# Homework Help: Weight & Mass (My Science Book is Wrong?)

1. May 29, 2006

### 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)?

2. May 29, 2006

### 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.

Last edited: May 29, 2006
3. May 29, 2006

### Andrew Mason

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

4. May 29, 2006

### 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?

5. May 29, 2006

### Ideologue

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!

6. May 29, 2006

### Chi Meson

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.

7. May 29, 2006

### Ideologue

Thanks for that, Chi.

8. May 30, 2006

### arunbg

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 .

9. May 30, 2006

### Hootenanny

Staff Emeritus
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

10. May 30, 2006

### arildno

Harrumph.

11. May 30, 2006

### Hootenanny

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

~H

12. May 30, 2006

### arunbg

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

13. May 30, 2006

### Integral

Staff Emeritus
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.