I need some help with a physics question I can't seem to figure out. The question is: Two trailers, A and B, is beeing held by a rope. They stand still. The mass of A is 1 kg and the mass of B is 3 kg. A spring is in between the two trailers. The rope is then cut, and the trailers move in a straight line away from each other with almost no friction. Trailer A have gotten a speed of 1.2 m/s. How much energy is stored in the spring when its given that 60% of the energy goes over to kinetic energy to the trailers.
And I am sorry for my terrible translation.
Initial momentum is equal to zero, so is the final momentum. For the energy part use 1/2mv^2
>>7673628
My native language isn't english, please tell me what you mean about initial and final momentum :P And if I am correct you refer to P=m'*v?
If so, I can't see how P=mass f
take away the last part of that comment
How do I find v_b?
E_k*0.6=1/2*(m_a*(v_a^2)+m_b*(v_b^2))=1.44/2+3/2(*v_b^2)
>>7673650
Yes, p= m*v but you have two momentums, so [eqn] p_1 = m_1*1.2m/s = p_2 = m_2*v_2 [/eqn] Then you solve for [eqn] v_2 [/eqn] and plug it into your energy kinectic equation. And you know that the sum of the energy kinetics is only 60% of the energy of the spring.
>>7673678
I'll guess that P_a=P_b only when its a spring in the middle?
>>7673687
No, I was lazy and didn't explain that [eqn] p_a = -p_b [/eqn] since the question didn't ask for a vector so it did not matter. But it is like that because the system initially has a momentum of 0, so in the end the momentum will also be equal to 0, meaning one velocity will travel in one direction, and the other in the opposite direction. Or have a negative value in the velocity. So in any circumstance with a system that has an initial momentum of 0 the final momentum has to equal 0, this is the law of conservation of momentum. Which worded in a different way states that the initial momentum of a system must be equal to the final momentum or [eqn] p_i = p_f [/eqn] This probably doesn't make much sense to you if you have a poor understanding of english, so just google conservation of momentum.
>>7673717
It made sense, thanks.
But just to make sure, the equation for v_b will be
v_b=-m_a*v_a/m_b
>>7673725
Yep