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May 5, 2016

# Homework Help: physics

Posted by kelly on Sunday, April 8, 2012 at 7:10pm.

An automobile traveling 95 overtakes a 1.10--long train traveling in the same direction on a track parallel to the road.

Q1: If the train's speed is 75 , how long does it take the car to pass it

Q2:How far will the car have traveled in this time?

Q3:What is the time if the car and train are traveling in opposite directions?

Q4:How far will the car have traveled if the car and train are traveling in opposite directions
• physics - bobpursley, Sunday, April 8, 2012 at 7:21pm

relative speed:20mph distance 1.1

time=distance/speed=1.1/20 hrs

• physics - kelly, Sunday, April 8, 2012 at 7:31pm

i got answers to some of the questions
Q1: 3.3 mins
Q2: ?km <- still need help with
Q3:23.3
Q4:?km still need help with

• physics - Elena, Monday, April 9, 2012 at 6:01am

It is easier to solve this problem if we introduce the relative velocity of the car (relative to the train). The velocity of the car relative to the train is (95-75) km/h=20 km/h.

In the relative description the train is not moving and the car is moving with constant speed 20 km/h.
a. The time the car needs to pass the train is
t = length of the train/relative speed = 1.1 km/20 (km/h) = 0.055 h =198 s.
b. To find the actual traveled distance of the car we just need to multiply the traveled time (0.055 h) by the actual speed of the car:
L = 95•0.055 = 5.225 km
c. The relative speed is v1= 95+75 =170 km/h
The time the car needs to pass the train in this case is
t1= length of the train/relative speed v1= 1.1 km/170 (km/h)=
= 0.00647h =23.3 s.
d. To find the actual traveled distance of the car we just need to multiply the traveled time (0.00647 h) by the actual speed of the car:
L = 95•0.00647 = 0.615 km.