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Physics Lab - Conservation of momentum

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Physics Planning Lab Report – Conservation of momentum

Introduction / Background information

A conserved quantity, momentum is a type of motion that is the mass of a body multiplied by the body’s velocity. The velocity of a body is the speed of the body in one stated direction. It is usually measured in kgm/s and is a vector, with both magnitude and direction involved.

The conservation of momentum is the principle that states: when two or more bodies act on one another, the total momentum remains constant, on the condition that no external forces act upon it. This means that when two bodies act on one another, as in a collision, the sum of the initial momentum of each of the bodies remains a constant, and so will be equal to the momentum of the two bodies combined, after the collision. The total momentum always remains a constant amount, provided no external forces are acting on the bodies (i.e. a closed system of objects).  


To investigate the principle of conservation of momentum, which states that the initial momentum of two bodies is equal to the final momentum of the bodies, combined.


  • 2 Metal carts – to represent the bodies taking part in the collision.
  • 4 Stop watches ± 0.01s – to measure the time for the bodies to travel a particular distance, to eventually calculation the momentum.
  • Meter rule ± 0.05cm – to measure the distance the bodies travel, to eventually calculate the momentum.
  • Pen & paper – to note down the readings.
  • Metallic track – to slide the carts on, to provide an almost friction-less surface.
  • Electronic balance ±0.01g – to measure the mass of the bodies.
...read more.


Fair test

  • Ensure the electronic balance is completely ‘zeroed’ before measuring the mass of the carts.
  • Ensure the same carts are used, which have the same mass.
  • Position the eye directly over the readings on the meter rule to avoid a parallax error.
  • Ensure the carts are completely stationary before the experiment is carried out.
  • Try and measure the time taken for the carts to travel as accurately as possible.
  • Do not push the carts with so much force that, the time taken cannot be measured.

Safe test

  • The carts are usually made of metal, and one should be careful to not drop them onto anyone’s feet. They also slide of the metallic track very easily.


Independent variable - initial velocity ( u )

Dependant variable – Final velocity ( v )

Controlled variable  - Mass of the cart, the type of the cart, surface.

Raw data


Data table #1 showing raw data collected during experiment.

Processed data

Data table #2 showing the initial and final velocity of the carts


Data table #3 showing the momentum before & after the collision.image02.png



Cart 1 :

d1 = 107cm

t1 =  1.12s

 initial velocity (u) = (107 / 100)meters / 1.12seconds

 = +0.955m/s

d2 = 55cm

t2 = 2.10s

final velocity (v) = (55 / 100)meters / 2.10seconds

= + 0.262m/s

Momentum before collision = mass x initial velocity

= (500.85 / 1000) kg x 0.955m/s

=0.478 kgm/s

Momentum after collision = mass x final velocity

= (500.85 / 1000) kg x 0.262m/s

=0.131 kgm/s

Cart 2 :

D1 = 0cm

T1= 0s

initial velocity (u) = 0m/s

d2 = 102cm

t2 = 1.

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Also, friction was not and could not be completely avoided. Due to friction the velocity of the carts would have been decreased and effected. Even though a near frictionless track was utilized, friction cannot be avoided, for if friction was negligible the cart could not be able to move. This could have clearly altered and effected the final results. The meter rule used to measure the distance traveled by the carts had a systematic error of ±0.05cm. Therefore values could not be measured more accurate than that. Like wise the stopwatch has a systematic error of ±0.01s. To be more accurate, more precise pieces of equipment can be used.A critical error in the experiment was not taking enough results. To extend my enquiry, I could have obtained more results.

From analyzing the data and comprehending particular lapse in the method I have been able to pin–point my errors and shall strive to avoid them next time.

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