Motion
vocabulary: position, distance, motion, speed
A moving object changes its position. Position is the location of an object. Everything has a position. Your nose has a position in the middle of your face. A race car has a position at the starting line. Some positions change. A race car moves from the starting line to the finish line. Other positions don’t change. Your nose stays in the middle of your face.
When an object changes position it is in motion. Motion is a change of position. Some motions are hard to see. A kicked ball may look like a blur. A snail moves so slowly that it doesn’t seem to be moving.
Some things move faster than others. Things that move faster have a greater speed. Speed is how fast an object moves. You can judge an object’s speed by how quickly it changes position. A fast-moving object changes position quickly. A slow-moving object takes longer to change position.
How is speed measured? You must measure two things: time and distance. Fast-moving objects go long distances in a short period of time. Slow-moving objects take longer to travel the same distance.
Forces
vocabulary: force, gravity, weight
When an object that was at rest starts to move, a change in motion occurs. A change in motion also occurs when a moving object speeds up, slows down, changes direction, or stops. A change in an object’s motion is the result of all the forces acting on the object.
Think of what happens when you get off a seesaw. The forces suddenly become unbalanced, and the seesaw moves.
Unbalanced forces also create a change of motion in a tug-of-war. When both sides pull equally, the forces balance, and nothing moves. If one side begins to pull harder, the forces become unbalanced, resulting in a change in the rope’s position.
Think about a ball rolling on the floor. Over time, the ball slows down, so a force must be acting on the ball. The force that slows down the ball is friction. Friction is a force that occurs when one object rubs against another object. In this case, friction occurs when the ball rubs on the floor.
Different materials produce different amounts of friction. Rough materials produce a lot of friction. Most smooth materials produce less friction. However, other materials, such as rubber, are smooth but still produce a lot of friction.
When you ride a bike, friction against the tires slows the bike down even if you are riding on a smooth sidewalk. You have to keep pedaling to keep the bike moving. How do brakes stop a bike’s motion? The brake pad squeezes against the rim of the wheel, causing friction. The wheel slows down, and eventually, the bike stops.
More Motion & Friction
vocabulary: friction
If you want to make a sound, you need to make something move. Try placing your hand to your throat while speaking. You can feel something moving inside. Sounds are made when an object vibrates, or moves back and forth quickly.
When you pluck a guitar string, you might see it vibrate. Other times, it may vibrate so fast that you won’t see it move. When a guitar string vibrates, it makes the air around it vibrate, too. Particles in the air also move back and forth quickly. This allows the sound to move through the air. You hear the sound when the air’s back-and-forth motion reaches your ear. Without air, you would not hear sound. Sound can move through solids, liquids, and gases.
A sound’s pitch is how high or low it is. Length can sometimes cause a change in pitch: shorter strings vibrate faster than longer ones. The faster a string vibrates, the higher the sound’s pitch. By tapping a piano’s keyboard, you can hear very low- or very high-pitched sounds.
Volume is how loud or soft a sound is. Making loud sounds takes more energy than making soft sounds. For example, a whisper uses less energy than a yell.
How are you able to hear sound? A sound takes less than a second to travel across a room to your ear. Then, the sound must pass through your ear to your brain.