Physics Problem; help me please

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Homework Help Overview

The discussion revolves around a physics problem involving a freight train's acceleration under a constant force, specifically examining the relationship between force, mass, and acceleration as described by Newton's laws. The original poster seeks to determine the time required for the train to reach a specified speed from rest, disregarding friction.

Discussion Character

  • Exploratory, Conceptual clarification, Mathematical reasoning, Problem interpretation

Approaches and Questions Raised

  • Participants discuss the application of Newton's second law to derive acceleration and explore kinematic equations to find the time needed to reach the final velocity. There are questions regarding the definition of "constant pull" and the importance of unit conversion from kilometers per hour to meters per second.

Discussion Status

The discussion includes attempts to clarify the problem setup and the necessary calculations. Some participants have provided guidance on using relevant equations, while others express confusion about the calculations and the expected answer, indicating a lack of consensus on the correct approach.

Contextual Notes

Participants note the importance of unit conversion in solving the problem, as the final velocity is given in kilometers per hour, which may affect the calculations for time. There is also mention of a discrepancy between the original poster's calculations and the answer provided in the textbook.

mike98020205
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Help, I just can't seem to get this.

A freight train has a mass of 15000000 kg. If the locomotive can exert a constant pull (what is a constant pull?) of 750000 N, how long would it take to increase the speed of the train from rest to 85 km/h? (Disregard friction.)
 
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Newtons second law: f=ma, a=f/m. You can use this to derive the acceration and use some kinematics formulas to find out how long it will take to reach a certain velocity with a known acceleration and a known initial velocity.
 
Last edited:
F = ma

750000 N = (15000000 kg) * a
 
Well, you have a fairly basic question that comes from Netwon's second law of motion. By constant pull, the system has a constant force that is pulling the train along. This means that the system also has constant acceleration, which is given by Netwon's Laws.

F = M A

You can easily find this acceleration by using the values already given.
You now can use this in a kinematics equation. You have acceleration,initial velocity,final velocity, and you need to find time.

I suggest using this equation:

V(f)=V(i)+at

By the way, make sure you watch your units. The final velocity that the problem gives is in kilometers per hour. Convert this speed to standard units.

Hope this helps
 
Ok... What is the answer? The book says 470 seconds. I did 750000N/15000000 and got .05m/s squared for the acceleration. Then, If I divide the final speed, 85 km/h by this acceleration, I get 1700 seconds which is wrong. How do they get 4.7 times 10 to the second (470 seconds)? Help please. I appreciate it.
 
conversion from m/s to km/h
 
They are looking for time in this problem.
 
yeah, but the conversion from m/s to km/h is essential in arriving at the proper time. You are dividing 85 km/h by m/s. The units do not cancel out.
 

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