I have two question that I don't know how to start. 1st question, three particles A, B and C of equal mass m lie on a horizontal suface at the vertices of an equilateral triangle; they are joined by light inextensible string. An impulse I is applied on C in the direction of BC. Find the speed of C just after the jerk, express the answer in terms of I and m. Calculate the ratio of the instantaneous speeds of B and A. [Hint: Backward impulses will act on particle C through the strings whenever there is an impulse I on C] 2nd question, A block of mass m1 is placed on the top of another block of mass m2 resting on the floor of an elevator, where m1<m2. The coefficient of static friction between the block is μ(s) and the coefficient of kinetic friction between the lower block and the floor is μ(k), where μ(s)<μ(k). A horizontal force F is applied on the lower block when the elevator is accelerating upward with a magnitude of a. As a result, the blocks move together without relative slipping.(F is in a rightward direction) (a) Draw the free body diagram of each block (b) Find the maximum magnitude of F and the acceleration a of the blocks for such case. My attempt: 1st question: I don't know what is the meaning of an impulse acting on an object, does it mean that there is a force or what? If yes, could I use force to calculate the speeds of the particles. However, for C, I am simply using change of momentum to calculate. So, speed of c= I/m. Am I correct? If yes, how could I link this up with the two remain speeds of A and B. 2nd question: For the free body diagram, am I drawing them right? If no, what are the problems? Also, I don't know how to link up F(max) with f(k) and f(s). Is it something like F(max)-f(k)=<f(s)?