How to Apply Force

Not enough information on the apparatus, but how about trickling water or sand into a container. Sand would have the lowest oscillation.

Imagine a weight on small wheels on an inclinable plane. It is connected with a string with the static mass via the force transducer. The string goes over a pulley. If you change the angle of the plane the tension in the string will grow proportional to angle sinus. The angular speed can be very low and thus the horizontal force pulling the mass can increase slowly till it is > adherence force and friction goes from static to dynamic.

It will have 2 aspects to show the friction evolution and the relationship between angle and force.

I thought about that, but the problem is that the velocity will not be linear because you are using the acceleration of gravity. Initial velocity will start slowly, but build in velocity as time goes on.

What about using a simple container into which you slowly pour sand. This will provide a steady increase in force which could be repeated.

Hi,

you do not need a force generator but a constant speed generator.

This can generate low force or high force depending on conditions of contact and local irregularities.

If this can maintain the velocity between mm/week (fingernail growth velocity) and cm/s this would be really useful.

In precision technical application this is realised (with a lot of $$) by either a hydraulic cylinder that is recircling its oil from one side to the other through a changeable orifice and being driven by a pneumatic cylinder or by a linear motor and amplifier and guiding stage and incremental encoder and servo loop and and servo electronics. Highest quality will be more than 100K$ per axis.

Cheapest solution I know is: buy a used screw-driven motorised stage with stepper motor and fine interpolation of steps. (see for example lasermotion.com). Connect to commanding computer that sets the velocity.

Or: get an old inkjet printer and use the head drive with modified drive frequency.

RHABE

I believe this represents all that is bad in teaching Physics that came in , in the UK, with the introduction of Nuffield or black box physics.

Having said that I feel a lot better.

First question is What are you trying to show? 2nd is at what level?

If it is the transition between static and dynamic friction then it is a good pupil experiment to do with weighted blocks and pans and pulleys. The pupil can then learn that the static friction builds up using the nudge meter or first digit as it is sometimes called, or even a calibrated compressible spring if you need to quantify.

I see your problem. The issue is you have is the manual data logging, not problems with the repeatability of the experiment.

What you need is a way to maintain constant speed of the block while you run the experiment.

Pulling the block is fine, but it needs to be done at a constant speed. I would recommend using a low speed motor with a pulley to pull a line from the force gauge toward the motor. You probably should tape the force gauge either to a wheeled trolley or mount it to the block itself and add the mass of the gauge to the block. By attaching the gauge to the block you remove any coupling effect. Think of how a train starts with a number of cars. The initial lurch forward first pulls the cars apart, then they rebound slightly and all converge as if they were tethered by a huge rubber band.

You want the motor to rotate at a constant speed and it should be slow, so either a gear box or a set of step down pulleys (pulleys are much better) should be employed to slow the output shaft enough so that the initial start does not jerk the block and spoil the experiment.

Mount or clamp the motor at the end of the table in place of your hand.

The choice of line used is a consideration. You want a small amount of stretch, but not an elastic rebound.

If you do what I suggest your experiment will be very repeatable.

Another tact is to consider the lack of repeatability as an opportunity for students to explain why this happens. This may be more educational than the idealized experiment. Students should suggest (and perhaps try) ways to improve the experiment.

compressible material that stays compressed. Try Silly putty... No joke this is a real material which has the properties you seek. It is sold under many names but the one I am familiar with is silly putty.

In my physics classes I have used the incline of a plane and vector resolution to illustrate the difference between static and kinetic friction. It works very well.

If you are using a data logger, why does the application of force need to be slow? Could you not do the experiment and scale the graph to illustrate (the students might miss the concept though)?

You also might try something big and heavy, forget the fancy equipment and just let the students push it and feel how the applied force changes.

Are you trying to show that the coefficient of kinetic friction is not linear and need a constant velocity for this? If so, several scientific suppliers offer constant velocity machines...or you could tinker around and build your own...work in torque and simple machines...electrical concepts...etc.

Ok, I've said too much...I get excited when it comes to students learning...

Thanks for all the input. This is a lab that I will be teaching and I'm trying to find a way of minimizing technique frustrations. Obviously a slow motor would do the trick, but that is not readily available and it might add another layer of non-conceptual mechanism between the student and the experiment. The challenge is to have the student see that linear (relatively so) buildup before the peak friction force. That really asks for constant velocity over a very short distance. In some respects, a spring scale has a great advantage over a force sensor since it spreads the distance out.

I like the silly putty idea. That, or a similar material might achieve the goal.

This is why I like CR4. Any question, no matter how odd, always gets a lot of brainpower and experience in the answers.

Thanks, guys.

I will allow myself a suggestion. The transition zone during which the force increases but no sliding occurs is for usual materials quite short since it depends on the two stiffnesses at the contact place. It could be possible to make it longer increasing the compliance of one of the surface in the sliding direction. Since your sliding body is not heavy why not use some kind of brush on which it is placed. The pushing or pulling force will bend/shear the support first and only when reaching the threshold a sliding will occur. This increases the stroke and make measurements more easy to do with slower sampling frequency.