Launching motion of Shenzhou-V |
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**Key Learning Points**

- Displacement-time graph
- Equations of uniformly accelerated motion
- Free body diagram
- Newton's second law

Typical results, suggested answers, students' worksheet.

**Introduction**

Changzheng 2F rocket |

On Oct 15, 2003 China reached a milestone in history with the launch of its manned spacecraft Shenzhou-V (神舟五號). The spacecraft was put into orbit and revolved around the Earth 14 times, taking 90 minutes to complete each cycle. After that, the returning cabin was separated from its orbiting component and returned to Earth safely. The successful launch and return of Shenzhou-V has made China the third nation in the world capable of sending a human into space.

You may have witnessed the launch of Shenzhou-V on television. Shenzhou-V was launched by Changzheng 2F rocket (長征二F火箭), which is specially designed to launch manned spacecraft. Changzheng 2F is so far the longest, with the highest carrying capacity, and the most complex rocket ever developed in China. In its 8-years' development, researchers have put a great effort to ensure the highest degree of safety and reliability. So the launch actually represents a great triumph of the scientific research in China.

The overall launching motion of Changzheng 2F rocket was very complicated. But basically, it can be described by the laws that you have learnt in the physics lessons. In this activity, we are going to use the Motion Video Analysis Software (MVAS) to study the launching motion, from which we can find out the initial acceleration of the rocket and compare with the value predicted by Newton's second law.

**Apparatus**

Video clip showing the launching motion of Shenzhou V. Motion Video Analysis Software (MVAS), Microsoft Excel, computer.

**Data**

The following data of Changzheng 2F are useful for your analysis:

Length of rocket = 58.3 m

Launching mass of rocket = 479.8 tons

Thrust provided by rocket = 5923000 N

**Procedure**

- Students are divided into small groups, each group is given a computer installed with MVAS, Microsoft Excel and the video clip showing the launching motion of Shenzhou V.
- Look at the launching motion of Shenzhou V on the computer.
- Your teacher will show you how to use MVAS to obtain the motion data. Use the length of the rocket as the reference length.
- Export the data to a text file. Import the data to Microsoft Excel.
- Plot a displacement-time graph of the motion with Excel. Describe the motion based on your knowledge of motion graphs.
- Your teacher will show you how to fit the motion data with a suitable equation, and find the acceleration of the rocket.
- Draw a free body diagram showing all the forces acting on the rocket during its launch.
- Your teacher will give you some data on the mass and the thrust of Changzheng 2F rocket. Using Newton's second law and the free body diagram that you have drawn, calculate the launching acceleration of the rocket. Compare this result with that found from the displacement-time graph.

Discussion

- What is the appearance of the displacement-time graph? How would you describe the motion of the rocket?
- How do you find the launching acceleration of the rocket from the displacement-time graph?
- Name the forces that are acting on the rocket during its launch. How do you find the net force acting on the rocket?
- In applying Newton's second law, , to the rocket, which force should correspond to? Explain.
- Does the acceleration you found from the displacement-time graph agree with your calculation?
- You can observe that hot gas is expelled from the rocket during its launch. How can this accelerate the rocket? Explain in terms of Newton's laws.
- In fact, since hot gas is being expelled, the acceleration of the rocket will not be a constant but will increase with time. Explain why.

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