what is the boy's mass?
A boy on a 2.4 kg skateboard initially at rest
tosses a(n) 8.9 kg jug of water in the forward
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
Answer in units of kg
- civil_av8rLv 79 years agoFavorite 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.
- D gLv 79 years ago
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
- TechnobuffLv 79 years ago
"AN" is fine, "a" is NOT an option.
(8.9 x 2.5) /.63 = 35.32kg.
Subtract 2.4kg. for the board, = 32.92kg.