- #1

- 256

- 18

## Homework Statement

The 75kg boy leaps off the cart A with a horizontal velocity of v' = 3 m/s to the left measured relative to the cart. Determine the velocity of cart A just after the jump. If he then lands on cart B with the same velocity that he left cart A, determine the velocity of cart B just after he lands on it. Carts A and B have the same mass of 50kg and are originally at rest.

## Homework Equations

Relative motion: Va = Va/b + Vb

Momentum: m1V1 = m2V2

## The Attempt at a Solution

the subscript j will represent the boy, while a/b represent the respective carts. the coordinate system: left represents negative, right positive

Relative motion

Vj = Vj/a + Va

(1) Vj = -3 + Va

Momentum

0 + 0 = ma*Va - mj*Vj

mj*Vj = ma*Va

(2) Vj = (ma)/(mj)Va = (50/75)Va

Substituting (1) and (2)

(50/75)Va = -3 + Va

Va(50/75-1)=-3

Va = 9 m/s (to the right)

Vj = -3 + 9m/s = 6m/s (to the left)

Now I find Vb (assume direction right)

mj*Vj + mb*Vb = (mj+mb)V

75*(-6) + 50*0 = (50+75)V

V = (75*6)/(50+75) = 3.6 (left)

But this is wrong,the answer is 0.720 m/s (left)