Question

Please define moment of arm (regarding physics) in a way I can understand. it is referring to the arm on a mousetrap car that was built. I need to discuss its effectiveness

Answer 1

A moment arm is simply the length between a joint axis and the line of force acting on that joint. Every joint that is involved in an exercise has a moment arm. The longer the moment arm is the more load will be applied to the joint axis through leverage.

Part of the gearing system of a mousetrap racers is the lever arm. Changing the length of a mouse trap's snapper (or lever arm) is how you control a mousetrap vehicle's acceleration and/or travel distance. Different lengths of lever arms can be used to increase and/or decrease the pulling force and change the amount of string that can be pulled off the drive axle. Changing the length of the lever arm does not change the total energy and/or the torque produced by the mouse trap but it does change the pulling force that is applied to the drive axle. Longer lever arms will have less pulling force than shorter lever arm but longer lever arms will pull more string from the drive axle than a shorter lever arm. Changing and/or attaching a lever arm to the mouse trap is the number one way to control a mousetrap vehicles performance.

When selecting a lever arm it is important to think about the strength of the material you are selecting. Some materials are not strong enough to be used for long lever arms and will bend under the tension of the mouse trap's spring. If a lever arm bends under the tension of the mouse trap's spring valuable energy will be wasted long before the mousetrap vehicle even starts moving. If you look carefully at the mouse trap of a full wound mousetrap vehicle that has a bending lever arm you will notice that the mouse trap's spring is not fully compressed and therefore is not starting with all it's possible potential energy. In most cases brass tubing and brass brazing rods make excellent lever arms because of their high-strength and light-weight. Wood can also make for a very strong and light-weight lever arm if it is used properly (see making a super long lever arm). Coat hangers make poor lever arms because they tend to bend easily under tension and should be avoided. Some times you may discover after you have assembled your mousetrap vehicle that the lever arm is bending under tension in which case you should try to replace it with a different material.