Does the size of wheels on toy car rolling down a hill, effect the speed?
one can settle this question by imagining really tiny wheels and really big wheels. In the case of small wheels the factor of friction with the axle bearing is a major one in inhibiting the motion of th car. Moreover the friction of the rim on the flat surface is also a factorr of friction. Large wheels will minimize the factor of friction in both cases. @Zardoz raises a point regarding the angular momentum which is valid, but realistically I believe that friction is the more important factor in this case, and so it would be the size of the wheel which matters. One must also consider that a toy car will have a low weight to volume ratio as compared to a real car. Again this would mean that that friction pl;ays a greater role in slowing down the vehicle... Are you playing with HotWheels ? ..."bigger is better" !
If I hold the clutch on my bike while going downhill, does it have any harmful effect on the bike?
No, it's called coasting. You can do it by disengaging the clutch or putting it into neutral, which saves your clutch hand. It won't damage your engine and will technically save fuel.Having said that it's not great practice. The amount of fuel saved is very small, unless you are coasting for significant time, and you have just disconnected from the drive train. If you have to take sudden evasive action you are not best placed. Engaging the clutch suddenly while swerving can make the bike act unpredictability.Coasting down hill may mean that you are using your brakes a lot this can overheat the brakes and cause Brake fade where they fail to function properly.You won't damage the bike but I don't use it as the tiny fuel saving is out weighed, for me, by the loss of control and added risk.
If a bicycle is traveling at 20mph?
You need to know the size of the wheel. small wheel, high RPM, large wheel, small RPM. I'll put in a number, you can adjust it. Say diameter is 1 meter. 20 mph = 8.9 m/s diameter of 1 meter, radius is 0.5 m and circumference is 3.14 m every time the wheel goes around, it travels 3.14 meters, per rev 8.9 m/s / 3.14 m / rev = 2.83 rev/ sec 2.83 rev/ sec x 60 sec/min = 170 rev/min .