Physics 105

Apparatus

Two pencils, thumb tacks or nails

Mass set with hanger

String

Protractor

Ruler

Spring scale

Tape or heavy book

Procedure

In this lab, we will use the hanging masses to examine the forces on a body in equilibrium. The body we will examine is the mass hanger. If we know two of  the individual forces on the mass hanger, then we can find the unknown force. We will do so both analytically and graphically, and give the results with some form or confidence - i.e. with errors attached.  (These will not be proper 90% confidence intervals because we are not doing any statistics.) Then we will compare our results of both the graphical and analytical solutions using the null hypothesis.

When the mass hanger reaches equilibrium (in this case, stops moving), the net force on the mass hanger is zero. We measure one of the forces with a spring scale. Some of the spring scales have Newtons on them, but some of them don't, so for simplicity in this lab, we will take grams to be some arbitrary unit of force, and call the units gram-force.We know that gravity acts on the mass hanger and this force is also proportional to the mass. This is not a unit conversion - there is no conversion factor to go from Newtons to grams.

1)      Tie a piece of string to the mass hanger.

2)      Attach the pencils (or thumbtacks or nails) to the edge of a table or other level surface about a foot apart (the actual distance doens't matter too much). You can tape them down or put a heavy book on top of them. This platform will be the support.

3)      Hang the spring scale from one of the supports. Hang the mass set with as much as you would like from the hook on the spring scale.

4)      Pull on the string and wrap it around the other support. You can hold the end of the string, tape it in place, or tie it with a knot. (Constrictor knots work particularly well if you know anything about knots--the knot gets tighter rather than looser if you pull on the ends.) The end result should look something like the picture shown below.

5)      Slip a piece of paper behind the string so that you can mark the position of the top of the mass hanger and the direction of each force (the direction each string is pulling and the direction gravity is pulling on the mass hanger).  You may want to put the paper on a clipboard or similar to help with the drawing. Alternatively, you can take and print a picture of the apparatus. Next to arrows for the known forces, record the grams-force reading or the mass of the weight.

6)      Pull out the paper and draw arrows to scale, representing each of the forces on the mass hanger both in magnitude and direction.  Choose your scale so that the arrows are long enough to take up a significant portion of your paper. You will have one arrow that you don't know the value for. So just draw a reasonable length.

7)      You should now have a piece of paper with a dot representing the ring and three arrows going out from that dot representing each of the forces.

Department of Physics