Homework Help: physics

Posted by srikar on Saturday, May 19, 2012 at 1:38am.

1.A train passes an electric post in 8sec and a bridge of length 1km in 108sec .Find the speed of the engine .

2.A runner travels at the rate o f25km/hr for 4min ;then at 50km/hr for 8minn and finally at 20km/hr for 2min .Find a) the total distance covered in km and b)average speed in m/sec

physics - Elena, Saturday, May 19, 2012 at 5:43pm
1. L is the length of the train.
The speed of the train is v = L/8,
The time of passing the bridge begins when head end of the train enters the bridge and ends when the rear end leaves the bridge, therefore, the speed of the train is
(s+L)/108.
L/8 = (s+L)/108
L/8 – L/108 = s/108.
s =1000 m
L = 69 m.
v = 69/8 = 8.6 m/s.
2. v1 = 25 km/hr = 25000/3600 =6.94 m/s.
s1 =v1•t1 = 6.94•4•60 =1666 m.
v2 = 50 km/hr = 50000/3600 = 13.9 m/s. (actually, it’s impossible since the world record of Usain Bolt is 9.58 s for 100 m. that gives speed 100/9.58 =10.44 m/s !!!!!)
s2 = v2•t2 = 13.9•8•60 =6667 m.
v3 = 20 km/hr =20000/3600 =5.56 m/s.
s3 = v3•t3 = 25.56•2•60 =667 m
Total distance is
s =s1+s2+s3 = 1666 + 6667 +667 =9000 m.
Total time is
t =t1+t2+t3 = 14•60 =840 sec.
v = s/t =9000/840 = 10.7 m/s.
(actually, all this is impossible since the world record of Usain Bolt is 9.58 s for 100 m. That gives the speed 100/9.58 =10.44 m/s !!!!!)

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