
Concept explainers
A roller-coaster car of mass 1.50 × 103 kg is initially at the top of a rise at point . It then moves 35.0 m at an angle of 50.0° below the horizontal to a lower point
. (a) Find both the potential energy of the system when the car is at points
and
and the change in potential energy as the car moves from point
to point
, assuming y = 0 at point
. (b) Repeat part (a), this time choosing y = 0 at point
, which is another 15.0 m down the same slope from point
.
(a)

Answer to Problem 63AP
The potential energy of the system on point B is zero.
The change in potential energy of the car when it moves from A to B is −3.94×105 J when y=0 at point B.
Explanation of Solution
Section1:
To determine: The potential energy of the system at point A.
Answer: the potential energy of the system at point A is 3.94×105 J.
Explanation:
Given Info:
The mass of the roller-coaster car is 1.50×103 kg.
The distance that the car travelled between point A and B is 35.0 m.
The path is at an angle 50.0° below the horizontal to the lower point B.
Since y=0 is considered at point B,
yB=0
yA=lsinθ (I)
- yB is the height to the point B from the point y=0
- yA is the height to the point A from the point y=0
- l is the length that the car moves
- θ is the angle of the path of the car to the horizontal
Formula to calculate the potential energy of the system at point A is,
PEA=mgyA(II)
- m is mass of the car
- g is acceleration due to gravity
Substitute equation (I) in (II),
PEA=mg(lsinθ)
Substitute 1.50×103 kg for m, 9.8 m/s2 for g, 35.0 m for l and 50.0° for θ to find the potential energy of the system at point A,
PEA=(1.50×103 kg)(9.8 m/s2)((35.0 m)sin50.0°)=3.94×105 J
Thus, the potential energy of the system at point A is 3.94×105 J.
Section 2:
To determine: The potential energy of the system on point B
Answer: The potential energy of the system on point B is zero.
Explanation:
Since y=0 is considered at point B, the potential energy of the system at point B is zero.
Conclusion:
From section1,
The potential energy of the system at point A is 3.94×105 J.
From section2,
The potential energy of the system on point B is zero.
Thus,
The change in potential energy when the system travels from A to B is,
ΔPEA→B=PEB−PEA
Substitute zero for PEB and 3.94×105 J for PEA to find the change in kinetic energy,
ΔPEA→B=0−3.94×105 J=−3.9×105 J
Therefore, the change in potential energy of the car when it moves from A to B is −3.9×105 J when y=0 at point B.
(b)

Answer to Problem 63AP
The potential energy of the system on point B is 1.69×105 J.
The change in potential energy of the car when it moves from A to B is −3.94×105 J when y=0 at point C which is 15.0 m down the slope from point B.
Explanation of Solution
Section1:
To determine: The potential energy of the system at point A.
Answer: the potential energy of the system at point A is 5.63×105 J.
Explanation:
Given Info:
The mass of the roller-coaster car is 1.50×103 kg.
The distance that the car travelled between point A and C is 50.0 m.
The path is at an angle 50.0° below the horizontal to the lower point B.
Since y=0 is considered at point C,
yB=lsinθ (I)
yA=lsinθ (II)
- yB is the height to the point B from the point y=0
- yA is the height to the point A from the point y=0
- l is the length that the car moves
- θ is the angle of the path of the car to the horizontal
Formula to calculate the potential energy of the system at point A is,
PEA=mgyA(II)
- m is mass of the car
- g is acceleration due to gravity
Substitute equation (I) in (II),
PEA=mg(lsinθ)
Substitute 1.50×103 kg for m, 9.8 m/s2 for g, 50.0 m for l and 50.0° for θ to find the potential energy of the system at point A,
PEA=(1.50×103 kg)(9.8 m/s2)((50.0 m)sin50.0°)=5.63×105 J
Thus, the potential energy of the system at point A is 5.63×105 J.
Section2:
To determine: The potential energy of the system at point B.
Answer: the potential energy of the system at point A is 1.69×105 J.
Explanation:
Given Info:
The mass of the roller-coaster car is 1.50×103 kg.
The distance that the car travelled between point B and C is 15.0 m.
The path is at an angle 50.0° below the horizontal to the lower point B.
Since y=0 is considered at point C,
yB=lsinθ (I)
yA=lsinθ (II)
- yB is the height to the point B from the point y=0
- yA is the height to the point A from the point y=0
- l is the length that the car moves
- θ is the angle of the path of the car to the horizontal
Formula to calculate the potential energy of the system at point B is,
PEB=mgyB(II)
- m is mass of the car
- g is acceleration due to gravity
Substitute equation (I) in (II),
PEB=mg(lsinθ)
Substitute 1.50×103 kg for m, 9.8 m/s2 for g, 15.0 m for l and 50.0° for θ to find the potential energy of the system at point A,
PEB=(1.50×103 kg)(9.8 m/s2)((15.0 m)sin50.0°)=1.69×105 J
Thus, the potential energy of the system at point B is 1.69×105 J.
Conclusion:
From section1,
The potential energy of the system at point A is 5.63×105 J.
From section2,
The potential energy of the system on point B is 1.69×105 J.
Thus,
The change in potential energy when the system travels from A to B is,
ΔPEA→B=PEB−PEA
Substitute 1.69×105 J for PEB and 5.63×105 J for PEA to find the change in kinetic energy,
ΔPEA→B=(1.69×105 J)−(5.63×105 J)=−3.94×105 J
Therefore, the change in potential energy of the car when it moves from A to B is −3.94×105 J when y=0 at point C which is 15.0 m down the slope from point B.
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