- #1
Karol
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Homework Statement
The masses C, of magnitude 2m, and B, of magnitude m, move with velocity V and hit a stationary mass A of magnitude m. the collision lasts a very short time and is plastic but the masses don't stick together. the spring with constant k is ideal and the surface is smooth.
What's the max acceleration of C during the contact between A and B
Box A continues, after it separates, with constant velocity, what's it.
What's the max contraction of the spring after A and B part?
How long did the contact between A and B take place?
Homework Equations
Kinetic energy, potential energy of a spring: ##E_p=\frac{1}{2}kx^2##
Impulse-momentum: ##mv=Ft##
Conservation of momentum: ##m_1v_1+m_2v_2=m_1v_1'+m_2v_2'##
The Attempt at a Solution
I divide into 2 collisions. the first is plastic between boxes A and B in which the members cling to one another. the final velocity of the boxes A and B is ##\frac{V}{2}## and i find it from conservation of momentum between box B and both boxes after the collision:
$$mV=2mv\;\rightarrow\; v=\frac{V}{2}$$
Now these 2 boxes collide elastically with C. the spring contracts, expands and reaches, again, the initial relaxed length L0. then and there the boxes A and B part and A continues alone.
One system consists of the boxes A and B and the other system is box C alone. conservation of energy+momentum:
$$\begin{cases}\frac{1}{2}(2m)V^2+\frac{1}{2}(2m)\left( \frac{V}{2} \right)^2=\frac{1}{2}(2m)v_1^2+\frac{1}{2}(2m)v_2^2 \\ 2mV+2m\frac{V}{2}=2mv_1+2mv_2 \end{cases}\;\Rightarrow\; v_1=\frac{V}{2},\; v_2=V$$
This is also A's velocity after it detaches, which happens at distance L0 again.
At some point, when the spring contracts to it's max, A and B halt momentarily. from the point of view of an observer on C all A and B's kinetic energy gets into the spring. A and B's velocity relative to C is ##\frac{V}{2}##:
$$\frac{1}{2}(2m)\left( \frac{V}{2} \right)^2=\frac{1}{2}kx^2\;\rightarrow\; x=\sqrt{\frac{m}{2k}}V$$
C's max acceleration:
$$ma=F=kx\;\rightarrow\;2m\cdot a=\sqrt{\frac{m}{2k}}V\;\rightarrow\;a=\sqrt{\frac{mk}{2}}\frac{V}{2m}$$
After A has left, B stretches the string and returns. this is another collision:
$$\begin{cases}\frac{1}{2}mV^2+\frac{1}{2}(2m)\left( \frac{V}{2} \right)^2=\frac{1}{2}(2m)v_C^2+\frac{1}{2}mv_B^2 \\ mV+2m\frac{V}{2}=2mv_C+mv_B \end{cases}\;\Rightarrow\; v_C=\frac{5}{6}v,\; v_B=\frac{V}{3}$$
Now it compresses the spring, how much?
The distance between B and C is x. i want to find x as a function of t: ##x=f(t)## so that i will be able to differentiate it and find the maximum contraction. i can't. i use a simplified setting: a spring attached to a wall.
Lets say x is at distance x0 from the wall, the velocity there is v0 and it advances distance dx. the acceleration is ##F=kx=ma\;\rightarrow\; a=\frac{k\cdot x_0}{m}##. the acceleration in the interval dx is considered constant, and the mean velocity in dx is:
$$v-\frac{k\cdot x_0}{m}dt$$
from kinematics:
$$dx=\left( v-\frac{k\cdot x_0}{2m}dt \right)dt\;\rightarrow\;\frac{dx}{dt}=v_0-\frac{k\cdot x_0}{2m}dt$$
$$\int \frac{dx}{dt} dt=\int dx=x=\int v_0 dt-\int \frac{k\cdot x_0}{2m}dt^2$$
I don't know to handle dt2, if everything is correct till there, of course.