Download Impulse and Momentum

Active Learning: Impulse and Momentum
rev. 5/12
I Impulse and Momentum
1. Two masses are initially at rest and acted on by identical forces over the same distance. Mass 1 is
twice mass 2. Be sure you can explain your answers.
A. Which force does the greater amount of work:
a. Force on mass 1
b. Force on mass 2
c. Both the same
B. Which force imparts the greater impulse?
a. Force on mass 1
b. Force on mass 2
c. Both the same
2. Now suppose you apply the identical forces for the same time on the two masses.
A. Which force does the greater amount of work:
b. Force on mass 1
b. Force on mass 2
c. Both the same
B. Which force imparts the greater impulse?
b. Force on mass 1
b. Force on mass 2
c. Both the same
3. Each of the following situations describes the initial and final velocities of identical baseballs. Rank
the magnitudes of the impulse for each case from greatest to smallest.
A. Vi = 0
Vf = 20 m/s to the right
B. Vi = 25 m/s to the right Vf = 40 m/s to the right
C. Vi = 30 m/s to the left
Vf = 10 m/s to the left
D. Vi = 15 ms/ to the left
Vf = 10 m/s to the right
4. An impulse is applied to a 2 kg object initially travelling 2 m/s to the left. The impulse lasts 20 ms.
Afterwards, the object is moving at 8 m/s to the right.
A. What was the average force applied to object?
B. If the impulse was triangular, rather than constant, what was the maximum force applied to the
object? (see the F vs. t graph below)
Force
Fmax
0
20 ms
Time
Active Learning: Impulse and Momentum
5. What force does a fire hose exert on a wall, assuming 50 kg/s of water strikes the wall at 15 m/s and
does not bounce back?
II Conservation of Momentum: Explosions
1. Two blocks (M1 = 2 kg, M2 = 5 kg) are forced against opposite ends of a spring as shown below.
Spring
M1
M2
A. The blocks are released simultaneously. What is the ratio vA/vB?
B. If the spring constant is k = 800 N/m and the initial compression is 0.20 m, what are the final
velocities of A and B after they leave the spring, assuming friction is negligible?
A. A 5 kg mass traveling at 10 m/s along the +x axis explodes into two pieces. The 4 kg piece goes
off at 25° in the first quadrant; the 1 kg piece goes off along the –y axis. What are the speeds of
the two pieces?
Active Learning: Impulse and Momentum
III Conservation of Momentum: Collisions
1. Three objects are traveling along the x axis as shown. A overtakes and sticks to B, and then the
combination collides head-on and sticks with C. What is the final velocity of the combined mass?
A(1 kg, 6.8 m/s)
B(4 kg, 2.5 m/s)
C(8 kg, 2.1 m/s)
2. Two identical pucks of mass M = 0.40 kg are traveling toward each at 4.0 m/s and 6.0 m/s,
respectively. They glance off each other and the slower puck goes off at 3.0 m/s @ 70° from its
original path.
A. What is the velocity of the other puck after the collision?
B. How much kinetic energy was lost in the collision?
3.0 m/s
#1
#2
#1
70°
4.0m/s
V’
6.0 m/s
#2
Active Learning: Impulse and Momentum
3. Two masses, M2 = 2M1, are traveling toward each other (V1 = +5 m/s; V2 = -8 m/s). They undergo a
head-on, elastic collision.
A. What is the relative velocity between the two masses before the collision (V2 – V1)?
B. What is the relative velocity between the two masses after the collision (V’2 – V’1)?
4. A mass M traveling at V collides head-on with a mass 2M traveling at V in the opposite direction.
The collision is elastic. What are the velocities of the two masses after the collision? Also, verify that
kinetic energy is conserved in the collision (i.e. K = 0).