## maheshmeghwal9 3 years ago In the arrangement shown in Fig. 1.24 the masses m of the bar and M of the wedge, as well as the wedge angle alpha , are known.The masses of the pulley and the thread are negligible. The friction is absent. Find the acceleration of the wedge M.

1. maheshmeghwal9

2. maheshmeghwal9

Answer is $\LARGE{\color{blue}{a=\frac{mg \sin \alpha }{M+2m(1-\cos \alpha)}.}}$$\color{red}{\text{But how to get this answer?}}$

3. Vincent-Lyon.Fr

Let x, v and a be displacement, velocity and acceleration of wedge. All these values are positive (to the right). Velocity of m wrt wedge is (first line x-component, second line y-component) -v cosα -v sinα Velocity of wedge wrt ground is: v 0 Adding them up gives velocity of m wrt ground: v (1 – cosα) -v sinα KE of wedge is KE1 = ½ M v² KE of mass is KE2 = ½ m v² [(1 - cosα)² + sin²α] = m v² (1- cosα) Altitude of m above ground is -x sinα , as it goes x down along the inclined when wedge moves of x. PE of mass is PE = -m g x sinα The system is conservative. So total energy is conserved. Adding up all derivatives of above energies must give zero: 0 = Mva +2mva(1 - cosα) - mg v sinα Simplifying by v and solving for a leads to the solution.

4. Vincent-Lyon.Fr

* altitude above ground is actually -x sinα + H where H is height of the pulley. H disappears in the derivation.

5. Vincent-Lyon.Fr

I also have a solution based on forces and N's laws, although I prefer the more global energy approach.

6. yrelhan4

also sir, what would be the displacement of the wedge? this is a common question, but i always end up getting stuck. @Vincent-Lyon.Fr