Here , i need to find out the range of the angle that can prevent scissor lifter to become tip-over. Before this , i had done some review,and i had read one of the paper state that the system will only experience tip-over during the kinetic energy lesser than the potential energy after impact of the lifter. Therefore, i was thinking to use this relation to sort out the range of the angle. Currently, i am using the center of mass of the whole system,and done the first derivative for the vertical component of the system, use this as the vertical velocity of the system. Follow by calculate the kinetic energy for this system. And i found out that, my kinetic energy is always lesser than potential energy of the system. So, i was wondering whether my solution got any problem. Or what i had missing out during the calculating of the total kinetic energy. Please give me some advise. Need to solve this urgent..Thankyou in advance!!