Question

a) A 59 kg physics student jumps off the back of her Laser sailboat (42 kg). After she jumps, the Laser is found to be travelling at 1.5 m/s. What is the speed of the physics student?

b) The same physics student jumps off the back of her Laser again, but this time the Laser is already travelling at 3.1 m/s before she jumps. If the physics student jumps off with a speed of 2.1 m/s, how fast is the Laser going after she jumps.

Answer 1

here,

a)

mass of student ,m1 = 59 kg

mass of Laser , m2 = 42 kg

final speed of Laser , v2 = 1.5 m/s

let the speed of student be v1

using conservation of momentum

Pi = Pf

0 = m1 * v1 + m2 * v2

0 = 59 * v1 + 42 * 1.5

v1 = - 1.07 m/s

the speed of physics student is 1.07 m/s

b)

the initial speed of Laser and student , u = 3.1 m/s

final speed of student , v1 = - 2.1 m/s

let the final speed of Laser be v2

using conservation of momentum

(m1 + m2) * u = m1 * v1 + m2 * v2

(59 + 42) * 3.1 = 59 * (-2.1) + 42 * v2

v2 = 10.4 m/s

the final speed of the Laser is 10.4 m/s