Conservation of MomentumA loaded cart is coasting along a road at a constant speed va. Suddenly, part of its load (equal to 20% of its mass) is projected forwards, causing the cart to some to rest. If, instead, the same amount was projected backwards, it would cause the cart’s velocity to increase to a value vb. What is the ratio between vb and va?
We will solve this problem, using conservation of momentum – in each of the two cases where the load is projected off the cart, the momentum of the cart – load system should remain unchanged.
If we take motion in the forward direction to be positive, the conservation of momentum for the first case can be written as:
- M va= 0.8*M*0 + 0.2*M*vc
Where vc is the velocity the projected load aquires.
Then, we can see that:
- vc = 5*va
Now, going to the second case (where the mass is projected backwards), conservation of momentum gives us:
- M va= 0.8*M*vb - 0.2*M*vc
The differences between this and the previous equation are the minus sign – this is to take into account that the velocity of the mass in this case goes in the other direction (the negative direction) – and the vb, because in this case, the cart is not stationary after the ejection of the mass.
Now, putting the vc we calculated earlier into this equation, we get:M va = 0.8 M vb – 0.2 M 5 va
2M va = 0.8 M vb
vb/va = 2/.8 = 1.25