Yes, I know it's probably a simple problem, but I'm retarded. So a person is standing on a raft that has no friction with the water, but still has friction with his feet. If he takes a step forward, what happens to his position in space and the position of the raft? I'd assume mass has something to do with it, so if it weighed 9999999 kg you would both move but the raft would move a small amount. If the raft weighed less than you, the raft would move a lot more than you. Can someone explain this concept?
I haven't done physics in a while but I think it's because the center of mass of the system {guy+raft} doesn't move
>>7669693
Also, I don't think the movement of the raft depends on the mass of the guy standing on it, it just moves the same amount in the opposite direction
It's easier to reason about this if you think about a skateboard.
But your assessment seems correct to me.
>>7669687
Newton's 3rd law
>>7669687
if he weighed 9999999kg he would be at the bottom of the river
>>7669687
Your right that's what would happen.
I like to look at it from the momentum perspective.
p = mv
momentum is conserved, so
p_final = p_initial
>>7669693
This. Unless an outside force is applied to the system, the center of mass of the system will not accelerate (Newton's first law). When the occupant moves, and equal force will be applied to both the raft and the occupant (Newton's third law). And finally, the distance each individual moves will depend on their mass (Newton's second law).
So the higher the ratio of the boat's mass is to the system's total mass, the less it will move per unit of force, and the same applies to the occupant as well.
>>7669687
In that intertial frame he'd still be applying force in whatever direction thought to his position (because we're on earth it's going to be 'down and to the back some).
On a raft experiencing zero friction it's going to move at a constant speed relative to the angle and force applied.
>>7670449
So why then can a skateboard move from a still position under the movement of weight to and fro from its center of mass?
>>7670449
Alright, thanks for the good explanation.
>>7670449
So, assuming that the mass of the guy and the raft is the same, if he took 1 step forward, would they both move 1/2 a step in opposite directions (from their initial position), therefore keeping their center of mass in the same position?
>>7670092
no volumes were specified
>>7670464
>So why then can a skateboard move from a still position under the movement of weight to and fro from its center of mass?
>from its center of mass?
read again
the center of mass of the SYSTEM stays fixed in place, not of the particular bodies linked together within that system
>>7670581
Correct.
>>7670092
Not if he occupies 100000000 L of volume
:^)
