Forces and Motion

1. Forces and Motion by, Drew Harris

2. Forces Every time you push or pull something, you use a force, or push or pull. Forces transfer energy. Examples: When you throw a ball, the force of your muscles moves your arm to push the ball into the air. When you pick up a book, the force of your muscles move your arm to pull the book off the desk. There are other, indirect examples, including: the force of a car’s engine turns the car’s wheels.

3. Motions Many of the things around you are motionless. Motion is a change of position. Position is a certain place. Objects stay in place until a force moves them. If something is moving, it means a force moved it. Mass is a large factor in motion. If you push with equal force on a toy car and a wagon filled with books, when you stop pushing the toy car will move faster. That is because the wagon weighs more. Weight is a measure of the force of gravity put on an object.

4. Force vs. Force The force needed to start an object moving also depends on other forces that are acting on the object. What if your coat is fastened closed with Velcro. To open it you would need more force than the Velcro. Once an object is moving, it moves until a force stops it. It’s easy to see where the stopping force comes from when a soccer goalie stops a kicked ball. Sometimes, however, the force is harder to name. You know that even if no player stops the ball it will stop on its own. That is friction. Friction is a force that keeps objects that are touching from sliding past each other quickly.

5. Changing Motion We know it takes a force to start a motion. It also takes a force to change a motion as well. Changes are things like speeding up, slowing down, turning, and stopping. Starting and slowing to a stop are examples of changing speed. Turning is an example of changing direction. Starting, slowing, speeding up, and turning are accelerations, or changes in the speed or direction of motion.

6. Gravity There are many kinds of forces. The force that pulls objects toward Earth is gravity. The larger the mass of an object- the stronger the gravity. Gravity is a force that pulls all objects toward each other. Gravity gets weaker the further out you get. Weight is a measure of the force of gravity upon an object. If you go to a place that has a weaker force of gravity, like the moon, your weight will be less, too. If you were at a place with stronger gravity, like Jupiter, you would weigh more.

7. Friction As you know, friction is a force that keeps objects that are touching from sliding past each other easily. However, friction can be useful. If there was no friction walking on a sidewalk would be like walking on perfectly smooth ice and the brake on a bike would be useless. Sometimes people want to make the force of friction smaller. Friction can rub away machine parts that rub against each other. People put oil on machine parts to make them slippery.

8. Frame of Reference Look around you, would you say you’re moving? How did you answer that question? You looked at the things around you. That is called frame of reference. Motion is based on a type of frame of reference called relative motion. Relative motion is a motion described based on a frame of reference.

9. Speed is the measure of an object’s change in position during a unit of time. For example, a swimmer has a time of about 1 yard per second. So, from this information you know that they will have a time of about 3 yards in 3 seconds. Speed

10. Newton Newton is the metric, or Systeme International (SI) unit of force. The abbreviation for it is N. One Newton is about the amount of force needed to lift a medium-sized apple.

11. Vocabulary • position – a certain place • motion – a change of position • frame of reference – the things around you that you can sense and use to describe motion • relative motion – a motion that is described based on a frame of reference • speed – a measure of an object’s change in position during a unit of time; for example, 10 meters per second • force – a push or pull

12. Vocabulary • newton – the metric, or Systeme International (SI), unit of force • acceleration – a change in the speed or direction of an object’s motion • gravity – a force that pulls all objects toward each other • weight – a measure of the force of gravity upon an object • friction – a force that keeps objects that are touching each other from sliding past each other easily

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