are different terms for the same thing.

Chet

8. Nov 9, 2015

### jbrandon

The formula I used was

T = mg + ma

T = tension, N, kg-m/s2

m = mass, kg

g = gravitational force, 9.8 m/s2

a = acceleration, m/s2

9. Nov 9, 2015

### SteamKing

Staff Emeritus
Using × in place of + suggests something else. × means multiplication. + means addition.

The formula T = mg + ma should be used only when there is some other acceleration a acting on m besides gravity g.

10. Nov 9, 2015

### jbrandon

Sorry I didn't catch that I'd used x rather than +, I used + In my actual calculation.

So using 9.8ms-2 and not applying gravity as the acceleration would give me

(420kg x 9.8ms-2) + (420kg x 0) =

Or since there is no acceleration not include it at all like this?

(420kg x 9.8ms-2) + 420kg =

11. Nov 9, 2015

### SteamKing

Staff Emeritus
According to this calculation, you're saying that 420 kg × 0 = 420. Is that some kinda Common Core math?

12. Nov 9, 2015

### jbrandon

Where did I say that? This is the new equation I've come up with following the advice from this thread

13. Nov 9, 2015

### Staff: Mentor

Compare you final two equations in post #10.

Chet

14. Nov 9, 2015

### jbrandon

Those are 2 different equations, the first is where I've left acceleration in and the second is where I've removed it leaving only mass

15. Nov 9, 2015

### Staff: Mentor

In that case, the first equation is correct and the second equation is incorrect.

16. Nov 9, 2015

### SteamKing

Staff Emeritus
Yeah, but force = mass × acceleration

Setting acceleration to zero means the force = 0, by definition.

You can't be careless with equations, math, or units. These will cost you points on exams and assignments when studying physics if you don't make sure they're all correct.

17. Nov 9, 2015

### jbrandon

This is why I'm here to gain a better understanding and learn

So the correct equation would be

(420kg x 9.8ms-2) + (420kg x 0) = 4.37kg m/s-2

18. Nov 9, 2015

### SteamKing

Staff Emeritus
I'm not sure how you can multiply 420 kg by 9.8 m/s2 and wind up with 4.37 kg-m/s2. That doesn't make sense arithmetically.

Also, kg m/s-2 is not the same as kg m/s2. Have you studied algebra yet? The laws of exponents?

19. Nov 9, 2015

### jbrandon

I haven't studied any algebra yet this is the first month of my distance learning degree, I multiplied it by 9.8m/s-2 [/SUP ] to get that answer

20. Nov 9, 2015

### SteamKing

Staff Emeritus
Well, 420 x 9.8 ≠ 4.37

You should be able to estimate what the answer should be without a calculator.

21. Nov 9, 2015

### jbrandon

The calculation I used wasw 420 x 9.8-2

So the correct way is the way you've presented to me?

22. Nov 9, 2015

### SteamKing

Staff Emeritus
You must learn to distinguish the units (ms-2) from the magnitudes (the numbers, like 9.8 or 420).

If you see 9.8 ms-2 written, that means 9.8 meters per second per second, not 9.8-2 meter seconds. If you're going to study physics, it is essential that you learn what units go with what physical quantities. For example, acceleration always has units of $\frac{distance}{time^2}$, or distance⋅time-2 as it is sometimes written.

If you want to indicate that something happens to the magnitude of a given unit, then you must show that operation directly on the magnitude.

For example, the area of a square is the square of the length of the sides, or A = L2. If a certain square has a side which measures 4 meters, or L = 4 m, then you would show the calculation of the area of the square as

A = L2 = 42 m2 = 16 m2, or sometimes
A = (4 m)2 = 16 m2

This is why I had asked in a previous post if you had studied algebra or the laws of exponents. A lot of these misconceptions would have been prevented from forming in your mind by this additional study.

23. Nov 10, 2015

### jbrandon

Thank you for the explanation, I have just got the mathematics for engineering book so I'll be studying that alongside my actual study to try and bring myself up to speed