Geotropism is the effect of gravity on plants. When a seed is planted and it germinates, the root will extend downward and a stem with leaves grows upward. The seed stores enough energy to sprout a root to obtain nutrients and water, and a stem with leaves collects sunlight. What happens if gravity is reversed -- that is, if you turn a newly germinated seed upside down? Will the stem and roots bend and turn so that the roots grow downward and the stem upward? Germinate several seeds such as radishes in pieces of cheesecloth that you keep moist. After roots and a stem appear, turn them upside down by mounting them on a piece of cardboard, using thumbtacks to secure the plants in cheesecloth to the cardboard. Keep the cheesecloth moist at all times. Talk to the kids about growing plants in a spaceship where there wouldn't be any gravity -- discuss how the seed would know which way to germinate.
Scientists compare the density of rocks by using a test to measure their specific gravity. Specific gravity is the relationship between the weight of an object and the water it displaces. Gather some local rocks and stones and determine their specific gravity. To measure specific gravity, weigh the rocks on a scale. Weigh a small cup. Fill a beaker that has a spout with water until it overflows. When no more water flows out, place the small cup underneath the spout on the beaker. Tie a thin thread around the rock and slowly lower it into the beaker until the rock is completely submerged. The water the rock displaces will flow into the cup. Weigh the cup containing the displaced water and subtract the weight of the cup when it was empty. Divide the weight of the displaced water into the weight of the rock. The result will be the rock's specific gravity.
Earth's gravity pulls equally on all objects. Construct a ramp with a "V"-shaped channel by nailing two 2-by-4, 8-foot long pieces of lumber together. Build a second ramp the same way. Set one end of each ramp up on the first step of a porch or entryway into your house, and lay the other end down on flat sidewalk or asphalt. Set a bowling ball at the top of one of the ramps and a golf ball at the top of the other. Let go of both balls at the same time. The balls will reach the bottom of the ramp at the same time. They will both be traveling the same speed when they exit the ramps because of the constant force of gravity. Though the initial speeds at the time the balls exit the ramps are the same, the mass, or weight, of the two balls is different. This will cause one ball to roll farther before coming to a stop. Experiment to discover which one rolls farther.
Meteors are rock-like chucks of metal and stone that travel through space at tremendous speeds. When they approach a planetary body, such a the moon, the force of gravity from that body attracts the meteors, causing them to slam into it. Because the Earth has an atmosphere, many meteors burn up before hitting the ground. We can see craters on our moon and planets such as Mars from the impact of meteors. Experiment to discover if you can tell the angle and direction that a meteor impacts a heavenly body by examining the crater it made. Fill a cookie tray that has a lip around it with wallboard compound. Stand at an angle to the tray and throw a small rock into the compound, simulating a meteor strike. Carefully remove the rock and examine the "crater" that it made. Examine the shape of the crater and look for ejecta material around one side, as these are clues as to the angle of entry.