2

i wanted to put a electrical motor into a toy car. i want to use an optimum motor with correct RPM (i know that with fixed power, higher RPM mean less torque). so here it goes. i have a toy car with mass of M, static friction of Fs. wheel with radius of 5 cm (0.05 m). if i want this car to have velocity of V. then what should be the wattage of the motor

user3760537
  • 21
  • 1
    Static friction isn't where you get losses. It's rolling friction and air resistance, and they're not easy to model from first principles. It would be easier to see what kind of motor other cars use or to just try some different sized ones. – BowlOfRed Nov 19 '15 at 22:50
  • Hi, welcome to physics stack exchange. Please note that engineering questions may be considered off topic. – AGML Nov 19 '15 at 22:53
  • AGML thanks for reading but my problem requires physics equation and i'm in the theoretical part of the work which i believe is in physics category. if it is absolutely necessary i can move it there – user3760537 Nov 20 '15 at 08:54
  • thanks BowlOfRed. but i dont want the car to require push too start moveing – user3760537 Nov 20 '15 at 08:55

1 Answers1

0

Assuming your friction and air resistance are not significant, we can figure it out easy without considering them and be pretty close. (I dont think you're going to go fast enough / have a large enough surface area to make air resistance significant)

E = 1/2 M * V^2

Power = E / t (Wattage)

V = V_0 + A*t

So, the wattage will depend on what force you want to apply to your mass (force = mass * acceleration) or how fast you want it to get to that speed.

To take into account static friction, use the definition of work. Work = Change in Energy, and for friction it equals E = F * d, where F is the force of friction and d is the distanxe over which that force was applied.

Ulad Kasach
  • 411
  • 1
    thank you very much. the use of kinetic energy equation was good. but i had this experience in the past that when i connected a motor to wheel the car requires first push to start moving. with this one i don't want it to happen. how can i find out how much more power do i require to avoid it – user3760537 Nov 20 '15 at 08:51
  • how can i take account of static friction – user3760537 Nov 20 '15 at 09:06
  • I updated it with the static friction equation. Will take a closer look at what you said in your coment after work – Ulad Kasach Nov 20 '15 at 11:21
  • From what I understand in your comment, you're looking to overcome the internal friction of your motor. For this I would recommend testing or guessing an acceleration that is great enough to over come the force of friction, so that Fmotor = (mass * acceleration) > Ffriction. – Ulad Kasach Nov 20 '15 at 21:00