A squirrel {mass 511 g) sits at rest at the bottom of the tree. At some moment it starts running vertically up the tree and reaches a speed of 4.9 mfs in 0.13 seconds. Assuming that the acceleration is constant; a] What is the magnitude of the acceleration of the squirrel? b) What is the magnitude of the force that the squirrel exerts on the bark of the tree during the acceleration phase