A light spring with a force constant of 22 N/m is connected to a wall and to a 1.8-kg toy bulldozer. When the electric motor in the bulldozer is turned on, it stretches the spring for a distance of 40 cm before its tread begins to slip on the floor and it accelerates back towards the wall at a rate of 0.784 m/s2. What is the coefficient of dynamic friction between the treads and the floor?

Emmy kicks a soccer ball up at an angle of 45° over a level field. she watches the ball's trajectory and notices that it lands, two seconds after being kicked, about 20 m away to the north. assume that air resistance is negligible, and plot the horizontal and vertical components of the ball's velocity as a function of time. consider only the time that the ball is in the air, after being kicked but before landing. take "north" and "up" as the positive ‑ and ‑directions, respectively, and use ≈10 m/s2 for the downward acceleration due to gravity.