But the mass of the bowling ball would never change. The bowling ball did not change size or shape- the only thing that changed was the amount of gravity that pulls on it. If you take that bowling ball to the moon, it would have a weight of only 2 pounds because their is less gravity on the moon than on earth. The difference between mass and weight is that mass is a measurement without the effects of gravity.įor example, letʻs say you have a bowling ball with a weight of 12 pounds. We could say “the hedgehog has 1 pound of mass.” Or we might say “the hedgehog has 2 pounds of mass after it has eaten 800 jelly beans.” When we talk about mass we use measurements of weight like pounds, ounces and grams. When I think of mass I think of the word “massive,” like “that is a massive elephant.” Mass is related to size and weight, but it isnʻt either one. When we learned the first law of motion we talked about an “outside force.” That outside force is something that does something to an object- either pushing it, or pulling it. But in order to understand Newtonʻs second law, we have to understand what those letter mean! Newtonʻs second law is an equation! When we remember his second law, we can just remember the letters and an equal sign.
But did you know that the ball would never stop if it was in outer space? As long as the ball never ran into anything, the ball would keep going in the same direction, at the same speed, forever. Do you know a force that would make the ball change speed and direction? If you guessed gravity, you would be right! Gravity is a force that is always at work on planet earth. Why is that? Newtonʻs first law tells us that some kind of force must be making the ball change direction and speed.
If you kicked a ball straight up, this law says it would keep going straight up and never stop, right? Wait- thatʻs not what a ball does! It goes up, but then slows down, and falls back down. If someone else kicks the ball, the ball hits something, or the ground is uneven, these things would affect the direction and speed of the ball. Once the ball begins to move the ball would go one direction, and continue going that way, until something makes it change direction. It will remain “at rest.” A little push, kick, or some other force, and the ball will move. Even though it is round, and easily rolls, it isnʻt going to move on its own.
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