#6 #10 #14 #18 #196. What average net force is needed to accelerate an 80-kg sprinter to a speed of 10 m/s in a time of one second? How does this compare to the sprinter's weight?
To find the average net force, we use the definition that the force is the change in momentum divided by the time to make the change.
The sprinter's weight is
Both of these forces are the same.
10. A pitcher throws a baseball (mass = 145 gm) at 45 m/s (100 mph), what is the average force exerted on the catcher's glove if it requires 0.2 seconds for the ball to stop?
The equation we want to use is that force is equal to the change in momentum divided by the change in time. Here, we are careful to convert the mass of 145 grams to kilograms before plugging in the values. There are 1000 grams in a kilogram, so 145 grams is 0.145 kilograms.
14. A woman with a mass of 50 kg runs at a speed of 6 m/s and jumps onto a giant skateboard with a mass of 25 kg. What is the combined speed of the woman and the skateboard?
The initial momentum of the system is from the running woman alone, since the skateboard is at rest.
The final system momentum is that of the woman on the skateboard.
We use conservation of momentum to find the final velocity of the woman and the skateboard.
18. A 1200-kg car traveling north at 10 m/s was rear-ended by a 2000-kg truck traveling at 26 m/s. What was the total momentum before and after the collision?
Since momentum is conserved, the initial and final momenta are the same.
19. If the truck and car in the previous problem (#18 above) lock bumpers and stick together, what is their speed immediately after the collision?
To determine the final speed, we employ conservation of momentum.
