Homework Help: Physics Kinematics Problem

1. Nov 29, 2004

albert611

Hi, I attempted this problem, but I do not see how it makes sense. Could somebody give me an idea of how to solve this problem? Thanks!

While traveling along at 24.3 m/s, a 13.7 kg car runs out of gas 16 km from a service station. Neglecting friction, if the station is on a level 14.8 m above the elevation of the car, how fast will the car be going when it rolls into the station?

The correct answer should be 17.33 m/s BTW.

Thank you very much for your help!

-Albert

2. Nov 29, 2004

Zlex

Well, lets see.

Find the component of gravity that is slowing down the car. That'll be your negative acceleration. You have all the other information. Use the appropriate kinematic equation, plug and chug.

3. Nov 30, 2004

aviv87

Also, if you know how to calculate potential gravitational energy and kinetic energy you could use the energy conservation law.

4. Nov 30, 2004

HallsofIvy

In other words:
1) find the kinetic energy of the car using Ek= (1/2)mv2.
2) find the increase in potential energy as the car moves up to the gas station using
Ep= mgh.
3) find the kinetic energy of the car at the gas station assuming conservation of energy
4) find the speed of the car from the kinetic energy using, again Ek= (1/2)mv2.

5. Nov 30, 2004

Tom McCurdy

I would advise setting up the problem as follows
$$KE_i+U+i=KE_f+U_f$$
KE=Kinetic Energy
U=Potential Energy
KE=1/2 m v^2
U=mgy

We can assume the $$U_f$$ will go to zero

$$1/2mv^2 = 1/2mx^2 + mgy$$ solving for x

the m's cancel out

$$1/2 v^2 = 1/2 x^2 + gh$$

solve for x and volla 17.3323

so end the end you did not need to know the cars mass or the 16 miles... go conservation of energy

6. Nov 30, 2004

tony873004

A 13.7 kg car? It'll be blown away by the wind before it gets there!