# Movement by impact

1. Oct 13, 2006

### moo

Hi all,

How does one calculate the point where impact of a moving object actually begins to move a stationary one, rather than just make noise?

Let's keep it simple and assume the objects are of the same material, the stationary one is not anchored, and they won't shatter or explode. Something like whacking a croquet ball with a mallet (lol, the things that keep me awake at night... )

If the answer is too time consuming, perhaps someone has a link?

Thanks, moo
__________________
moo (moo') adj. Of no practical importance; irrelevant, such as a moo point (i.e. a cow's opinion).

2. Oct 13, 2006

It depends on the geometry of the objects.

3. Oct 13, 2006

### moo

Hmmm... ok that makes sense.

How about two identical cubes impacting on flat surfaces then (one stationary, one moving)?

Thanks, moo
__________________
moo (moo') adj. Of no practical importance; irrelevant, such as a moo point (i.e. a cow's opinion).

Last edited: Oct 13, 2006
4. Oct 13, 2006

### moo

I guess friction must be considered on this as well, otherwise the slightest touch would cause the stationary one to move... wouldn't it?

moo
__________________
moo (moo') adj. Of no practical importance; irrelevant, such as a moo point (i.e. a cow's opinion).

5. Oct 13, 2006

Well, that depends on the 'orientation' of the cubes. There can be either a plane of impact, or a line of impact.

6. Oct 13, 2006

### moo

Lol, this is kinda like pulling teeth.

Maybe you can ask the question so I can get an answer?

Thanks, moo
__________________
moo (moo') adj. Of no practical importance; irrelevant, such as a moo point (i.e. a cow's opinion).

7. Oct 13, 2006

State your question more clear, and it won't be like pulling teeth.

8. Oct 13, 2006

### moo

Lol, ok I'll try again and change it up a bit...

Let's say we have two blocks (A and B).
Block dimensions are in inches, weights are in pounds (hey I tried to get you to ask the question).

A is 2x2x2 (a cube) and weighs 1 pound.
B is 2x2x4 and weighs 2 pounds.
B is stationary, A is moving.
One of A's flat 2x2 sides perfectly impacts B's flat 2x2 end and moves it.

What is the relationship between A's mass & velocity and B's mass & distance moved? Btw, feel free to fill in any blanks I've prolly left...

Thanks, moo
__________________
moo (moo') adj. Of no practical importance; irrelevant, such as a moo point (i.e. a cow's opinion).

9. Oct 13, 2006

10. Oct 13, 2006

### moo

Thanks, that should get me started.

moo
__________________
moo (moo') adj. Of no practical importance; irrelevant, such as a moo point (i.e. a cow's opinion).

11. Oct 13, 2006

### tim_lou

well... momentum is always conserved. if you assume elastic collision, then you can assume that mechanical energy is also consered.
Last but not least, angular momentum is conserved if there is no external torque. (so you can calculate rotations and stuff..)

momentum is defined as $$\vec{p}=m\vec{v}$$ in newtonian physics. now, force=change in momentum per time, and according to newton's third law, the force of object 1 on object 2 is equal and opposite direction of the force of object 2 on object 1. so the total change of momentum (a system of these two objects) is zero, momentum is conserved.

in linear case,
$$m_1v_1+m_2v_2=m_1v'_1+m_2v'_2$$

since there are 2 unknowns, there must be some constrains or assumptions in the system in order to calculate both of these unknowns... maybe you can assume that the objects stick together, or assume that kinetic energy is conserved... or other stuffs...

Last edited: Oct 13, 2006
12. Oct 13, 2006

### moo

Thanks Tim.

I realize you guys deal with incredible precision (and therefore my questions may drive ya crazy sometimes), but I'm often just looking for a general estimate. Such as...

Does a half-size croquet mallet need to move roughly twice the speed of a regular one to whack a ball the same distance?

Sheesh, it's just croquet. Lol, and I don't even play...

Thanks, moo
__________________
moo (moo') adj. Of no practical importance; irrelevant, such as a moo point (i.e. a cow's opinion).