Review - Forces

• All objects are either in motion or still.

• To change an object's movement, apply a force.

• Forces can be strong or weak.

• As the mass of an object increases, you need to exert more force to change its movement.

Classifying Forces

• Scientists classify forces to make them easier to study.

• The classifications of forces are:

  • Contact Forces

  • Non-contact Forces

• Contact forces involve one object touching another object.

• Non-contact forces do not require the objects to touch. They work across a long distance.

Gravity is a Force

• Gravity is a non-contact force.

• Gravity pulls two objects towards each other.

• We can also say, "Gravity attracts two objects."

• The two objects do not need to be touching to be attracted by gravity.

Gravity's Pull Changes

• Two factors affect Gravity's pull:

  • How close the objects are

  • The mass of the objects

• Objects that are closer together can pull more strongly than objects far apart.

• As objects get farther apart, gravity's pull gets weaker.

Gravity's Pull Changes

• Gravity's pull is also affected by mass.

• Objects with more mass create a stronger gravitational pull than objects with less mass.

Gravity on Earth

• On Earth, the biggest object will always be Earth!

• So on Earth, gravity pulls things towards Earth's center.

  • Water is pulled down as rain.

  • Cooling magma is pulled down towards the core.

  • People who jump are pulled back to Earth's surface.

• When Earth's gravity pulls on us, we pull a little bit on the Earth too!

Measuring Gravity's Pull

• We can measure how much gravity pulls on any object.

• We measure the pull of gravity using Weight.

• Weight is affected by mass and gravity.

• In places with a different amount of gravity, like other planets, weight will change, but mass will stay the same.