What would happen if you threw a ball in the air? Of course, it would go up, and then it would quickly come back down. But why does it come back down? Because of Earth’s gravity.

What would happen if you threw the same ball up while you were in outer space? It would just keep going up, up, up. It wouldn’t come back down because there is no gravity in outer space.

What would happen if you rolled a ball along the floor in your classroom? It would eventually stop because of friction (the rubbing of a surface or object against another).

What would happen if you rolled the same ball in outer space? It would just keep going and going like the Energizer Bunny because there is no gravity or friction in outer space to stop it.

Isaac Newton came up with an idea to explain all of this. He said that an object will continue moving unless some force acts on it. In other words, if you kick a ball in outer space from a spacecraft, it will keep going until something hits it (like a comet) or acts upon it (like gravity from another planet or the Sun). This works on Earth, too. A ball thrown in the air would keep going up, but the Earth’s gravity acts upon it, pulling it back down. A ball rolled along the floor doesn’t keep going along because friction between the ball and the floor acts upon it, stopping it. This is Newton’s First Law of Motion.

Have you ever kicked a ball forward and it went backwards? Of course not! The ball travels in the direction you kick it.

Have you ever thrown a ball up and it went down instead of up? Of course not! The ball travels up because that is the direction you threw it. The ball goes up because your throw was a force sending the ball up. It came back down because Earth’s gravity pulled it down.

Newton came up with a second law to explain all of this. It says that if a force acts on an object, the object moves in the direction of that force. His second law also says that the speed of an object and the distance it travels are related to the strength of the force and the mass of the object.

Wow, that’s tough to understand! Here’s an example to help explain it. Say you’re kicking a ball. The strength of the force means how hard you kick the ball. If you kick the ball hard, it will go further than it would if you kick it softly. The mass (sort of like the weight, but I don’t want to confuse you!) of the ball also plays a part in how far it will go. Kick a small, light ball and it goes faster and further than if you kick a big, heavy ball. If you kick a bowling ball it will not go as far as if you kick a soccer ball the exact same way. You can make a bowling ball and a soccer ball roll the same distance if you kick the bowling ball hard (with a lot of force) and the soccer ball softly (with only a little force). Newton realized that both the mass (remember, it’s kind of like the weight) of the ball and the strength of the force acting on it influences how far and fast the ball will move.

Newton also noticed that for every action there is an equal and opposite reaction. Sounds scientific! Let’s look at an example. If you lean against a wall with your hands and push on the wall lightly, your body pushes away from the wall softly. If you push on the wall hard, your body pushes away from the wall harder. This is what Newton meant when he said the reaction is equal.

So what was Newton talking about when he said there was an opposite reaction? Again, let’s look at an example. If you’re floating in the water and you push against the pool wall, your push makes you go in the opposite direction, away from the wall. Another example is if you’re floating on a raft and you want to go the direction your head is pointing you put your hands in the water and push your hands towards your feet. This makes you and the raft go in the direction your head is pointing. Your hands pushing against the water in one direction is an action and you and the raft moving in the other direction is an opposite reaction.