Find some physics problems in the first volume of the second day of junior high school

1. A man is standing in the valley. He gave a cry. 1S, he heard an echo, which passed 0.5.

S heard the echo again and asked how wide the valley was.

2. The car drove to the tunnel at the foot of the mountain at a uniform speed, honked its horn at a distance of1750m, and heard an echo after10s. What is the speed of the car? How many meters away from the hole is the echo?

1) Let the distances between people and both sides of the valley be x and y respectively, and the time from shouting to hearing the echo =2* distance/speed of sound.

Because the speed of sound = 340m/s.

So 2x/340= 1, x= 170m.

2y/340=( 1+0.5)= 1.5，y=255m

Valley width = x+y =170+255 = 425m.

2) Let the vehicle speed be V, t= 10s, and the forward distance of the vehicle be S 1= 10V.

The sound propagation distance is s =1750+(1750-10v) = 3500-10v.

And S= speed of sound * time =340* 10=3400.

So 3500- 10V=3400.

Vehicle speed v = (3500-3400)/10 =10m/s

The distance from the echo to the hole is s' =1750-10v =1750-10 *10 =1650m.

33. A large transport vehicle, with a length of 40m and a mass of 30t, takes 30s to pass through a cave with a length of 260m at a uniform speed, and 24s to pass through a bridge at the same speed. Find the length of the bridge.

Xiaoming and Xiaogang both study in the same middle school, which is 2400 meters away from home. After school, they both went home from school along the same road at the same time. Xiaoming rides a bike at a certain speed, Xiaogang walks at a speed of 2m/s, and Xiaoming waits 10min when he gets home. Ask Xiao Ming what the speed of cycling is.