Question

You are going sledding with your friends, sliding down a snowy hill. friction can't be ignored. riding solo on your sled, you have a certain acceleration. would the acceleration change if you let a friend ride with you, increasing the mass?

Answer 1

If the sloped plane were frictionless, then the mass of the object would not have any effect on the acceleration of the object as it slides down the slope. However, if there is friction, the weight would be a necessary force to overcome which results in the decrease of the acceleration. In equation form:
a = g sin θ (frictionless)
a = g sin θ - W cos θ (with friction)