I am conducting an experiment using a cart and pulley system. I am trying to measure the force that is accelerating the cart by using a dynamo-meter and pulling in the opposite direction. I think that in this way I can measure the gravity of the weight used to pull the cart and frictional forces. In order to calculate the magnitude of the force of friction, I think that I should use the formula $$ F_{pull} - F_{weight} = F_{friction}.$$ What do you think?

Here you can see it, so imagine now that I am measuring the force by pulling with dynamo-meter in the opposite direction.

  • $\begingroup$ Friction does depend on weight in this case (in general it depends on the reaction force by $Fr = \mu R$ where $\mu$ is a constant specific to the surface). However your equation is incorrect since the force due to weight is downwards (and is perfectly balanced by the reaction force of the ground), so is not related to the other two forces in your equation, which act horizontally. Hopefully this helps you on your way. The description of the experiment you wish to perform is not complete enough to provide a more specific answer. Are you pulling it so it doesn't move at all, for instance? $\endgroup$
    – Joe Iddon
    Mar 8, 2020 at 17:27
  • $\begingroup$ I m pulling it so it moves slowly and uniformly. $\endgroup$ Mar 8, 2020 at 20:53
  • $\begingroup$ If your equation were true then the cart won't accelerate. Is this the case? $\endgroup$
    – user249968
    Mar 9, 2020 at 5:50
  • $\begingroup$ Well I am holding the cart on purpose to see what is the force needed to hold it. Moving it just a bit very slowly. I am holding it with dynamometer in the oposite direction than the one on the picture. $\endgroup$ Mar 9, 2020 at 6:06

1 Answer 1


I would use $$\sum F = ma = W_{hanging-mass} - f$$ because the force from the hanging mass is the force accelerating the cart, and the friction is acting in the opposite direction.

It is possible I have misinterpreted the experiment. Mind attaching a diagram?


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