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Homework answers / question archive / 1) A 43
1) A 43.0 kg cross country runner runs up a slope in 60.0 s. The vertical rise is 25.0 m. Approximately what power does the runner exert? .
a. 150 W
b. 175 W
C. 1.11 x 10' W
d. 2.22 x 10' W
2. A book falls off a table and is now 1 cm from the floor. Which statement is correct?
a. The kinetic energy of the book is at its greatest.
b. The potential energy of the book is at its greatest.
c. The kinetic and potential energies of the book are equal.
d. Both the kinetic and potential energies of the book are at their greatest.
3. A ball is thrown in the air. At the start of the throw, the kinetic energy of the ball is 2.2 x 10' J and the gravitational potential energy is 0 J. What is the gravitational potential energy when the ball reaches its highest point?
a. 0J
b. 1.1 x 10
c. 2.2 x 10' J
d. -4.4 x 10" J
4. Modern tennis players are able to hit the ball with greater speed than was possible with older wooden tennis racquets. This is due to .
a. tennis balls of greater mass.
b. greater muscle development of players.
C more contact time between the ball and the racquet.
d. changes in the rules of tennis.
5. Two carts each with mass 1.2 kg are on a frictionless track. The first cart travelling at 3.0 m/s collides with the second stationary cart. The two carts latch together and continue rolling down the track. The velocity of the carts is now
a. 3.0 m/s.
b. 4.2 m/s.
c. 3.6 m/s
d. 1.5 m/s.
6. Momentum is conserved when
a. An insect collides with the windshield of a moving car.
b. An electron splits an atom into many subatomic particles.
c. A rifle fires a bullet and the gun recoils.
d. all of the above
7. A child with a mass of 15 kg runs at a horizontal velocity of 3.6 m/s [forward] and jumps onto a stationary tree swing of mass 3.0 kg. The child "sticks" on the tree swing and swings forward. What is the horizontal velocity of the child and swing?
a. 54 m/s [forward]
b. 6.6 m/s [forward]
c. 2.9 m/s [forward]
d. 2.7 m/s [forward]
8. Two objects undergo a head-on collision. The objects have masses, m, - 2kg and my - 5 kg. The velocities before and after the collision are as follows, V,, - 7m/s; V,2 - 0 m/s, Vy - 2m/s; Vra - 2 m/s. Identify the type of collision.
a. completely inelastic
b. inelastic
c. elastic
d. none of these
Answer:
1. b. 175 W
2. a. The kinetic energy of the book is at its greatest.
3. c. 2.2x10^2 J
4. c. more contact time between the ball and the racquet.
5. d. 1.5 m/s
6. c. A rifle fires a bullet and the gun recoils.
7. c. 2.9 m/s
8. a. completely inelastic
Step-by-step explanation
1. b. 175 W
Power =(43kg x 9.8m/s^2 x 25.0 m) / 60.0 s = 175 W
2. a. The kinetic energy of the book is at its greatest.
It has gained enough velocity (from the idea of free fall) so it has its greatest kinetic energy right before hitting the ground. Potential energy will be greatest at its peak or highest height.
3. c. 2.2x10^2 J
By Law of Conservation of Energy.
4. c. more contact time between the ball and the racquet.
Old racquets have weaker (more stretchable) strings so more contact time between ball and racquet means reduced power, thus reduced speed.
5. d. 1.5 m/s
m1v1i+m2v2i = (m1+m2)v2f
v2f = (m1v1i+m2v2i)/ (m1+m2)
v2f = (1.2 kg x 3.0m/s + 0)/(1.2 kg + 1.2 kg)
v2f = 1.5 m/s
6. c. A rifle fires a bullet and the gun recoils.
Other cases are non-elastic collisions.
7. c. 2.9 m/s
v2f = (m1v1i+m2v2i)/ (m1+m2)
v2f = (15 kg x 3.6m/s + 0)/(15 kg + 3.0 kg)
v2f = 3.0 m/s (closest is 2.9 m/s)
8. a. completely inelastic
Since both objects have the same velocity after collision which means they "stick" together.
