## general motion mechanics

1. The problem statement, all variables and given/known data
The resultant force acting on a train of mass m starting from rest on a level track is a constant P for speeds less than V . For speeds greater than V the power exerted by the resultant force has a constant value PV . Find the time taken to reach a speed 2V from rest, and the corresponding distance travelled.

2. Relevant equations

Force= power * velocity
Power= dw/dt

3. The attempt at a solution

for some reason, i dont know how to go about doing it.
 PhysOrg.com science news on PhysOrg.com >> King Richard III found in 'untidy lozenge-shaped grave'>> Google Drive sports new view and scan enhancements>> Researcher admits mistakes in stem cell study

Recognitions:
Homework Help
 Quote by bernvall 1. The problem statement, all variables and given/known data The resultant force acting on a train of mass m starting from rest on a level track is a constant P for speeds less than V . For speeds greater than V the power exerted by the resultant force has a constant value PV . Find the time taken to reach a speed 2V from rest, and the corresponding distance travelled. 2. Relevant equations Force= power * velocity Power= dw/dt 3. The attempt at a solution for some reason, i dont know how to go about doing it.
Since W = Fs, Power = dW/dt = Force * ds/dt = Force * velocity

What does the resultant force do to the train? (hint: Newton's 2nd law). That should allow you to determine time and distance in reaching V in terms of P and m (mass of the train). Does anything change when it hits speed V?

AM
 Yes, i realised that i needed to use Newtons 2nd Law. What i did was split it up in two, from 0 to V and then from V to 2V. the first part i managed to find the time and distance easily using equations of motion. However for the second part i didn't know what the resultant force was. It gives you the power at 2V NOT the resultant, so i got stuck again.

Recognitions:
Homework Help

## general motion mechanics

 Quote by bernvall Yes, i realised that i needed to use Newtons 2nd Law. What i did was split it up in two, from 0 to V and then from V to 2V. the first part i managed to find the time and distance easily using equations of motion. However for the second part i didn't know what the resultant force was. It gives you the power at 2V NOT the resultant, so i got stuck again.
Let speed = v. Since Power = PV = Fv for v>V, then what is the force F?

What is the kinetic energy at speed 2V? What is the relationship between change in kinetic energy, power and time where the power is constant? That should give you the time. And from that you should be able to work out the distance.

AM

 Tags mechanics, motion, power