When a wave crosses a surface at an angle from one medium into another, why does it change directions as it moves across the boundary into the new medium

 This pivot is called refraction. It happens because different materials have different densities. The more dense the material the more atoms the light collides with and the slower it travels; the less dense, the fewer the collisions and hence a faster velocity. This pivoting, or refraction, is caused by the light either slowing down or speeding up.

Electoral current force mechanical advantage if the wheel and axle is 2 n and the output force is 20 n