Concept explainers
(a)
The amount of energy dissipated by friction.
(a)

Answer to Problem 100P
The amount of energy dissipated by friction is 935 J_.
Explanation of Solution
Figure 1 represents the free body diagram of the crate.
For an isolated system the total energy of the system (the sum of mechanical energy and its internal energy) is conserved.
ΔK+ΔU+ΔEint=0
Here, ΔK is the change in kinetic energy, ΔU is the change in potential energy, and ΔEint is the change in internal energy of the system.
In this situation, the work done by the frictional force is equal to the change in internal energy of the system.
The above equation is reduced to.
(12mv2f−12mv2i)+mgΔy+ΔEint=0
Here, ΔEint is the energy dissipated by friction, m is the mass of the crate, g is the acceleration due to gravity, and Δy is the distance travelled along the incline.
Since the final velocity of the crate is zero and the distance travelled along incline is given by −dsinθ, the above equation is reduced to
−12mv2i−mgdsinθ+ΔEint=0 (I)
Here, d is the distance travelled along the incline.
Rearrange the above equation.
ΔEint=12mv2i+mgdsinθ (II)
Conclusion:
Substitute 100 kg for m, 2 m/s for vi, 9.8 m/s2 for g, 1.5 m for d, and 30° for θ in equation (I), to find ΔEint.
ΔEint=12(100 kg)(2 m/s)2−(100 kg)(9.8 m/s2)(1.5 m)sin30°=935 J
Therefore, the amount of energy dissipated by friction is 935 J_.
(b)
The coefficient of the sliding friction.
(b)

Answer to Problem 100P
The coefficient of the sliding friction is 0.73_.
Explanation of Solution
Write the expression for net force acting along y direction.
∑Fy=0
Apply the above condition in Figure 1.
N−mgcosθ=0N=mgcosθ (III)
Write the expression for internal energy dissipated by friction.
ΔEfric=μkNd
Here, μk is the coefficient of kinetic friction, N is the normal force, and d is the distance travelled along the incline.
Use the above equation in equation (III).
ΔEfric=μkmgcosθd
Rearrange the above equation to find μk.
μk=ΔEfricmgdcosθ (IV)
Conclusion:
Substitute 935 J for ΔEint, 100 kg for m, 9.8 m/s2 for g, 1.5 m for d, and 30° for θ in equation (IV), to find μk.
μk=935 J(100 kg)(9.8 m/s2)(1.5 m)cos30°=0.73
Therefore, the coefficient of the sliding friction is 0.73_.
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