Problem with the power of car

AI Thread Summary
The discussion focuses on the performance parameters of an electric car, specifically its acceleration, speed, and power dynamics. After reaching the rated power at 72 km/h, the car's acceleration decreases due to the balance between the driving force and the constant resistance. Despite this, the car continues to accelerate because the motor maintains power output, allowing it to overcome resistance. The maximum speed at rated power is determined by the relationship between thrust and resistance. Understanding these dynamics is crucial for analyzing electric vehicle performance.
rickyw2777
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Homework Statement
Some of the power performance parameters of an electric car are shown in Table 7-1. If the car starts from rest and moves along a straight road with a uniform acceleration of a = 1.5 m/s2, when the speed reaches v1 = 72 km/h, the motor works at the rated power, and the power of the motor remains unchanged. Assume that the resistance of the car during driving is constant.

(1) Analyze and explain the changes in the acceleration and speed of the car after the motor reaches the rated power.

(2) What is the maximum speed of the car when the motor is working at the rated power?

(3) After the car is fully charged, how long does it take to move in the above manner?
Relevant Equations
P=Fv P=W/t
Some of the power performance parameters of an electric car are shown in Table 7-1. If the car starts from rest and moves along a straight road with a uniform acceleration of a = 1.5 m/s2, when the speed reaches v1 = 72 km/h, the motor works at the rated power, and the power of the motor remains unchanged. Assume that the resistance of the car during driving is constant.
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(1) Analyze and explain the changes in the acceleration and speed of the car after the motor reaches the rated power.

(2) What is the maximum speed of the car when the motor is working at the rated power?

(3) After the car is fully charged, how long does it take to move in the above manner?

I don't quite understand why the driving force decreases after the car reaches the rated power? Why does the car still accelerate after reaching the rated power? Why does the resistance equal the power when the car maintains the rated power?
 
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rickyw2777 said:
I don't quite understand why the driving force decreases after the car reaches the rated power? Why does the car still accelerate after reaching the rated power? Why does the resistance equal the power when the car maintains the rated power?
For homework problems, we require some effort on your part. Please read the forum rules.
 
The questions seem half baked…but give them a try so we can see what you know.
 
Start by thinking through what is happening before it reaches ##v_1##.
If the resistance is ##F_r##, what is the thrust from the engine during that time?
What, therefore, is the thrust when it reaches ##v_1##? How does that relate to the rated power?
 
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