# Need Help

1. Oct 9, 2006

### Katie17

Hey everyone,

Just wondering if anyone could help me with my physics homework. It's my first time ever doing physics and I was always terrible at maths. It needs to be in soon so I'm kind of freaking out If anyone could either show me how to answer these questions or even pm I'd be very grateful.

1. [a] The radius of a circle is 2mm. Write down an expression for its area A in scientific notation of the form A = a x 10 to the power of -b m to the power of -2, where a and b are constants.
A sphere of lead has a mass of 0.1 kg. If the density of lead is 11.3 x 10 to the power of 3 kg m to the power of -3, find the radius of the sphere [remembering that density = mass over volume]

2. [a] What is the speed in kilometres per second of a car travelling at a constant speed of 60 miles hr to the power of minus 1? [1 mile = 1.609 km]

What is the speed of the car in metres per second? [1km = 10 to the power of 3 m]

[c] What is the average speed for a trip of 157 km that requires 2.45 hr?
[d] How far in km will a car go in 3.5 hr at a constant speed of 95 km hr to the power of -1?

I know this might seem really easy to people who are into physics but to me this is really hard.

2. Oct 9, 2006

### staceyw

well for parts 2 a) b) and c) you could use speed = distance
--------
time
and then just rearrange the formula for d), that is how i would do that anyway! Hope it helps!

3. Oct 9, 2006

### Hurkyl

Staff Emeritus
What have you tried on these? Have you at least had some ideas, even if they didn't work?

P.S. the typical way to write

$$a^b$$

in a text format is as a^b. (Make sure to use parentheses if your base or exponent are expressions, and not just a single symbol)

4. Oct 9, 2006

### Katie17

I can't even attempt them. Speed = distance??

I haven't got the slightest clue what you're trying to say. I've always been very slow when it comes to anything involving maths. I took a year out before college so even basic maths is dusty. Really going to be stuck if this is like a different language.

5. Oct 9, 2006

Hurkyl meant that you can write 'a^b' instead 'a to the power of b'.

6. Oct 9, 2006

### Katie17

Oh okay thanks

7. Oct 9, 2006

### mbrmbrg

and I think staceyw meant to write $$speed = \frac{distance}{time}$$

8. Oct 9, 2006

### mbrmbrg

Actually #2 parts a and b are giving you a value in one type of units and asking you to convert it into another type of units. (This is sometimes called dimensional analysis.)

You want to set up a multiplication problem to cancel units, like so:
to get 3 weeks into days
$$\frac{3weeks}{1}\times\frac{7days}{1week}=21days$$

Do you see what happened? Take your value (in this case 3 weeks) and multiply it by a convenient form of 1(in this case 7days/1week). The weeks canceled, and you're left with days.