# About expt re Newton's 2nd law

1. Jul 28, 2011

### grzz

hello all

Consider an experiment to prove that the acceleration in a certain direction on a given mass is directly proportional to the resultant force in that direction given that the given mass is constant. The acceleration can be measured using light gates. This resultant force can be varied by pulling on the given mass by elastic bands taking care to keep the extension of the band constant for a given resultant force.
I heard a discussion re this experiment where the resultant force was provided by hanging a mass on a pulley. i do not think this is correct Physics.
I would appreciate any reply whether I was correct in this.
Thanks.

2. Jul 28, 2011

### Staff: Mentor

What's your objection to it? It's not quite as direct as the example you gave. Certainly the force on the primary mass does not simply equal the weight of the hanging mass, if that's your point.

3. Jul 28, 2011

### grzz

My objection is, IMHO, that in finding the tension on the hanging mass which is the resultant force on the given mass ( we are assuming that contact between pulley and string is smooth so that pulley does not rotate) one has to use Newton's 2nd law itself. And we were always taught that in proving some law one cannot make use of that same law.

4. Jul 28, 2011

### Staff: Mentor

I agree. Were I to do that 'experiment' I would not calculate the tension. Instead I'd use a force of m2g (the weight of the hanging mass) acting on the total mass of m1 + m2. Still not quite as direct as your first example, but workable.

But even if you calculate the tension, you can always think of it as applying Newton's 2nd law to predict the acceleration, then checking your prediction.

The two methods--with and without the tension calculated--are entirely equivalent, of course.

Last edited: Jul 28, 2011
5. Jul 29, 2011

### grzz

Now I have a better insight into this experiment.
Thank you.