# Choosing of Motor

Hi there, i am doing a school project to build a car(4 wheels) that can carry a payload with a weight up to 9 kg(Total) with gear motors(4 Motors in total)

Based on my calculations, assuming there is no friction:
Force required to move= Mass* Acceleration
= 9kg* 9.81m/s-2
= 88.29 N

Torque required to move wheels = Force * Radius of wheels(120mm Diameter wheels)
= 88.92N * 0.06m
= 5.3352Nm
Torque required per motor = 1.3338Nm = 188.881990814 oz.in = 13.60097480898 kgcm

After browsing the 25D gear motors, do i choose this motor for my project?
pololu.com/product/3206

Hello.

9kg sounds heavy for those gearmotors and would probably strain the components of the gear motor mechanically. For comparison, the 6WD Dagu Wild Thumper has a recommended max payload of 5kg and uses 6 gearmotors of that size. You should probably consider using larger more powerful gearmotors. I do not have a recommendation for a more powerful motor.

- Grant

This suggests that you plan for the cart to accelerate as if it were in free fall, which is unrealistic.

An acceleration of 1 m/s/s would be more realistic. Starting from zero, after just 1 second the cart would be moving 1 m/s (a walking pace). After two seconds, it would already be moving faster than most people can comfortably walk.