After the cylinders are dropped, the only force on each is the force of gravity. Use Newton's second law. FNet on A aA aB = FNet on B MA MB MA g aA = - = g MB g MA aB = MB = g Substitute FNet on A = MAg and FNet onB = MBg. -9.8 m/s2 -9.8 m/s2 Substitute g = -9.8 N/kg = -9.8 m/52. 3 EVALUATE THE ANSWER . Are the units correct? N is the correct unit for force. m/s2 is the correct unit for acceleration. . Does the sign make sense? The direction of the fall is downward, the negative direction, and the object is speeding up, so the acceleration should be negative. . Is the magnitude realistic? Forces are 1-5 N, typical of that exerted by objects that have a mass of one kg or less. The accelerations are both equal to free fall acceleration. PRACTICE PROBLEMS Do additional problems. Or 16. You place a watermelon on a spring scale calibrated to measure in newtons. If the watermelon's mass is 4.0 kg, what is the scale's reading? 17. You place a 22.50-kg television on a spring scale. If the scale reads 235.2 N, what is the gravitational field at that location? 18. A 0.50-kg guinea pig is lifted up from the ground. What is the smallest force needed to lift it? Describe the particular motion resulting from this minimum force. 19. CHALLENGE A grocery sack can withstand a maximum of 230 N before it rips. Will a bag holding 15 kg of groceries that is lifted from the checkout counter at an accelera- tion of 7.0 m/s2 hold? Section 2 . Weight69. You toss a ball straight air resistance is negligible. a. Draw a free-body diagram for the ball at three points: on the way up, at the very top, and on the Figure way down. Specifically identify the forces and agents acting on the ball. b. What is the ball's velocity at the very top of the 77. Bas motion? from c. What is the ball's acceleration at this point? ma 70. When receiving a basketball pass, a player doesn't a. V hold his or her hands still but moves them in the b. V direction of the moving ball. Explain in terms of acceleration and Newton's second law why the C. player moves his or hands in this manner. Mixed Review 78. An 24 71. A dragster completed a 402.3-m (0.2500-mi) run in wl 5.023 s. If the car had a constant acceleration, what was its acceleration and final velocity? 79. Ai ai 72. Space Station Pratish weighs 588 N on Earth but is P currently weightless in a space station. If she pushes a off the wall with a vertical acceleration of 3.00 m/s, O push off. determine the force exerted by the wall during her P a 116 Chapter 4 . Assessment