A boy on a 2.4 kg skateboard initially at rest tosses a(n) 7.9 kg jug of water in the forward direction. . .  If the jug has a speed of 2.6 m/s relative to the ground and the boy and skateboard...

A boy on a 2.4 kg skateboard initially at rest tosses a(n) 7.9 kg jug of water in the forward direction. . .

 

If the jug has a speed of 2.6 m/s relative to the ground and the boy and skateboard move in the opposite direction at 0.58 m/s, find the boy’s mass.

Answer in units of kg

 

Asked on by pops2

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ndnordic | High School Teacher | (Level 2) Associate Educator

Posted on

This is an example of an inelastic collision where the two objects start together and then come apart.

general equation is:

(m1 + m2)v = m1v1 + m2v2

in this case, let the boy + skateboard be object 1 and the jug of water object 2.

initially everything is at rest, so v = 0

The jug of water is thrown with a velocity of 2.6 m/s and has a mass of 7.9 kg

The boy plus board move in the opposite direction at 0.58 m/s

Now choose one direction as positive. Let it be the direction the boy plus skateboard are moving. Then the velocity of the jug will be negative since it is going in the opposite direction.

0 = -7.9* 2.6 + m* 0.58, where m is the total mass of the boy + skateboard.

m  = 35.41 kg

subtract mass of skateboard of 2.4 kg, means the boy has a mass of 33.01 kg

 

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