Education and Manpower Bureau
Using Datalogger in the Teaching of Physics


Collision and Conservation of Momentum

Experiment Profile:

1. Background

An inelastic collision is one where the total kinetic energy of the colliding particles is not the same after the collision as it was before. An example is a toy car smashing towards a wall. Before the collision, the car moves fast and has lots of kinetic energy - and afterward, it has zero kinetic energy. An elastic collision is when the sum of the kinetic energy of all the colliding particles is the same after the collision as it is before. An example of elastic collision is a basketball bouncing off a backboard. The ball comes back with the same level of kinetic energy as it had when it went forward, which means it was an elastic collision. In this experiment, a group of students will use 2 collision carts with a data-logging system to study the characteristics of collision and the conservation of momentum during the collision. The group will then present their findings to the class.

2. Objective

To study an elastic collision.
To study the conservation of momentum during the collision.

3. List

Datalogger interface connected to a PC
2 motion sensors / 2 pairs of photogates
2 collision carts with magic tape
Air track

  Collision and Conservation of Momentum
  Simple Harmonic Motion
  Acceleration due to Gravity
  Pushing and Pulling a Dynamics Cart
  Acceleration of a Dynamics Cart

Collision and Conservation of Momentum


1. Connect the datalogger interface to a PC with the software installed.

2. Place the motion sensors at the two ends of the air track. Please refer to Figure 1 for the experiment set-up using the motion sensors.
3. If photogates are used in this experiment, place the photogates between the two collision carts. Figure 2 show the experiment set-up are using photogates.
4. The first collision cart is 30cm away from the photogates with the magic tape on the opposite side of the photogates.

5. The second collision cart should be 50 cm away from the first collision. the magics tape on one side of each cart should face each other to ensure the two carts will stick after collision.

6. Start the datalogger to record the velocity of the two collision carts.

7. Push the second collision cart to move towards the first collision cart at a steady speed in a straight line.

8. Wait for the second collision cart to stick to the first collision cart and move for a small distance.

9. Stop recording the data after the two collision carts are still.


1. Plot a velocity graph of the two collision carts and study how the velocity changed before and after the collision.

2. Is there any energy lose shown in the experiment? If yes, state the reason.

3. What types of collision occurred during the experiment? Elastic or inelastic? Please give a reason.

4. Group presentation to the class. The group of students who used the datalogger to do the experiment will show their results to the class and give explanations to their classmates.


Replace the magic tape with magnet ends of the same polarity to ensure the collision carts will repel and repeat the previous experiment.

Add a force sensor to the experiment to measure the force before and after the collision.

Ask the students to calculate the conservation of momentum using the data collected.

Video Download:

We are particularly grateful to SKH Kei Hau Secondary School for their help in the production of this video.
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