Question

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Answer 1

Is this the problem you messaged me about?

Answer 2

Yikes...sorry hun, but this isn't something that I've learned yet. I wish I could help. :(

Answer 3

This might help though:
http://answers.yahoo.com/question/index?qid=20100129094747AAjjqJk
the person solves it out and everything.

Answer 4

use \[\Large v_f^2 = v_i^2 + 2a \Delta x\]to find it's final velocity. vi is zero, a=-9.8m/s^2, delta x is 5.0 m

Answer 5

Get the velocity of the rock from the increase in kinetic energy
due to gravity (1/2) m v^2 = m g h,

the momentum of the rock will then be m v.

That momentum will add its tiny bit to the momentum of the Earth.
Starting with a stationary Earth would give you mv + 0 = (m+M) v'
so v' = [m/(m+M)] v

with m = rock mass and M = earth mass and v' = final velocity.

Answer 6

No problem.

Answer 7

I wrote a in my post.

Answer 8

Yes, since it's freefalling

Answer 9

Yes

Answer 10

^that's much too high... you should get \[\Large v = \sqrt{ 2gh}\] then you just plug in g and h

Answer 11

Too high.
v = sqrt[2(9.8)(5)]

Answer 12

You're squaring too many things..

Answer 13

You can either use this \[\Large v_f^2 = v_i^2 + 2a \Delta x\] or this

(1/2) m v^2 = m g h

Answer 14

It's not, do the calculation from scratch. You're squaring a when you shouldn't be

Answer 15

Yes

Answer 16

Yes

Answer 17

Change in momentum is mass times velocity (change in velocity, but it started from rest anyway).

Answer 18

Yes, the one with the negative should do, since you used g as negative

Answer 19

It doesn't really matter, as long as you specify direction.

Answer 20

idk, sure, whatever 14*9.8 equals

Answer 21

give that added momentum to the Earth and calculate its new velocity
from
added momentum = (m + M) v'

Answer 22

@douglaswinslowcooper are you sure about that?

Momentum before = momentum after

momentum before the object drops is zero, momentum after = mv + MV (using mv for the rock, MV for earth)

Answer 23

so the momentum of rock = momentum of earth

14*9.8 = M*v

where M is earth's mass. Find v.

Answer 24

the point of the problem is that the tiny momentum you calculated for the rock, mv, when added to the Earth produces a very, very tiny change in the Earth's velocity

m v = (m+M) v'

v' = [m/(m+M)] v

Answer 25

^the rock had zero momentum before, and they also aren't sticking together (as your equation implies) the rock hasn't hit the ground. It's falling.

Answer 26

In an inelastic collision, the objects fuse and share the same final velocity.

Answer 27

It isn't. Where are you reading that they collided?

Answer 28

A 14 kg rock starting from rest free falls through a distance of 5.0 m with no air resistance. Find the momentum change of the rock caused by its fall and the resulting change in the magnitude of earth’s velocity. Earth’s mass is 6.0 × 1024 kg. Show all your work, assuming the rock-earth system is closed.

^nowhere does it imply that the rock has hit the ground.

Answer 29

Shall we assume there is only the gravitational interaction with no impact?

Answer 30

And if the rock did hit the ground, the change in momentum would be zero. Before the rock falls, net momentum is zero. After it hits the ground, net must still be zero.

Answer 31

Impact would mean both objects come to rest, since before the rock fell, they were both at rest.

Answer 32

Yes, but I doubt this was the intent of the question. Who knows?

Answer 33

It is, there's no reason to assume the rock collided.

Momentum before = momentum after

Before, the objects are at rest. After, the rock has gained 14*9.8 kgm/s, thus the earth must have gained that same amount, in the opposite direction.

Answer 34

Momentum before = momentum after

0 = mv + MV

so little letters for rock, big for earth

-mv = MV

Answer 35

All you're concerned about is momentum before the rock falls, and after it's fallen 5.0 m.

Answer 36

-mv = MV

where m is mass of rock, v is velocity of rock

M is mass of Earth, V is velocity of earth (solve for V)

Answer 37

You found it earlier, velocity of the rock was sqrt(2*9.8*5)

Answer 38

Doesn't really matter, like I said above. You choose, since you're the one deciding which direction is negative or positive.

Answer 39

Yes

Answer 40

I don't know how you got that, unless that 23 is -23

Answer 41

-23