關於物理的簡單問題

1. For this type of problem involving plane mirrors, it is always wise to draw out the position of image, so that the reflected light ray becomes a straight line. Then use similar triangle to find the unknow.

Just take the knee of Peter a a point, its first image formed by the "floor mirror" is 50 cm below the floor and 1 m in front of the wall. Mark this first image point on a sketch diagram.

The final image formed by the "wall mirror" of the first image is located 1 m behind the wall and 50 cm below the floor. Mark this second image point.

Join the second image point to the eye of Peter. Complete a right angle triangle by joining the eye of Peter with the first image point, the first image point and the second image point.
Hence, by similar triangle,
(h+50)/1 = (150+50)/2
solve for h gives h = 50 cm


2. Magnification = image size/object size = 5/1 = 5
But magnification = image distance/object distance
hence, image distance = 5 x (object distance) = 5 x 5 cm = 25 cm
Distance between image and object = (25 - 5) cm = 20 cm

3. Tine taken for the stone to reach the water, using the equation of motion s = ut + (1/2)a.t^2, with u = 0 m/s, a = g(=-10 m/s2), s = - 15 m, is
-15 = (1/2)(-10).t^2
i.e. t = 1.732 s

Hence, time taken for sound to travel from water to top of well
= (1.77 - 1.732) s = 0.038 s
Speed of sound = 15/0.038 m/s = 395 m/s