# I'm Physics Illiterate

1. Oct 7, 2004

### matchboxdude

I have a few questions that I desparately need help with. I'm new to this physics thing and it is really getting me.

On Earth, you have two parts of a space ship with weights 19000N and 5100N. They are separated center to center by 10m and are uniform spherical objects. what is the magnitude of the gravitational force that each part exerts on eachother out in space, far from everything?
I figure i would use F=(G*m1*m2)/r2 / F=(6.67E-11*19000*5100)/10^2 , but i got the question wrong. So, I don;t know my other options...

Then another question is...
A car is towing a boat on a trailer. The car starts from rest and accelerates to a speed of 12m/s in 18seconds. The combined mass of the boat and trailer is 490kg. What is the tension in the hitch that connects the trailer to the car?
I have an equation from a different problem, but I'm not sure if it works for this problem. T + 9.8 = mass*accelleration. But using this gives me an incorrect answer. So I assume I am going in the wrong direction again.

Thanks ahead of time for any help.

2. Oct 7, 2004

### Leong

#1 m1 and m2 are the masses of the parts of the space ship not their weights.

3. Oct 7, 2004

### matchboxdude

ahh that would make a difference. so would i use Weight = (G* Mass of Earth* mass of object)/radius of the earth^2 to get the masses then plug then into the formula i mentioned earlier?

4. Oct 7, 2004

### Leong

i mean you have to convert their weights into masses first before you put them into the formula. W=mg where
W= weight of part
m= mass of the part
g = the gravity acceleration on earth = 9.81 meter per second per second

5. Oct 7, 2004

### matchboxdude

ok i did that, plugged the numbers back into the original equation and got 6.73e-7 N does that sound like it could be correct?

6. Oct 7, 2004

### matchboxdude

i went ahead and used that answer and got it correct. thank you very much. how about that second problem? i dont have any clue where to go with it

7. Oct 8, 2004

### Leong

The acceleration a for the car and the trailer is given by (Assume a is constant through the towing):
$$a=\frac{v-u}{t}$$
$$a=\frac{12-0}{18}$$
a=2/3 meter per second per second
Consider the boat & trailer :
$$\Sigma\vec{F}=m\vec{a}$$
$$T=m_{trailer\&boat}*a$$
T= 490*(2/3) = 327 N

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