By: Drew Harris

force 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.

Motions Motion Position 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.

Motion

Position – a certain place

Motion – a change of position

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. frictionFriction 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.

Friction

Which one has more friction?

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. accelerations 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.

Acceleration

gravity Weight 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.

F RICTION 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.

W HICH ONE HAS MORE F RICTION ?

H OW CAN WE REDUCE FRICTION ? By using bearings By using lubrication (usually oil) By having a smooth surface such as skis or ice By using a aerodynamic shape that air slips over easily

frame of reference relative motion 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.

Speed 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.

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.

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

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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