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Forces: Pushes and Pulls

Lesson 8: What Causes Objects to Move or Stop Moving?

  • A force is a push or a pull.
  • Sometimes, you see a force acting, such as when a racket hits a tennis ball. Other times, you do not see a force acting but you can see the result of the force.
  • You know a force is acting because something moves.
  • The wind, magnetic, and electric forces are all examples of forces that can act on objects but cannot be seen.

Gravity

  • Gravity is a force of attraction that exists between any two objects. The force of gravity between two objects acts equally on both objects.
  • Many people think of gravity as the pull of Earth on an object. In reality, just as Earth exerts a pull on an object, the object also exerts a pull on the Earth.
  • The pull of gravity between any two objects depends on the amount of matter, or mass, each object has.
  • The greater the masses of the objects, the greater the force of gravity between them.
  • The closer the centers of the two objects are, the greater the force of gravity between them.
  • When rockets are sent to the moon, Earth's pull decreases as the distance of the rocket from Earth increases.

Mass and Weight

  • Mass is the amount of matter an object contains.
  • In the metric system, mass is measured in grams (g), kilograms (kg), or milligrams (mg).
  • Use a triple-beam balance scale.
  • The mass of an object is constant, no matter the location.
  • Weight how much force gravity pulls on an object.
  • The metric unit of force or weight is the newton (N).
  • Spring Scale
  • The weight of an object varies as it travels through space.
  • Objects weigh less on the moon than on Earth.
  • Why does weight change?
  • The moon has less mass than Earth, and it is smaller in size. Because of this difference, the force with which the moon pulls on an object is only about one-sixth as strong as the pull of Earth.

Forces

  • Most objects have many forces acting on them at all times.
  • When you ride a bike, gravity is pulling you down, the wind might be pushing you toward one side or the other, and the forces between the bike's tires and the road are pushing you forward.
  • In science, forces are represented by arrows.
  • The length of an arrow shows the strength of the force.
  • The direction of the arrow shows the force's direction.

Balanced and Unbalanced Forces

  • Net Force: the overall force acting on an object (expressed in newtons (N)).
  • Balanced Forces: when two equal forces act on an object in opposite directions.
  • If two forces are equal in the opposite direction, the forces balance each other out and have a net force of zero (the object will not move).
  • Unbalanced Forces: when unequal forces cause an object to move.
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