Calculating the time needed until a known velocity for a vehicle with friction as a function of velocity

Click For Summary

Homework Help Overview

The discussion revolves around calculating the time required for a vehicle to reach a known velocity while considering friction as a function of velocity. The problem involves dynamics and the effects of friction on motion, specifically focusing on the integration of a differential equation derived from forces acting on the vehicle.

Discussion Character

  • Exploratory, Mathematical reasoning, Assumption checking

Approaches and Questions Raised

  • Participants discuss the setup of the problem, including the forces acting on the vehicle and the form of the friction function. There are inquiries about alternative methods to solve the integral without using partial fractions, and some participants express confusion about the complexity of the problem relative to its context.

Discussion Status

The discussion is ongoing, with participants exploring different methods to approach the integral. Some guidance has been offered regarding the method of partial fractions, but there is no consensus on the best approach to solve the integral without it. The original poster seeks alternative solutions while others question the assumptions made in the problem setup.

Contextual Notes

There is a mention of potential discrepancies in the physical assumptions regarding friction and vehicle dynamics, particularly the nature of tire-asphalt interaction. The problem is noted to have varying levels of difficulty compared to typical high school questions.

Patrick Herp
Messages
5
Reaction score
0
Homework Statement
A truck with a mass of 1 ton is travelling at a constant speed of 36 km/h. It is known that the total friction force of all wheels with the asphalt can be expressed by the function ##f(v) = 70v + 6v^2##, where v is measured in m/s.
a. Determine the power output of the engine at that moment!
b. The maximum speed of the truck is 108 km/h. Determine the maximum power of the truck's engine!
c. If the truck is initially at rest and the accelerator pedal is pressed as hard as possible, determine the time it takes for the truck to reach half its maximum speed!
Relevant Equations
$$\sum{F} = ma$$
The solutions for (a) and (b) are pretty straightforward, which I got 13 kW and 225 kW each, but when I try to solve for (c), I get stuck with this:
$$
\begin{align}
a &= \frac{F}{m} \nonumber\\
&= \frac{F_\text{max}-f(v)}{m} \nonumber\\
&= \frac{7(30)+6(30)^2 -70v-6v^2}{1.000} \nonumber\\
\frac{dv}{dt} &= \frac{7.500-70v-6v^2}{1.000} \nonumber\\
\frac{dv}{7.500-70v-6v^2} &= \frac{dt}{1000} \implies t = 1.000\int_0^{15} \frac{dv}{7.500-70v-6v^2} \nonumber
\end{align}
$$
Is there any way I could solve for (c) without directly solving that integral?
 
Last edited:
Physics news on Phys.org
Why don't you want to solve the integral? Have you not seen the method of partial fractions?
 
Patrick Herp said:
Homework Statement: A truck with a mass of 1 ton is travelling at a constant speed of 36 km/h. It is known that the total friction force of all wheels with the asphalt can be expressed by the function ##f(v) = 70v + 6v^2##, where v is measured in m/s.
That looks more like air resistance. The point of having wheels is that they roll and avoid friction. Also, tires are generally made of high-friction rubber, which would be absurd if tire-asphalt friction were constantly acting against a vehicle's motion.
 
  • Agree
Likes   Reactions: kuruman
kuruman said:
Why don't you want to solve the integral? Have you not seen the method of partial fractions?
For more clarification, I don't really "know" where this question comes from, I just found it randomly on the internet, among other questions that I think belong to high school questions, so I find it really weird that this particular part of the question suddenly jumps in difficulty.
I did get the partial fraction of the integral to this:
$$
\begin{align}
\frac{t}{1.000} &= \int_0^{15} \frac{dv}{7.500 -70v -6v^2} \nonumber \\
&= -\frac{1}{6} \int_0^{15} \frac{dv}{\left( v-30 \right)\left( v+\frac{125}{3} \right)} \nonumber \\
&= -\frac{1}{6}\cdot \frac{3}{215} \int_0^{15} \left( \frac{1}{v-30} - \frac{1}{v+\frac{125}{3}} \right) dv \nonumber \\
\frac{t}{1.000} &= -\frac{1}{430} \left[ \ln{ \frac{v-30}{v+\frac{125}{3}} } \right]_0^{15} \nonumber \\
t &\approx 2.33\text{ s} \nonumber
\end{align}
$$

Basically, I just want to know if there is any way to solve that part without partial fraction or integral of 1/x
 
Last edited:

Similar threads

  • · Replies 27 ·
Replies
27
Views
1K
Replies
6
Views
2K
Replies
6
Views
1K
  • · Replies 6 ·
Replies
6
Views
3K
  • · Replies 5 ·
Replies
5
Views
2K
  • · Replies 15 ·
Replies
15
Views
2K
  • · Replies 0 ·
Replies
0
Views
2K
  • · Replies 2 ·
Replies
2
Views
2K
  • · Replies 10 ·
Replies
10
Views
2K
  • · Replies 3 ·
Replies
3
Views
3K