If you are spinning around with your arms flung-out to the sides and then pull them in it makes you spin faster. I think that the conventional explanation for this is that pulling your arms closer to your body makes your MOI get smaller and in order to conserve angular momentum you have to spin with a greater angular velocity. My question is - if the force applied to pull your hands to your body was always prependicular to the velocity (the force was applied along the radius of the circle that your hads trace as you spin around) how can it affect the angular velocity, in other words, how can a force that applies no torque create angular acceleration? I think that the answer could be related to the centripital force - that force is in the same diretion as the force pulling your arms together (along the radius) and it is the force that causes circular motion. could it be that increasing this force increases the angular accelaration? Any explanation of this would be very helpfull. Thanks.