Impulse. Observations

Transcription

1 Impulse Observations What is the physics behind breaking an egg? If we look at the situation in terms of force, an egg dropping seems to have the same force (of gravity) as an egg sitting on a counter, but the dropped egg breaks while the sitting egg does not. Why does that dropped egg break hitting a hard counter but not if it lands on a pile of towels? To stop a dropped egg, its velocity and therefore momentum must change. It takes an impulse to change momentum. In the case of the counter, the impulse force comes from the normal force of the counter. An impulse, however, is a force applied through a time. For a hard counter the time is short. With padding, such as towels, the time is spread out. In this experiment you will investigate another momentum-impulse example. You will see how a dropped mass will break a thread. The time it takes to break that thread is very short too short to measure with a stopwatch, but you can determine that time using the momentum-impulse equation. Procedure Equipment Meter Sticks Carpet Pad Diagram Mass Hanger Thread Set of Masses Scissors Page

2 Setup: Step. Place one meter stick on the table with only a short amount sticking out. Be sure you have someone holding that stick to the table at all times. Step. Cut a length of thread and tie it to the meter stick. Tie the other end to a mass hanger. The thread should be long enough so that the bottom of the mass hanger is about 0 cm above the floor. Step. Place the carpet square on the floor so the middle is below the mass hanger. Part : Thread Strength Step. With the mass hanger simply hanging from the thread, start adding masses. Step. Continue adding masses until the thread breaks. Step. Record the amount of mass it takes to break the thread. Be sure to include the mass of the mass hanger itself (0 g). Step. Repeat Steps - at least times. Part : Dropping masses Analysis.Part. Step. Begin with the same setup for Part. Record the distance from the table edge to where the thread is attached to the meter stick. Step. With an empty mass hanger, raise the hanger so you will have a drop of about cm. Measure the beginning height with the second meter stick and record this information. Step. Drop the mass hanger and see if the thread breaks. Step. Add more mass in increments of 0 g to the mass hanger and repeat steps -. Be sure to start from the same height each time. Continue adding masses until the thread breaks. Record the mass it takes to break the thread by dropping. Repeat times. Step. Move the first meter stick out until about only 0 or 0 cm is in contact with the table. Measure and record the distance from the table edge to the thread. Repeat times. Step 6. Repeat Part using one more different overhang distance between the table edge and the thread. The force is takes to break the thread in Part is simply the weight of the mass. Compute the weight for each of the times. W = m g where g = 0 m/s Calculate the average weight. Page

3 Part.. Start with the impulse-momentum equation: F t = m v. We can solve for time, changing the equation to t = m v/f.. For our situation, force is the breaking weight of the thread from Part, F = W. The mass is the maximum falling mass found in Part. We keep the symbol m.. The velocity is the final velocity right before the thread breaks in Part. This velocity is a result of acceleration due to gravity (dropping). From the motion equations, v = a t, but time in this case is the time it takes for the mass to drop. We do not have that time but we have measured a distance which is the height from which the mass is dropped. Distance (height) is related to time by h = ½ g t. (Acceleration is due to gravity so g = 0 m/s.) Combining these two equations we find that v = hg. The distance is the thread length minus the height from which the mass hanger was dropped.. Putting the results of to back into equation, it becomes t = m hg where m is W the maximum falling mass from Part, W is the average breaking weight from Part, h is the height from which the mass is dropped from Part, and g is acceleration due to gravity or 0 m/s. Use equation to compute the breaking time of the thread for each data set. A data set is for a given overhang of the meter stick to which the thread is attached. For each data set compute the average time and its standard deviation. Create a scatter graph of breaking time on the vertical axis and overhang length of the meter stick on the horizontal axis. Page

4 Data Sheet Name: Approximately, what will be the breaking time of the string? (On the order of: seconds, 0. s, 0.0 s, 0.00 s, or even smaller?) Will it take more, less, or the same weight to break the string when the ruler has a large overhang compared to when it has a small overhang? Why? Part. No dropping, just hanging Number Mass Weight Average: Average Weight W = units Part. Dropping the mass Height of the hanger at the end of the string: units: Height of hanger before drop: units: Distance hanger dropped: h = units: Overhang Distance (from table edge to where the thread is tied to the stick): units: Velocity of hanger at the end of the string v = hg: units: Page

5 Equation: number Average Largest Dropped Mass m/w Breaking time Overhang Distance from table edge to thread: units: Equation: number Average Largest Dropped Mass m/w Breaking time Overhang Distance from table edge to thread: units: Equation: number Average Largest Dropped Mass m/w Breaking time Page

6 Questions. Why do you make multiple measurements?. What is changing as you change the length of the first meter stick, impulse or force? Why?. Why is it important to always drop the mass from the same height?. Show that equation produces the correct units.. What trend does your graph show? Why? 6. In Part, what is physically happening to the meter stick, particularly for a long overhang, from which the thread is hanging when the mass drops and pulls on the thread? 7. What would happen if you replaced the thread with a thin rubber band? Bonus: Create a discussion question of your own. Page 6