
Concept explainers
A 60.0-kg athlete leaps straight up into the air from a trampoline with an initial speed of 9.0 m/s. The goal of this problem is to find the maximum height she attains and her speed at half maximum height. (a) What are the interacting objects and how do they interact? (b) Select the height at which the athlete’s speed is 9.0 m/s as y = 0. What is her kinetic energy at this point? What is the gravitational potential energy associated with the athlete? (c) What is her kinetic energy at maximum height? What is the gravitational potential energy associated with the athlete? (d) Write a general equation for energy conservation in this case and solve for the maximum height. Substitute and obtain a numerical answer. (e) Write the general equation for energy conservation and solve for the velocity at half the maximum height. Substitute and obtain a numerical answer.
(a)

Answer to Problem 22P
Explanation of Solution
When the athlete jumps and when he is in the air, the interacting objects are the athlete and the earth. The gravitational force between the athlete and the earth will be the interaction between the objects.
Thus, the interacting objects are the athlete and the earth and the interaction is gravitational force.
Conclusion:
The interacting objects are the athlete and the earth and the interaction is gravitational force.
(b)

Answer to Problem 22P
Explanation of Solution
Section 1
To determine: The kinetic energy of the athlete at h=0 level.
Answer: the kinetic energy of the athlete at h=0 level is 2.4×103 J.
Explanation:
Given Info:
The speed of the athlete at h=0 level is 9.0 m/s.
The mass of the object is 60.0 kg.
Formula to calculate the kinetic energy of the athlete is,
KEi=12mvi2
- vi is the speed of the athlete at h=0 level
- m is the mass of the athlete.
Substitute 60.0 kg for m and 9.0 m/s for vi to find the kinetic energy of the athlete,
KEi=12(60.0 kg)(9.0 m/s)2=2.4×103 J
Thus, the kinetic energy of the athlete at h=0 level is 2.4×103 J.
Section 2
To determine: The potential energy of the athlete at h=0 level.
Answer: The potential energy of the athlete at h=0 level is zero.
Explanation:
Given Info:
The speed of the athlete at h=0 level is 9.0 m/s.
The mass of the object is 60.0 kg.
Formula to calculate the potential energy of the athlete is,
(PEg)i=mghi
- hi is the height of the level
- m is the mass of the athlete.
Substitute 60.0 kg for m and zero for hi to find the potential energy of the athlete,
(PEg)i=(60.0 kg)(9.8 m/s2)(0)=0
Thus, the potential energy of the athlete at h=0 level is zero.
Conclusion:
The kinetic energy of the athlete at h=0 level is 2.4×103 J and the potential energy of the athlete at h=0 level is zero.
(c)

Answer to Problem 22P
Explanation of Solution
Section 1
To determine: The kinetic energy of the athlete at maximum height.
Answer: the kinetic energy of the athlete in the maximum height is zero.
Explanation:
Given Info:
The mass of the object is 60.0 kg.
Formula to calculate the kinetic energy of the athlete is,
KEf=12mvf2
- vf is the speed of the athlete at the maximum height
- m is the mass of the athlete.
When the athlete reaches its maximum height in the jump, the athlete will be in the rest at the maximum height. Thus, the kinetic energy of the athlete will be zero in the maximum height.
Thus, the kinetic energy of the athlete in the maximum height is zero.
Section 2
To determine: The potential energy of the athlete at maximum height.
Answer: the potential energy of the athlete at maximum height is 2.4×103 J.
Explanation:
Given Info:
The mass of the object is 60.0 kg.
When the athlete reaches the maximum height, according to conservation of energy; the total energy will be converted in the form of gravitational potential energy.
ΔPEg=−ΔKE
Thus,
PEf=PEi+KEi−KEf
- PEi is the initial potential energy
- KEi is the initial kinetic energy
- KEf is the final kinetic energy
Substitute zero for PEi, 2.4×103 J for KEi, and zero for KEf to find the potential energy of the athlete,
PEf=0+2.4×103 J−0=2.4×103 J
Thus, the potential energy of the athlete at maximum height is 2.4×103 J.
Conclusion:
The kinetic energy of the athlete at maximum height is zero and the potential energy of the athlete at maximum height is 2.4×103 J.
(d)

Answer to Problem 22P
Explanation of Solution
Since the athlete’s total energy is conserved,
ΔKE+ΔPE=0
Thus,
KEf+PEf=KEi+PEi
Hence, the general equation for energy conservation is;
12mvf2+12mghf=12mvi2+mghi
Formula to calculate the final height of the athlete is,
hf=mghi+12mvi2−12mvf2mg=hi+(vi2−vf2)2g
Substitute zero for hi, 9.0 m/s for vi , zero for vf and 9.8 m/s2 for g to find the final height,
hf=0+(9.0 m/s)2−02(9.8 m/s2)=4.1 m
Thus, the maximum height of the jump of the athlete is 4.1 m.
Conclusion:
The maximum height of the jump of the athlete is 4.1 m.
(e)

Answer to Problem 22P
Explanation of Solution
Since the athlete’s total energy is conserved,
ΔKE+ΔPE=0
Thus,
KEf+PEf=KEi+PEi
Hence, the general equation for energy conservation is;
12mvf2+12mghf=12mvi2+mghi
Formula to calculate the velocity at half the maximum height is,
vf2=2m(12mvi2+mghi+−mghf)vf=√vi2+2g(hi−hf)
Substitute zero for hi, 9.0 m/s for vi , hmax/2 for hf and 9.8 m/s2 for g to find the final speed,
vf=√(9.0 m/s)2+2(9.8 m/s2)(0−4.1 m2)=6.4 m/s
Thus, the velocity at half the maximum height of the athlete is 6.4 m/s.
Conclusion:
The velocity at half the maximum height of the athlete is 6.4 m/s
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