Forces and Motion

What Is a Force?#

Every time something moves, stops moving, or changes direction, a force is at work. A force is a push or a pull that can change the motion of an object.

Forces Are All Around You#

You use forces constantly without thinking about it:

• You push open a door
• You pull your backpack zipper closed
• You kick a soccer ball (a push with your foot)
• The Earth pulls everything downward (gravity)

Describing Forces#

When scientists describe a force, they think about two things:

1. Strength (magnitude): How hard is the force? A gentle breeze is a weak force; a hurricane is a very strong force.

2. Direction: Which way dös the force push or pull? Forces have direction — left, right, up, down, or any angle.

What Forces Do to Objects#

Forces can:

• Start motion — a push makes a stationary ball roll
• Stop motion — friction slows a rolling ball to a stop
• Speed up motion — a bigger push makes a ball roll faster
• Slow down motion — air resistance slows a parachute
• Change direction — hitting a tennis ball changes which way it travels
• Change shape — squeezing clay changes its shape

Balanced and Unbalanced Forces#

When two equal forces push or pull in opposite directions, the forces are balanced and the object dös not move (or keeps moving at the same speed).

When forces are unbalanced — one force is stronger — the object will move in the direction of the stronger force.

Example: In a tug-of-war, when both teams pull equally, nothing moves. When one team pulls harder, the rope (and the other team!) moves toward the stronger side.