# Forces & Equilibrium

BHS -> Staff -> Mr. Stanbrough -> Physics -> Mechanics -> Newton's Laws -> Newton's First Law -> this page

## Question:

When an object is in equilibrium, what can you say about the forces that are pushing and pulling on it?

## Procedure

### Part 1 - Forces in One Dimension

1. Tie a loop in both ends of a short piece of string.
2. Calibrate (adjust the scale) your spring scales so that they both read the same force when they support the same object.
3. Put a loop over each spring scale hook, and have a person pull on each spring scale. The string should remain at rest, or move at constant velocity, while the people pull on it.
4. In a data table, record the force on each spring scale.
5. In order to get an idea of the precision of your measurements, check to see how much one person can vary their spring scale reading without affecting the reading on the other scale. Record this information.
6. Repeat for a few other "pulls".

### Part 2 - Forces in Two Dimensions

1. Your teacher will show you how to set up the force table apparatus so that 3 forces are pulling on the central ring.
2. Adjust two of the forces so that they are directed at 0o and 90o respectively. The direction of the third force will vary, depending on the number of washers you place on the weight hangers.
3. Adjust the direction of the third (non-perpendicular) force (by moving the string), and the amount of each force (by changing the number of hanging washers) until the central ring does not touch the post.
4. Record the magnitude of each force (number of washers) and direction of each force in a data table.
5. In order to test the precision of your measurements, check to see about how much you can move a string, or change the number of washers, without disturbing the central ring.
6. Repeat the procedure so that each person in your group has at least one experimental trial to analyze.

## Results:

### Part 2 - Forces in Two Dimensions

For each set of forces:

1. Determine the resultant (magnitude and direction) of the two vectors at 0o and 90o - you many use either algebra and trigonometry or a scale drawing.
2. Make a scale drawing showing your resultant vector from step 1 and the third vector in the set. Draw the vectors with their "tails" together.

## Conclusions:

### Part 1 - Forces in One Dimension

How do the forces on the 2 spring scales compare? In particular, given the variation that you noted in step 5 above, could the two forces be equal in size?

### Part 2 - Forces in Two Dimensions

For each set of vectors, how does the third vector in each set compare with the resultant of the other two vectors? What does this tell you about the net force on the ring? Why?

BHS -> Staff -> Mr. Stanbrough -> Physics -> Mechanics -> Newton's Laws -> Newton's First Law -> this page
last update November 10, 2007 by JL Stanbrough