"a problem involves a car of mass m going down a track from a height h, and round a loop of radius r. the loop is frictionless.
it asks for the minimum cut-off speed required, at the highest point in the loop (call it point d), such that the car makes it round the loop without falling. i know the solution; i should set the centripetal accleration equal to 9.81. in other words, contact force with the track at point d is equal to zero.
but i tried solving it by conservation of energy. at point d, the car is at a height 2r from ground level. therefore, in order for the car to reach that height at point d, it must initially have a potential energy of mg(2r). meaning, it should be released from a height h = 2r.
i got the wrong answer and i'm confused why that happened. isn't that how conservation of energy work? clarify, where's the error in my solution? "

The ballistic pendulum was invented in 1742 by english mathematician benjamin robins. it consists of an initially stationary pendulum that moves after being struck by a bullet, and it is used to measure the original velocity of the bullet. the known variables are the bullet's mass m, the pendulum's mass m, and the height to which the block and bullet swing, determined by the length of the pendulum l and final angle θ. two principles of physics are necessary to solve for the bullet's original velocity. what are these principles? conservation of momentum and newton's third law conservation of energy and conservation of angular momentum grade summary deductions 0% potential 100% conservation of momentum and newton's second law. newton's second law and conservation of angular momentum. conservation of energy and newton's third law conservation of momentum and conservation of angular momentum. conservation of momentum and conservation of energy conservation of energy and newton's second law.

Which of the following is an example of motion in two dimensions?

If an athlete runs once around a track, back to the starting line, her average velocity is zero. true or false?