# Equation to find density of metal

1. Nov 4, 2005

### RMHAZARD

i need help please what si unit do i use to messure the mass of a coin i believe that you would have to use grams but i believe i could be mistaken
Thanks
also how woul i calculate the weight of a lump of metal of mass 3.4 x 10 to the power of -six
I forgot what is the equation to find density of metal and weight of metal under water
i do not want the answer just a step in the right direction

Last edited: Nov 4, 2005
2. Nov 4, 2005

### zanazzi78

The SI unit for mass is the Kilogram, so using grams is ok.

Weigh is a force, Newtons 2nd law states; Force = Mass x Acceleration.

The mass of the object will not change if placed in water. Similarly the acceleration due to gravity does not change in water, therfore its wieght is unchanged if on land or in water.

Q. What is the definition for density?

3. Nov 4, 2005

### ranger

The SI unit for mass is kg.
The weight of and object is proportional to the mass and the acceleration of gravity.

4. Nov 4, 2005

### RMHAZARD

10 to the power -6 would be what in normal digits

5. Nov 4, 2005

### zanazzi78

not sure what you mean but ...

1 x 10 ^ -6 = 0.000001

6. Nov 4, 2005

### RMHAZARD

kk thanks i have got a test and are finding out how to do these types of question how would i do this one pleae

A 2g mass falls 10m. Calculate the kinetic energy of the mass after falling this distance. Gravitational acceleration = 10 m/s ^ 2

7. Nov 4, 2005

### zanazzi78

Do you know the equation for Kinetic energy?
Do you know the relationship between Potential energy and kinetic energy?

8. Nov 4, 2005

### RMHAZARD

The equation for Kinetic energy is

K.E = 1/2 Mv ^ Kinetic energy +1/2 Mass x Velocity

Potenial enery and kinetic energy forgot what they had in common

Last edited: Nov 4, 2005
9. Nov 4, 2005

### zanazzi78

$$K.E. = \frac{mv^2}{2}$$
P.E. = mass x gravity x height
Q. How are P.E. and K.E. related?

Last edited: Nov 4, 2005
10. Nov 4, 2005

### RMHAZARD

i think i have got the answer is it 5.67893 ami right if i am wrong can you show the steps u did to do it

11. Nov 4, 2005

### zanazzi78

How did you get that value?

if you tell my what you did, ill show you where you went wrong.

What are the units for Kinetic Energy?

Last edited: Nov 4, 2005
12. Nov 4, 2005

### RMHAZARD

the units for kinetic energy are K.E?

how u find velocity i think i know what to do now thanks

Last edited: Nov 4, 2005
13. Nov 4, 2005

### zanazzi78

Mass is mesured in Kg
Time in seconds,
Energy in ...

14. Nov 4, 2005

### RMHAZARD

i done it twice i don't know if either one is correct

First attempt

K.E = ? Mass = 2

Gravitational acceleration = 10

so i worked out i need to find velocity so the formula for velocity is V = M x A
V = 2 x 10 = 20 so velocity is 20 now i have done that i need to do
K.E = MV ^2 all divided by 2 so thats 40 divide by 2 equal 20 is this right?

Second atempt

I think Kinetic Energy = 1/2 Mass x Velocity squared.

Therefore: 1/2 x 0.002 x Velocity squared.

We need to work out velocity.

V = U squared + 2 accel x distance
V= 0 + 2 x 10 x 10
V = 200 but that doesnt at all sound right.

Therfore = 1/2 0.002 x 200 squared
= 40

15. Nov 4, 2005

### zanazzi78

When you read a question think "What information do i have? how can i use this info?" If a question gives you a height then you need to use it some where, teachers dont give you info in a question to confuse you!

Fact; "The lost in potential energy is equal to the gain in Kinetic energy"

The question gives you the height of the falling object! - 10m

It gives you mass 2g or 0.002kg

It gives you acceleration due to gravity - 10m/s^2

PE= m.g.h

Therefore you know all the values on the right hand side!

But for the K.E expression you only know the mass

The question is designed to test your knowlede of the relationship between K.E. and P.E.

so you work out the P.E. of the object when it is 10m above the ground and then state that once it has fallen 10m, then the very same, amount of energy it "transfered" into Kinetic energy.

so we now have ...

P.E. = 0.002 kg x 10 m x 10m/s^2
= 0.2 ?, you never answered my question "what is energy measured in?"
so ill ask it again, what are the units here? You will loose marks if you dont get the units right!

16. Nov 4, 2005

### RMHAZARD

Energy is messured in joules

17. Nov 4, 2005

### RMHAZARD

so what did i get wrong?

Last edited: Nov 4, 2005
18. Nov 4, 2005

### zanazzi78

only your approach to the question

You made the assumption that since the question was asking about Kinetic energy you needed to use $$\frac{mv^2}{2}$$ you dont. The question gave you all the info you needed to work out the potential energy.

you first attemp was allmost right you need to use si units i.e. kg inorded to get teh correct value of joules, there is a big dirffrence between 20 and 0.2!

hope this helped.

19. Nov 4, 2005