# Download A block slides across a rough horizontal table top. The work

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```Phys101
Term:131
Online HW-Ch08-Lec02
Q1:
A block slides across a rough horizontal table top. The work done by friction changes:
A.
B.
C.
D.
E.
only the kinetic energy
only the potential energy
only the internal energy
only the kinetic and potential energies
only the kinetic and thermal energies
Ans:
E
Q2:
A 0.50-kg block attached to an ideal spring with a spring constant of 80N/m oscillates
on a horizontal frictionless surface. The total mechanical energy is 0.12 J. The
greatest extension of the spring from its equilibrium length is:
Ans:
At the greatest extension all the mechanical energy is converted to spring potential
energy.
1
∆𝐸 = 𝑘𝑥𝑚 2
2
2∆𝐸 0.5
2 × 0.12 0.5
𝑥𝑚 = �
� =�
� = 0.054 𝑚
𝑘
80
Q3:
A 5.00 kg projectile is fired over level ground with a velocity of 200m/s at an angle of
250 above the horizontal. Just before it hits the ground its speed is 150m/s. Over the
entire trip the change in the thermal energy of the projectile and air is:
Ans:
𝑊 = ∆𝐸 + ∆𝐸ℎ
W = 0 because there is no external force to do the work
0 = ∆𝐾 + ∆𝑈 + ∆𝐸ℎ
ΔU = 0 because there is no change in height.
∆𝐸ℎ = −∆𝐾 = 𝐾0 − 𝐾 =
KFUPM-Physics Department
1
1
𝑚(𝑣0 2 − 𝑣 2 ) = × 5 × (2002 − 1502 ) = 44.0 kJ
2
2
1
```