1-A 7.80-g bullet moving at 575 m/s strikes the hand of a superhero, causing the
ID: 1785600 • Letter: 1
Question
1-A 7.80-g bullet moving at 575 m/s strikes the hand of a superhero, causing the hand to move 5.50 cm in the direction of the bullet’s velocity before stopping. (a) Use work and energy considerations to find the average force that stops the bullet. (b) Assuming the force is constant, determine how much time elapses between the moment the bullet strikes the hand and the moment it stops moving.
2-The two masses in the figure are released from rest. After the 3.0-kg mass has fallen 1.5 m, it is moving with a speed of 3.8 m/s. What is the amount of work done on the system during this time interval by the frictional force on the 2.0 kg mass?
Explanation / Answer
(A) Applying work - energy theorem,
Work done = change in KE
- F (0.0550) = 0 - (0.00780) (575^2)/2
F = 23.4 x 10^3 N Or 23 kN
(b) a = F / m = 3 x 10^6 m/s^2
vf = vi + a t
0 = 575 - (3 x 10^6) t
t = 1.91 x 10^-4 sec
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