Please someone of you help me in this concept. My Prof told us that if we need to move with contact with respect to an object, then friction is essential. I cannot think the dynamics behind it. There was a question was like somone jumped from a platform and we had to clculat the back velocity of the platform due to the impulsive force of the man jumped. While explaining this he said thatfriction is essential forthe manto jump. I don't understand the basis behind this.