# Help needed for experimental design - Force

1. Jul 30, 2009

### jozef

1. The problem statement, all variables and given/known data

I need a method for my experimental lab procedure that allows me to use the same force multiple times. I need to provide a constant force to object A that causes it to collide with object B -- on a horizontal plane. I thought of using a spring on object A where F=kx but assuming I have the maximum compression how would I determine the spring constant.

Another method I thought was to place object A on an inclined plane where it would roll down. For this method I thought of using the constant acceleration equations and then using the equation F=ma to find the applied force (I have displacement, initial velocity - 0, and I can time the length at which it rolls down the inclined plane thus solving for acceleration) However, the problem with this method is that I have to factor in the friction.

Can anyone help me work out my problems or provide me with new methods?

2. Relevant equations

The five constant acceleration equations (?)
F=ma

3. The attempt at a solution

See above.

2. Jul 30, 2009

### slider142

Access to an air track will allow you to ascertain that friction is neglible for most setups. If you still want a spring, then with two known masses, determining the spring constant is just algebra.

3. Jul 30, 2009

### jozef

For experimental purposes I will need to include friction in the investigation. As for the whole algebra part for the spring constant I'm not sure I understand what you mean. I know Hooke's Law: F=-kx but how do I determine the spring constant with two known masses. (if it helps both masses are the same)

4. Jul 30, 2009

### slider142

If hanging a known mass without stretching the spring beyond the application of Hooke's law is feasible, you can solve for k by applying knowledge of the force due to gravity on the mass balancing the force due to the spring, giving a net force of 0.
If that is not an option, you would definitely need an air track to measure momentum using two masses.
Otherwise, if you really need friction and you can't hang a mass, you would need to use two different masses or two different spring compressions to get two equations in two unknowns (friction constant and spring constant).

Last edited: Jul 30, 2009
5. Jul 30, 2009

### jozef

I think you are misunderstanding what I meant when I said use a spring. I meant use a horizontal spring and compress it. I can measure the max. compression but I need the spring constant to determine the force. By doing this I can keep my force constant correct? Perhaps you can point me in the right direction I'm not sure. I'm still open to other options if possible if this doesn't work.

6. Jul 30, 2009

### slider142

The spring constant does not depend on the orientation of the spring. Hanging the mass is the simplest method of getting the spring constant. If that is not possible, then you would either have to measure the velocity of masses accelerated by the spring, or ascertain the force of friction due to the surface slanting the surface and using gravity as the constant known force (easiest), or using the unknown force provided by the spring in two scenarios so that you can solve two equations in two unknowns.

7. Jul 30, 2009

### jozef

Oh I understand your point now. Thanks for all your help but I just want to clarify one point you said:

"or ascertain the force of friction due to the surface slanting the surface and using gravity as the constant known force (easiest)"

I'm not too sure about what you wrote here. Are you referring to determining the co-efficient of friction where Fnet = Fgravity + Ffriction but how will that help me with regards to the lab