The Story of Motion Back

Kindergarten to 2nd Grade

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The story of motion starts out with position. Where is something exactly- near or far, in front of or behind, under or on top of? There are many ways we can describe position.

So what does position have to do with motion? Motion tells us how the position of something is changing over time. Where is it going- toward or away, up or down? There are many ways we can describe motion. Did something move in a straight line or zig-zag; did it go around in a circle or move back and forth; did it go fast or slow? Position Project

It’s important to think about exactly how something moves so we can figure out why it’s moving that way. How do you make something move? You push or pull it! Pushes and pulls are forces; forces start, stop and change motion. For example, when we kick a ball we are pushing the ball away from us. During a tug-of-war two teams are pulling on a rope, each trying to win by pulling the other team toward them. Kicking and tugging are two examples of forces. Boat Project

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Whenever you pull or push something it’s always in a specific direction. Direction is an important part of describing motion and forces. When you kick a ball you push it away from you. In our tug-of-war, each team is pulling the other toward the center. Pushing always move things away from the push and pulling always move things toward the pull. All forces act in a specific direction.

Think about the tug-of-war. Both teams are pulling, but the winning team has to pull harder. So all pushes or pulls are not created equal. The harder you push or pull, the more you change the motion. Softer pushes and pulls generate less force and change motion less.

There are specific patterns of motion including rolling, sliding, swinging and spinning. If we understand these patterns we can learn a lot about motion. Balls roll around and around, swings move backward and forward, baseball players slide along the ground to the bases and dancers spin around and around. Every pattern of motion occurs because of forces and the direction that they act. Paper Rockets Project

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Let’s recap: Motion is a change in position, so we need to understand position to understand motion. Motion occurs because of forces or pushes and pulls. The harder we push or pull something the more we change its motion. Pushes and pulls are always in a direction, understanding the direction of the push or pull tells us where something will move.

3rd to 5th Grade

At any moment there can be many forces – pushes and pulls – acting on something. For example, when you ride your bike, the bike pushes you forward, and the wind pushes you backward. Those are both forces.

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Forces cause motion to change. In the example with your bike, the bike pushes you forward more than the wind pushes you back, and so you start moving forward. As long as the bike pushes forward harder than the wind pushes back, you’ll keep moving forward faster and faster.

Of course, when you move faster the wind in your face pushes back harder. At some point the wind pushes back on you just as hard as the bike pushes you forward. The two pushes – or forces – are exactly the same but act in opposite directions. They cancel each other.

When forces cancel each other we say that they are balanced. Balance is an important part of understanding how forces change motion. When forces are balanced things just keep doing whatever they were doing. If they were moving they keep going the same way, and if they are stopped they stay still. On your bike you keep going forward at the same speed – doing what you were doing.

But when there is an unbalanced force it will change what something is doing; it changes motion. When forces are unbalanced things happen! Things like starting, stopping, or changing direction.

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Another example of a force is a kick. Kicks push objects. When you kick a ball there is no opposite force to balance it, so the ball moves away from you. What happens if your friend kicks the ball on the other side at the same time as you? If the kicks are equal, the ball won’t go anywhere. The forces cancel each other. They are balanced so the ball keeps doing what it was doing – sitting there. But if one kick is harder than the other the forces don’t exactly cancel. They are not balanced and the ball starts to move in the direction of the harder kick.

Two forces commonly at work are gravity and friction. Gravity is the force that pulls somethings toward the Earth. Friction is a force that happens when two things rub together, friction is a force that works against motion.

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Gravity is constantly pulling us toward the center of the earth. Whatever else is going on, Earth gravity is always pulling them down. If you are sitting on a chair gravity is pulling you toward the earth and the chair is pushing you up, balancing the force of gravity. Since the forces are balanced, your motion doesn’t change. You just sit there. But if the chair is yanked away, you will fall to the ground because there is no more force pushing you up. The force of gravity is unbalanced and you fall down to the ground because when forces are unbalanced things happen.

Things will keep doing whatever they were doing until there is an unbalanced force to change what things are doing. This means that if a ball is rolling along the floor it should roll forever. But we know eventually the ball stops. That’s because there’s a force slowing it down - the force of friction. Friction is why you have to keep pushing on something to keep it moving; you have to balance the friction trying to make it stop. If it weren’t for friction, things that moved would want to keep on moving – like a skater or a hockey puck moving across the ice.

Friction happens whenever two objects are touching – or are in contact – with each other and one moves against the other. It creates a force that is opposite to the direction of the motion. When a ball is rolling, friction acts opposite to its roll. If there are no other forces to keep it going, the unbalanced friction stops the ball.

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All of this talk about how something keeps doing what it was doing when the forces acting on it are balanced is called Newton’s first law of motion. It describes inertia. Inertia is the tendency of something to resist changing its motion. The more inertia something has, the harder it is to change its motion.

It turns out that the inertia of something is the same as its mass, or how much of it there is. Mass refers to the amount of stuff packed into something. Which would be worse – standing in front of a running elephant or standing in front of a running mouse? An elephant has a lot more mass than a mouse, so it’s a lot harder to stop (change the motion of) an elephant than to stop (change the motion of) a mouse. More massive things really want to keep doing whatever they were doing! A bowling ball is much harder to get rolling then a ping-pong ball, it’s a lot harder to make it turn, and it’s a lot harder to stop it. All those are changes in the motion, and they’re harder for the bowling ball because it has so much more mass and inertia. Boat Project

North Carolina Science Standards

Kindergarten K.P.1 Understand the positions and motions of objects and organisms observed in the environment. EX.K.P.1 Identify positions and motions of familiar objects in the environment.
First Grade 1.P.1 Understand how forces (pushes or pulls) affect the motion of an object. EX.1.P.1 Understand how pushes or pulls change the motion of an object.
Second Grade 2.P.1 Understand the relationship between sound and vibrating objects. EX.2.P.1 Understand that vibrations create motion.
Third Grade 3.P.1 Understand motion and factors that affect motion. EX.3.P.1 Understand the factors that affect motion.
Forth Grade 4.P.1 Explain how various forces affect the motion of an object. EX.4.P.1 Understand how force affects the motion of an object.
Fifth Grade 5.P.1 Understand force, motion and the relationship between them. EX.5.P.1 Understand how force can change motion of objects.