The discussion revolves around a physics problem involving momentum conservation when Mr. Green, with a mass of 80 kg, steps off a boat weighing 35 kg at a speed of 3 m/s. The total momentum before and after Mr. Green steps off must remain zero. The correct calculation for the boat's velocity is derived from the equation: momentum of Mr. Green (240 kg m/s) equals the negative momentum of the boat, leading to a boat velocity of approximately -6.86 m/s. The participants clarify the importance of using vector signs in momentum calculations.
PREREQUISITESStudents preparing for GCSE physics, educators teaching momentum concepts, and anyone interested in understanding the principles of motion and forces in physics.
nonthesecond said:Mr Green, mass 80 kg, stepped off a boat of mass 35 kg at a speed of 3m/s. how fast did the boat move in the opposite direction?
i have no idea how to work it out?
PeterO said:I think you will find that Mr Green and the boat are assumed to be at rest initially, so the total momentum was - and will remain - zero.
PeterO said:I think you will find that Mr Green and the boat are assumed to be at rest initially, so the total momentum was - and will remain - zero.
nonthesecond said:i still don't understand i don't understand what formula to use as well.
PeterO said:If you define the direction Mr Green goes as positive, he will have some positive momentum after he leaves the boat.
To maintain a total of zero, the boat must have an equal sized, but negative momentum - so it will be traveling at an appropriate speed in the negative direction (ie the other way)
nonthesecond said:i think i remeber my teacher saying that it's: 80 + 35= 115
115 x 3 = 375
375/35 = 1.3...m/s
is that correct?
nonthesecond said:i think i remeber my teacher saying that it's: 80 + 35= 115
115 x 3 = 375
375/35 = 1.3...m/s
is that correct?
PeterO said:answer these in order:
What is Mr Green's mass?
What is Mr Green's velocity?
What is Mr Green's momentum?
What is the boat's momentum?
What is the boat's mass?
What is the boat's velocity?
nonthesecond said:you are very kind for helping me.
80kg correct
(+3) - but you didn't answer that question
+240 kg m/s almost correct
105 kg m/s incorrect
35kg correct
no idea for velocity i know formula is f=d/t but you aren't given time or distance.
PeterO said:For that final velocity - think about how you calculated the momentum of Mr Green - and don't forget to use + and - signs correctly.
Velocity is a vector; so can be positive or negative.
EDIT: Hint - answers 3 and 4 add up to zero
PeterO said:For that final velocity - think about how you calculated the momentum of Mr Green - and don't forget to use + and - signs correctly.
Velocity is a vector; so can be positive or negative.
EDIT: Hint - answers 3 and 4 add up to zero
nonthesecond said:is the answer 240/35?