Conservation of momentum - Explosions

Experiment contributed by Peter Adams

Purpose

To show that momentum is conserved during an explosion.

Equipment Required

• Two Light Gates (Tainlab) - U-light gate OR Gooseneck light gates.

• Two dynamics trolleys (one with a built in spring loaded plunger), with masses.

Procedure

1. Attach a piece of cardboard to each of the trolleys. The cardboard must be 20 cm long (Use Blu-tack to attach).

2. Set up the equipment as shown below. The trolleys should be touching so that when the plunger is released they are pushed apart. The light gates should be positioned approximately 100 mm from the cardboard and at a height so that the cardboard will pass through the gate.

3. From the QuickStart menu select Conservation of Momentum - Explosions. Click Open Selection.

4. Attach the light gates as shown in the Connections screen.

5. Click Run. The Test Readings will be shown. To check that you have connected the light gates correctly, place something through each light gate and observe the Test readings screen.

6. Click Start Experiment. Now release the plunger.

7. The WinTec software can measure two velocities at each of the light gates, but in this experiment, only one velocity reading is required from each gate.

8. Press Esc or click Cancel or Stop Readings to display the final results.

9. Record the velocities for each object in the results table as Trial 1. Remember that velocities in one direction should be recorded as positive, and velocities in the other direction as negative. Click Main Menu to return to the connections screen. Save the results for each trial if you wish to view them again later.

10. Now repeat the experiment four times, each time adding a 500g mass to the right hand side trolley, and complete the table.

Discussion

It should be evident from the results that the total momentum before the explosion (zero) equals the total momentum after the explosion. Obviously if your trolleys have significant friction, this will not be absolutely true.

Discuss the question ‘Why should rifles be significantly more massive than the bullets that they fire?’.