Question

Imagine that at some point during the Ice Age a boulder with a mass of 380kg rolls off a cliff and onto a smooth (approximately frictionless) glacier so that just before it hits the glacier its

̂
velocity is ??? = (68m/s)î + (52m/s)k where the z-axis is the positive vertical direction

(perpendicular to the surface of the earth) and the x-axis is parallel to the surface of the earth.

̂1. (a) (4 points) What is this boulder’s initial momentum (just before it hits the glacier) in î, ĵ, k

notation?

̂
(b) (6 points) What is its momentum just after it hits the glacier in î, ĵ, k notation? How do you

know (think about and discuss force directions and conservation of momentum in component form)?

Answer 1

Given,

mass of the boulder, m = 380 kg

Now,

Initial velocity, v_1i = (68 m/s) + (0) + (52 m/s)

(a)

Now,

initial momentum, p_i = m*v_1i

                                   = (380*68 kg*m/s) + (380*52 kg*m/s)

                                   = (25840 kg*m/s) + (0) + (19760 kg*m/s)

(b)

Now,

Since glacier is frictionless, there will be momentum change in x-axis only,

after hitting the glacier, velocity in x-axis will be -x-direction now with same magnitude,

thus,

momentum after hitting the glacier will be,

p_f = (-25840 kg*m/s) + (0) + (19760 kg*m/s)