Question

a train passes an electric post in 10 second and a bridge of length 2 km in 110 seconds. the speed of the engine is?

Answer 1

Answer on Question #42627 – Physics - Mechanics | Kinematics | Dynamics

a train passes an electric post in 10 second and a bridge of length $2\mathrm{km}$ in 110 seconds. the speed of the engine is?

Solution:

$\mathrm{d} = 2000\mathrm{m}$ – length of the bridge;

L – length of the train;

$\mathbf{v}$ – speed of the train;

$t_1 = 10s;\quad t_2 = 110s;$

Train passes an electric post in time $t_1$:

$$\frac {\mathrm {L}}{\mathrm {v}} = \mathrm {t} _ {1} \Rightarrow \mathrm {L} = \mathrm {v t} _ {1}$$

Train passes a bridge in time $t_2$:

$$\frac {\mathrm {L} + \mathrm {d}}{\mathrm {v}} = \mathrm {t} _ {2}$$

(1) in (2):

$$\frac {\mathrm {v t} _ {1} + \mathrm {d}}{\mathrm {v}} = \mathrm {t} _ {2}$$ $$v t _ {1} + d = v t _ {2}$$ $$v = \frac {d}{t _ {2} - t _ {1}} = \frac {2 0 0 0 m}{1 1 0 s - 1 0 s} = 2 0 \frac {m}{s} = 7 2 \frac {k m}{h}$$

Answer: speed of the engine is $72\frac{\mathrm{km}}{\mathrm{h}}$.

http://www.AssignmentExpert.com/