# Forces for Differing Masses and Accurately Reporting Them

1. Feb 14, 2013

### KayCup

Hey all,

I'm currently working on a chemistry lab report analyzing different ratios of reagents in polymers and their abilities to bounce. These different samples were dropped from a constant height and their bounces were recorded. I've calculated the force each struck the ground with.

My question is since different masses yield different forces, how should I account for this in my final report of bounce heights? What if I reported this in height per unit of mass? How can I analyze the effects of the ratios of reagents without influence from different forces?

I'd appreciate any direction someone could offer. :)

Last edited: Feb 14, 2013
2. Feb 14, 2013

### Staff: Mentor

Welcome to Physics Forums!!!

Just report the ratio of the bounce heights for each material. This should be independent of the mass.

Incidentally, how did you calculate the force each struck the ground with?

3. Feb 14, 2013

### KayCup

Thanks!

I'm sorry, I don't think I understand what you're saying. Why don't different masses affect the force?

I used F=KE/D, where D is the distance traveled after impact, or, in my case, how high the polymer ball bounced initially after impacting the ground. To calculate KE, I used: KE=1/2(mass*velocity), and velocity was V=square root(2*initial height*gravity)

So for my sample with mass of 1.85 g dropped from a height of 50 cm, with an initial bounce height of 1.85 cm, my force was 25.2 N.

If I'm doing something wrong, please tell me. Unfortunately, I haven't taken a physics class yet, so this is all information that I'm getting from the internet.

4. Feb 14, 2013

### ZapperZ

Staff Emeritus
Were you given permission to seek external help to do your lab report?

BTW, this should have been done in the HW/Coursework forum, where this thread will be moved to.

Zz.

5. Feb 14, 2013

### KayCup

In the information handout that we're given about the experiment, it encourages us to utilize resources available (i.e. book for the course, internet, etc.), and my TA suggested the tutoring center on campus for help. I'd go there now, but I'm not on campus at the moment, and I'd like to avoid working on this last minute.

Either way, sorry about using the wrong forum. I thought the HW forum was reserved specifically for physics classes.

6. Feb 14, 2013

### Staff: Mentor

I'm sorry, KayCup, but this is basically wrong. The equation for the force makes no sense.

If I understand what you are trying to do, you are trying to determine which formulation results in the highest elastic recovery (least dissipation of energy). To do this, you can take the ratio of the bounce height on subsequent bounces, including after you first release the ball. The closer this ratio is to 1.0, the higher the elastic recovery of energy. Properly determining the force (or really, the time variation of the force during contact with the ground) is a very complicated problem in deformational mechanics, including the so-called viscoelastic response behavior of the polymer.

I see from your data that, for example case you cited, the bounce height was only 1.85 cm for a drop height of 50 cm. This would only be a bounce ratio of 0.037, and would indicate a very large amount of energy dissipation, with not much elastic recovery. Are these figures correct? If you bounce a basketball, the bounce ratio will typically be on the order of 0.8, indicating a high degree of energy recovery.

Chet