Physics Lab 2.2: Tug-of-War

Transcription

1 Physics Lab 2.2: Tug-of-War Name Period Purpose: To investigate the tension in a string, the function of a simple pulley, and a simple tug-of-war. Materials: 1 75 cm string 2 30-cm strings 1000 g of assorted slotted masses 2 mass hangers (or extra strings) 2 low friction pulleys 2 ring stands with rings or 2 physics stands with rods g spring scales 4 large paper clips or 4 ring clips for hangers Discussion: Suppose you attach a string to a wall and pull on the free end. Anyone can see that you pull on the string, but only careful observers will see that the wall also pulls on the string. If the wall is replaced with a person pulling on the string, a tug-of-war takes place, and it is easier to see that the string is being pulled on both ends. Place two fingers inside a rubber band and and use them to stretch the band. Which of the fingers is doing the stretching? Both fingers are required to stretch the rubber band. You know both fingers are involved because both have a force you can feel, and this force should be the same on each finger, and according to Newton s 3rd Law of Motion, the force of your finger on the rubber band is the same as the force of the rubber band on your finger. And because the rubber band is pulling evenly on itself, the forces on and from your fingers should be the same.

2 Procedure: 1) Suspend a 450 g load on a 50 g mass hanger from a spring scale as shown in Figure A. What is the force (Newtons) reading on the scale? 2) Suspend the same load on from a 75 cm string connected to the spring scale. How does this force (the tension in the string) compare to the force from Step 1? 3) Drape the string over a pulley such that both ends of the string hang vertically, as shown in Figure B. Pull down on the scale with as much force as you can WITHOUT making the load move. What does the scale read, and how does this force compare to the weight of the load (Step 1)? How does this force compare to the tension in the string (Step 2)? 4) Move the spring scale so it is at a higher position than in Step 3. Remember to keep the strings vertical! Again, pull down on the scale with the max you can WITHOUT making it move. How does the force compare to that from Step 3?

3 5) Move the spring scale to a lower position than in Step 3. Keep the strings vertical! How does the force compare to to that from Step 3? Explain these results. 6) Move the spring scale to various angles to the vertical, until the scale is held horizontally, as shown in Figure C. Continue pulling on the scale with the max force that keeps the load stationary. Does the reading on the scale ever deviate from what you measured in the previous steps? Briefly explain your results. 7) Remove the string from the pulley and drape it over a horizontal rod or ring, as shown in Figure D. Repeat Step 6, again reading the max force that keeps the load stationary. Is there a difference between the force from this step and from Step 6? Explain these results. 8) Drape the string over the pulley, and attach a spring scale to each end of the string. Hang 500 g loads from each spring scale, as shown in Figure E. What do the scales read?

4 What role does friction play in the function of the pulley? 9) Have your partner hold one end of a spring scale while you pull horizontally on the other end. Pull until the scale reads the same amount of force as it did with the suspended mass. Record the following observations: The force you exert on the scale: The force the scale exerts on you: The force your partner exerts on the scale: The force the scale exerts on your partner: 10) Attach strings to both ends of a spring scale. Fasten one end to the wall or a steady support. Call this String A. Pull horizontally on the other string, String B, until the string reads the same as in Step 9. Record the following observations: The force you exert on String A: The force String A exerts on the scale: The force the scale exerts String B:

5 The force String B exerts on the wall: The force the wall exerts String B: Is there any difference between the situations in Step 9 and Step 10? If so, what is the difference? From a microscopic point of view, how does the spring or string transmit the force you are exerting on your partner or on the wall? 11) Study Figure F and make a prediction for the reading of the scale when when one 500 gram load is suspended at the end of each string. Once you have recorded your prediction, assemble the apparatus and check your prediction. Use short strings between the spring scale and the masses this time. Predicted scale reading: Actual scale reading: