# Momentum and movement basic GCSE Quest

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
Homework Helper
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?

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.

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.

ok thanks but how do i answer the question?

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.

i still don't understand i don't understand what formula to use as well.

PeterO
Homework Helper
i still don't understand i don't understand what formula to use as well.

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 travelling at an appropriate speed in the negative direction (ie the other way)

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 travelling at an appropriate speed in the negative direction (ie the other way)

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
Homework Helper
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?

When you added the masses and multiplied by 3 (presumably the speed of Mr Green), you calculate the momentum if both he and the boat were travelling at that veocity; only he is.

PeterO
Homework Helper
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?

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?

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?

you are very kind for helping me.

80kg

240 kg m/s

105 kg m/s

35kg

no idea for velocity i know formula is f=d/t but you aren't given time or distance.

PeterO
Homework Helper
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.

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.

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.

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.