18. A pony is pulling a sled lled with rocks along a horizontal snow-covered path at a steady speed. The combined mass of the sled and rocks is 850 kg. The coefcient of kinetic friction between the sled and snow is 0.12. If the pony is pulling with its maximum power of 600 W, how fast is the sled moving? A) 0.07 mfs B} 0.60 m/s C} 1.7 m/s D) 5.9 m/s E) Not enough information given 19. A system consists of two carts and a massless spring. Initially the two carts are at rest and the compressed spring is between them. The cart on the left has a mass m and the cart on the right has a mass 3m. After the spring is released, which statement below is true? BEFORE A) The speeds of the carts are the same. AFTER B) The velocities of the carts are the same. C) The total kinetic energy of the system is zero. D) The total momentum of the system is zero. E) More than one of the above is true. 20. A block is tied to a string that is wound around a pulley. The pulley has non-zero mass. The radius of the pulley is 0.10 m. The block is released from rest and is observed to be accelerating downwards at 1.5 m/s2. How many revolutions will the pulley have made 10 seconds after the block is released (to two significant figures)? A) 75 revolutions B) 120 revolutions C) 150 revolutions D) 240 revolutions E) 750 revolutions 25. A pottery wheel, with a moment of inertia of 1.6 kg ma, is rotating at an angular speed of 25.1 radians per second. A torque of 0.4 N.m is applied in a direction opposite the spinning direction. How long does it take the wheel to stop spinning? A) 3s B) 16 s C) 60 s D) 100 s E) 200 s