# Tensile force calculation

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. Homework Statement

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).

## Homework Equations

i calculated it as tension assuming tensile force means tension[/B]

T = mg x ma

## 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)

## Answers and Replies

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

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

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

Chestermiller
Mentor
what about the gravitational force ?
The gravitational force is the same thing as the weight.

Chet

The gravitational force is the same thing as the weight.

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

SteamKing
Staff Emeritus
Science Advisor
Homework Helper

## Homework Equations

i calculated it as tension assuming tensile force means tension[/B]

T = mg x ma

It's not clear where you got this equation. BTW, it's meaningless.

## 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)

(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.

Chestermiller
Mentor
so basically the tensile force is 420N or 0.42Kn, are tension and tensile force 2 different measurements or the same ?
They are different terms for the same thing.

Chet

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.

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

SteamKing
Staff Emeritus
Science Advisor
Homework Helper
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

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.

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 =

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

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

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

Chestermiller
Mentor
Where did I say that? This is the new equation I've come up with following the advice from this thread
Compare you final two equations in post #10.

Chet

Compare you final two equations in post #10.

Chet
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

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

SteamKing
Staff Emeritus
Science Advisor
Homework Helper
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
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.

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.

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

SteamKing
Staff Emeritus
Science Advisor
Homework Helper
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
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?

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?
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

SteamKing
Staff Emeritus
Science Advisor
Homework Helper
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
Well, 420 x 9.8 ≠ 4.37

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

Well, 420 x 9.8 ≠ 4.37

You should be able to estimate what the answer should be without a calculator.
The calculation I used wasw 420 x 9.8-2

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

SteamKing
Staff Emeritus
Science Advisor
Homework Helper
The calculation I used wasw 420 x 9.8-2

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

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.

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