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qr code reader for java mobile What is the baseball s final velocity Because pf vf yields the following: vf in ObjectiveC
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Analyze and Sketch the Problem
Sketch the system Include a coordinate axis and select the positive direction to be the direction of the velocity of the car Draw a vector diagram for momentum and impulse Known: m vi vf 2200 kg +26 m/s +00 m/s tgentle braking t hard braking t hitting a wall 21 s 38 s 022 s
94 km/h Vector diagram
Unknown: Fgentle braking F hard braking F hitting a wall
Solve for the Unknown
Determine the initial momentum, pi pi mvi (2200 kg)( 26 m/s) Substitute m 57 104 kg m/s Determine the final momentum, pf pf mvf (2200 kg)( 00 m/s) Substitute m 00 kg m/s 2200 kg, vi 26 m/s
Impulse
2200 kg, vf
00 m/s
Apply the impulsemomentum theorem to obtain the force needed to stop the vehicle F t pf pi F t ( 00 kg m/s) (57 104 kg m/s) Substitute pf 00 kg m/s, pi 57 104 kg m/s 57 104 kg m/s F 57 104 kg m/s t 57 104 kg m/s 21 s
Substitute tgentle braking 21 s
Fgentle braking
27 103 N F hard braking
57 kg m/s 38 s 104
Substitute thard braking 38 s
Math Handbook
Operations with Significant Digits pages 835 836
15 104 N F hitting a wall
57 104 kg m/s 022 s
Substitute thitting a wall 022 s
26 105 N Evaluate the Answer
Are the units correct Force is measured in newtons Does the direction make sense Force is exerted in the direction opposite to the velocity of the car and thus, is negative Is the magnitude realistic People weigh hundreds of newtons, so it is reasonable that the force needed to stop a car would be in the thousands of newtons The impulse is the same for all three stops Thus, as the stopping time is shortened by more than a factor of 10, the force is increased by more than a factor of 10 9 Momentum and Its Conservation
1 A compact car, with mass 725 kg, is moving at 115 km/h toward the east Sketch the moving car a Find the magnitude and direction of its momentum Draw an arrow on your sketch showing the momentum b A second car, with a mass of 2175 kg, has the same momentum What is its velocity 2 The driver of the compact car in the previous problem suddenly applies the brakes hard for 20 s As a result, an average force of 50 103 N is exerted on the car to slow it down a What is the change in momentum; that is, the magnitude and direction of the impulse, on the car b
Force (N) 5 0 5 1 2 Force (N) 5 0 5 1 2 Time (s) b Complete the before and after sketches, and determine the momentum and the velocity of the car now 3 A 70kg bowling ball is rolling down the alley with a velocity of 20 m/s For each impulse, shown in Figures 93a and 93b, find the resulting speed and direction of motion of the bowling ball 4 The driver accelerates a 2400kg snowmobile, which results in a force being exerted that speeds up the snowmobile from 600 m/s to 280 m/s over a time interval of 600 s a Sketch the event, showing the initial and final situations b What is the snowmobile s change in momentum What is the impulse on the snowmobile Time (s) Figure 93 c What is the magnitude of the average force that is exerted on the snowmobile 5 Suppose a 600kg person was in the vehicle that hit the concrete wall in Example Problem 1 The velocity of the person equals that of the car both before and after the crash, and the velocity changes in 020 s Sketch the problem a What is the average force exerted on the person b Some people think that they can stop their bodies from lurching forward in a vehicle that is suddenly braking by putting their hands on the dashboard Find the mass of an object that has a weight equal to the force you just calculated Could you lift such a mass Are you strong enough to stop your body with your arms

