Help with physics investigation! Newtons second law

[timtamboy63]

Okay so here's my problem.

What we did for our investigation was we got a cart with two of those weight things: http://www.thesciencefair.com/Merchant2/graphics/00000001/SltdWtSt90-1_M.jpg

Okay now we basically had a piece of string attached from the end of the cart to another weight(the empty holder things from above). This was hanging over the desk.

We let that weight go and the cart accelerated.

We then changed the amount of weight on the end(which we were dropping), taking care to keep the total mass of the body constant. We did this by taking one of the small round weights(which weigh 50g) and taking that off the cart and putting it on the end weight.

Here are some numbers for one of our trials:

Mass of end weight(which was hanging over the edge):150g

Mass of Cart(+ the weights on it):955.41g

Distance the Cart Accelerated over(same as the distance the end weight fell):58.5cm

Time taken for the end weight to hit the ground(same as the time the cart accelerated over): 1.87s

So what I need to do is find out both the Force and Acceleration. I've tried, but the relationship doesn't seem to be F=ma, which it should be. Can anyone guide me on where to go?

Diagram:

http://pastie.org/1169743

Edited September 20, 2010 by timtamboy63

[Heisenberg]

Did you make sure to write the mass in kilograms instead of grams and length in meters? That's one of the most usual problems in physical formulae - remembering the fundamental units

[timtamboy63]

I did, it made really messy numbers, but I definately did.

[timtamboy63]

Erm, I am really confused. I calculated the Acceleration downwards by the weight hanging on the side of the table to be 0.334ms^-2. I thought it should have been 9.8ms^-2?

[Heisenberg]

Not if the exercise states that the acceleration does not equal 1g, which it doesn't. The acceleration should be 9.81 indeed. If you have the acceleration and length, force is easy to calculate.

[timtamboy63]

As far as I can tell, the tension is reducing the acceleration. I had to do half an hours research on tension before I finally worked out what was going on here. My physics teacher went over tension in about 10 minutes. But I had it worked out, only now ive forgotten again ;P

But yeah, cheers for the help

[Sublime Sunshine]

Just to clarify. The force acting on the cart is the force of gravity that is acting upon the weights suspended over the table.

F = mg

Therefore the force that is accelerating the cart is equal to the mass of the weights hanging over the edge multiplied by 9.8 ms^-2

For the cart:

F = ma

We now know that the force acting upon the cart is equal to the force of gravity upon the weights, therefore you can substitute the equation to form:

mg = Ma

(Remembering that these masses are not the same, to illustrate this a capital M has been used for the mass of the cart)

Then, for each of the tests, you can find the acceleration through the formula

a = (mg)/(M)

For example, for the data you provided

a = (0.150 kg)x(9.8 ms^-2)/(0.955 kg)

acceleration of the cart is approximately 1.5 ms^-2

[timtamboy63]

Thanks

I worked it out somehow before, but now I understand it properly, thanks again.