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If I have an object of 90kg, moving along the ground with motors attached to wheels, how would I calculate how much torque, rpm and power those motors would need to have to accelerate the object to 20 mph in 10 seconds? If you have any resources like videos or readings, or just an explanation of how to do it, I would appreciate it. I realize this may be a duplicate question, but I cannot find something that explains this simply. I am a novice to this sort of thing and through all of my googling I just can't figure it out.

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Assuming that:

  • there is no rolling resistance
  • there is no air resistance
  • the acceleration is constant during the 10[s]

Then the total force between the wheels the ground would be equal to :

$$\sum F = m \cdot a $$

where:

  • $a=\frac{20 [mph]}{10 [s]}= 0.891 [m/s^2]$
  • $m=90 [kg]$ Therefore, $F= 80.47 [N]$

Then you can calculate the properties you are after can be easily calculated as:

quantity formula Comments
rpm $n = \frac{60}{2\cdot \pi \cdot r_{wheel} }u $
Torque $F\cdot r_{wheel}$
Power $P = F\cdot u $ ~720[W]

The rpm and the torque are depended by the Radius of the wheel ($r_{wheel}$

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  • $\begingroup$ Thank you so much! I got as far as finding the force (from acceleration and friction with the ground) as well as the torque from that, but I could not find equations for the other things. What is the 'u' variable in the power and rpm equation? $\endgroup$
    – Sebastian
    May 3, 2021 at 15:34
  • $\begingroup$ U is the velocity. Preferably in m/s $\endgroup$
    – NMech
    May 3, 2021 at 15:40
  • $\begingroup$ Sorry, 1 more question. If I had 2 wheels would I need to put about half the power, torque etc on each one? $\endgroup$
    – Sebastian
    May 3, 2021 at 17:49
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    $\begingroup$ @Sebastian if you had two motors to two wheels each would only need half the power. Otherwise it makes no difference. $\endgroup$
    – Tiger Guy
    May 4, 2021 at 19:21
  • $\begingroup$ @Sebastian, just as TigerGuy said. I will only add that the way the force is calculated is the total force on all wheels that are contributing to the motion. $\endgroup$
    – NMech
    May 4, 2021 at 19:29

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