A moving sidewalk has a velocity of 1.7 m/s north. If a man walks south on the sidewalk at a speed of 1.1 m/s, how long does it take him to travel 15 m north in relation to a stationary observer?

When you drop a pebble from height a it reaches the ground with speed v if there is no air resistance. from what height should you drop it so it will reach the ground with three times the speed? (a) 9m (b) 3m (c) 6m (d) 18m

One day, after pulling down your window shade, you notice that sunlight is passing through a pinhole in the shade and making a small patch of light on the far wall. you see that the patch of light seems to be a circular diffraction pattern. it appears that the central maximum is about 1 cm across and you estimate that the distance from the window shade to the wall is about 3 m. what is (a) the average wavelength of sunlight? (b) the diameter of the pinhole?

If the chemical properties of a substance remain unchanged and appearance or shape of a substance changes it is called a ?