Weight & Mass (My Science Book is Wrong?)

[Ideologue]

According to a science book I have a person with a mass of 90kg will weigh 900 Newtons on Earth. As 1kg = 10 Newtons I think this science book is making a mistake. Surely weight is the force of gravity * mass? So, as gravity on Earth is 9.81m² and the person’s mass is 90kg their weight on Earth should be 882.9 Newtons (9.81 * 90)?

 

[dav2008]

Your book just rounded the value of 9.81 m/s² to 10 m/s².

Also, notice that 9.81 m/s² is the acceleration due to gravity near the Earth's surface. It's not the force due to gravity. The force due to gravity on an object is its weight.

 

[Andrew Mason]

According to a science book I have a person with a mass of 90kg will weigh 900 Newtons on Earth. As 1kg = 10 Newtons I think this science book is making a mistake. Surely weight is the force of gravity * mass? So, as gravity on Earth is 9.81m² and the person’s mass is 90kg their weight on Earth should be 882.9 Newtons (9.81 * 90)?

Your book is rounding g = 9.81 m/sec^2 to g = 10 m/sec^2. Using fewer significant figures is often done to simplify calculations.

AM

 

[Ideologue]

I see. Thank you for that.

I did think they rounded off the number, but being a science book they really should not have done. I am new to this and very confused!

So weight = mass * gravitational acceleration, because weight is gravitational force?

 

[Ideologue]

Your book is rounding g = 9.81 m/sec^2 to g = 10 m/sec^2. Using fewer significant figures is often done to simplify calculations.

AM

Thank you also.

I understand the usefulness of rounding-off, but in science having exact numbers is critical. This book (a basic-level book), in my opinion, is wrong to round-off such figures as it causes confusion in neophytes such as myself!

 

[Chi Meson]

I see. Thank you for that.

I did think they rounded off the number, but being a science book they really should not have done. I am new to this and very confused!

So weight = mass * gravitational acceleration, because weight is gravitational force?

That is correct. Another name for "g" is "gravitational field strength." I prefer this name since (as you must have noticed) things still have weight even though they are not accelerating in free fall. But since "acceleration due to gravity" is equivalent to "gravitational field strength" the terms are used interchangably.

 

[Ideologue]

Thanks for that, Chi.

 

[arunbg]

Thank you also.

I understand the usefulness of rounding-off, but in science having exact numbers is critical. This book (a basic-level book), in my opinion, is wrong to round-off such figures as it causes confusion in neophytes such as myself!

Perhaps they were measuring weights at the Earth's poles ( of course they should have mentioned that ), where g is much closer to 10m/s^2 .
But for all practical purposes acceleration due to gravity on the surface of the Earth is taken as 9.81m/s^2 .

 

[Hootenanny]

Just further to what arunbg said, the gravitations field strength (g) varies greatly on the surface of the earth. For example the standard acceleration due to gravity is given as 9.80665 m.s^-2 and at the poles g is 9.832 m.s^-2. So you see in reality 9.81m.s^-2 is not significantly more accurate than 10m.s^-2.

~H

 

[arildno]

As 1kg = 10 Newtons

Harrumph.

 

[Hootenanny]

As 1kg = 10 Newtons

Harrumph.

I would be appauled if any physics textbook made this statement. Which text are you reading from?

~H

 

[arunbg]

I don't think that statement was in the textbook, rather it was the OP's interpretation .
Well it's all clear now .

 

[Integral]

You might check the forward, or read closely the first chapter, the author may state that he will define g to be 10 to simplify calculations while attempting to convey the concepts.