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Conservation of momentum: Internal force.

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Conservation of momentum: Internal force

Tim Hoffmann

Physics 11th IB


11: Lab-group:

Tim Hoffmann, Sagar Manilal

III: Theory:

Momentum always remains constant. This can be set down to the law of the “conservation of momentum” which states that the momentum before a collision is identical to the momentum afterwards. This is Newton’s third law. It states:

The total momentum of an isolated system of bodies remain constant.”


Planning A:

  1. Aim: To find the relationship between the force of an explosion from weighted carts and the momentum.
  2. Due to the law of the conservation of momentum, I predicted before the lab that when the two momentums were added, the final product would be 0, as it is 0 before any collision occurs.
  3. Variables:

Independent: Mass of trolley


Control: Force of collision, type of trolley, ticker-tape.


Planning B:


-2 trolleys (springs included)

-100g masses

-Ticker-tape machine


-2 metre rulers



  1. Place two trolleys facing opposite directions lined up parallel to each other
  2. Add 500g to one trolley, (assign this one a (+) velocity.
  3. Attach ticker-tape to trolley 1
  4. Turn on ticker-tape machine
  5. Push spring to create acceleration
...read more.


Trolley: Trolley 1 (Lighter):

What is shown?: Measurements

Extra: Every dot on the ticker-tape represents .02s intervals. Because we already know this information, 4cm intervals will be used for each cart to measure the velocity of each.

Trial 1: Measurement

Trial 2:Measurement

Trial 3: Measurement


1.) 2.2cm

1.) 2cm

2.) 2.2cm

2.) 2.1cm

2.) 2cm

3.) 2cm

3.) 1.9cm

3.) 1.8cm

4.) 1.9cm

4.) 1.8cm

4.) 1.8cm

So, for Trial 1, the average distance taken over .02s is 2cm. So, if we use the constant of 4cm to measure our velocity it is as follows (4 is used because it is a rounded number which can be used as a control for both trolleys):

Velocity= displacement / time

V= 4cm/ .04s

V= 100cm/s

This value is going in the direction we have labeled (+), therefore our velocity is:


Now, momentum is a product of velocity and mass.

We can now plug in our values

Momentum= velocity x mass

P= 100cm/s   x   1kg


Trolley: Trolley 2 (Heavier):

What is shown?: Measurements

Extra: Every dot on the ticker-tape represents .

...read more.


There was an error however in the final product. Reasons for this could possibly have been:
  1. Human error: the readings for the measurements could have been slightly inaccurate and could have provided the problem.
  2. Calculations: There could have been error in the calculations
  3. Ticker-tape: The ticker-tape machine could possibly have been used or be working inaccurately.

The near accuracy of the procedure however suggests that the information was quite accurate.

  1. Procedure evaluation:

Weaknesses: The materials should have included “friction-free surface” such as waxed-wood or something with limited friction. Also, the procedures have the weakness of using ticker-tape which is placed along-side the trolley, this gave some problems.

Errors: The error in the data analyzing probably came from incorrect values. This could have been due to the incorrect mass of the trolleys or merely by interpreting the values incorrectly.

Limitations: The ticker-tape, although quite accurate, is not very easy to interpret or represent in a table, so the values received from this were quite limited.

  1. Modification suggestions

The weakness of the ticker-tape being put along-side the trolleys could have been improved through holding it directly above the apparatus so that there would be no error in the tape’s accuracy (being at a slight angle.)

...read more.

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