*o Question 4: Rocker Rob enters from stage left at a speed of 5 m/s and slides to a rest over a distance of 2 m. What is the coefficient of kinetic friction between his socked-feet and the marley stage floor below?

In a laboratory model of cars skidding to a stop, data are measured for four trials using two blocks. The blocks have identical masses but different coefficients of kinetic friction with a table: k = 0.2 and 0.8. Each block is launched with speed vi = 1 m/s and slides across the level table as the block comes to rest. This process represents the first two trials. For the next two trials, the procedure is repeated but the blocks are launched with speed vi = 2 m/s. Rank the four trials (a) through (d) according to the stopping distance from largest to smallest. If the stopping distance is the same in two cases, give them equal rank. (a) vi = 1 m/s, = 0.2 (b) vi = 1 m/s, k = 0.8 (c) vi = 2 m/s, = 0.2 (d) vi =2 m/s, k = 0.8

Unreasonable Results A mother pushes her child on a swing so that his speed is 9.00 m/s at the lowest point of his path. The swing is suspended 2.00 m above the child's center of mass. (a) What is the magnitude of the centripetal acceleration of the child at the low point? (b) What is the magnitude of the force the child exerts on the seat if his mass is 18.0 kg? (c) What is unreasonable about these results? (d) Which premises are unreasonable or inconsistent?

Chris, a recent physics major, wanted to design and carry out an experiment to show that an objects mass determines its inertia. He used an ultrasound device to measure acceleration of a low-friction cart attached to a hanging block to provide the same force on the cart during each run (Fig. P6.76A). Chris varied the mass of the cart by varying the number of lead rods placed in it. Chris used Newtons second law Fx=FT=Max to predict his results. He reasoned that because FT is the same for each run, the carts acceleration should be inversely proportional to its mass: ax=FTM=constantM(1) Chriss goal was to show that his data fit Equation (1). He decided to analyze his results by plotting ax as a function of 1/M; Equation (1) predicted that he should get a straight line, passing through the origin with a slope equal to the tension (red line in Fig. P6.76B): Chris ran several trials for each run, averaged his results and estimated the error. He then plotted his data (green line in Fig. P6.76B). Chris was excited to see that he correctly predicted that the data fell along a straight line: ax=(0.27N)1M(0.048m/s2) According to the straight-line fit to the data, the slope of the line is 0.27 N, which was close to the weight of the hanging mass and therefore close to the tension in the string. Chris, though, was disappointed to see that the line had a negative intercept. Mathematically, as M, 1M0. Chris was confused because he believed that as the mass increased, the carts acceleration should approach zero. He was quite sure that he did not discover some new property of inertia or mass. After convincing himself that he was not being careless in the laboratory and that his data were correct, he started to search for an explanation for the discrepancy between his prediction and his data. Help Chris find an explanation. FIGURE P6.76 A. Chriss experimental apparatus. B. Chriss prediction (red line) and experimental results (green line).