what is the boy's mass?

Question

A boy on a 2.4 kg skateboard initially at rest

tosses a(n) 8.9 kg jug of water in the forward

direction.

If the jug has a speed of 2.5 m/s relative to

the ground and the boy and skateboard move

in the opposite direction at 0.63 m/s, find the

boy’s mass.

Answer in units of kg

Answer 1

Favorite Answer

I'll show you the method, you can plug in the numbers.

Conservation of momentum

m1*v1i + m2*v2i + .... = m1*v1f + m2*v2f + .....

Since everything is initially at rest, all the vi's are 0

0 = m1*v1f + m2*v2f + ......

Set a direction as positive... I'll choose forward as the postivie direction.

m1 = 8.0 kg

v1f = 2.5 m/s

m2 = mass of the boy + skateboard

v2f = -.63 m/s (negative because it is moving in the opposite direction

0 = m1*v1f + m2*v2f

Solve for m2

m2 = -(m1*v1f) / v2f

mass of the boy = m2 - 2.4 kg

Plug and chug.

Answer 2

m1v1 + m2v2 = m1fv1f + m2fv2f

since the objects are initially at rest

0 = m1fv1f + m2fv2f

let m1f be the mass of the jug

v1f is the velocity of the jug

v2f is the velocity of the boy and the skateboard

m2f = -m1fv1f/v2f

m1f = 8.9kg

v1f = 2.5m/s

v2f = -0.63m/s

m2f = 8.9*2.5/0.63

the minus signs cancel

m2f = 35.35 kg

this is the boy and the skateboard

m2f = mb + msb

35.35 = mb + 2.4

mb = 35.35 - 2.4

mb = 32.95 kg

so I think the boy will be 32.95 kg ..

i used momentum is conserved to find out the mass of the boy and skateboard then i subtracted the given mass of the skateboard from the solved value for the mass of the boy..

hope that helps

Answer 3

"AN" is fine, "a" is NOT an option.

(8.9 x 2.5) /.63 = 35.32kg.

Subtract 2.4kg. for the board, = 32.92kg.