Balanced & Unbalanced Forces

A force is a push or a pull. A force has both strength and direction. Forces cause objects to change their motion.

Balanced forces. Gravity is pulling the student down and the chair is pushing the student up with equal force, resulting in no motion.

The ball is in motion. The forces acting on the ball are unbalanced (Gravity is pulling it down).

Both sides need to be pulling with the same amount of force in opposite directions for the forces to be balanced.

Izzy hit the ball too hard. He used too much STRENGTH when applying a force.

In Zoe’s first attempt she used the right amount of STRENGTH when applying a force to the ball, but did not know the right direction because she was wearing a blindfold. In Zoe’s second attempt she applied just the right amount of strength and knew the right direction to win. (A force has both strength and direction.)

When Zoe pulled with a lot of force while Izzy wasn’t pulling at all, the strength of the force was greater in Zoe’s direction. That made Izzy move in that direction.

Before Dr. Jeff cut the rope, the forces were balanced. The force from Izzy pulling in one direction was cancelled out by the force of Zoe pulling with the same amount of force in the other direction. When the rope was cut, suddenly the forces were no longer balanced and the result was motion!

The moving air from the hair dryer pushes the ball upward and the force of gravity pulls the ball downward with a force that is equal and opposite. The upward and downward forces on the ball cancel each other out so it is not in motion. When the hairdryer is turned off, the upward force is removed and the force of gravity causes the ball to fall down.

Zoe’s hovercraft only moved in one direction. The force pushed her forward. If she wanted to stop, she needed to apply an equal force in the opposite direction, which she could have done by aiming the fire extinguisher in front of her.

When the egg is sitting on top of the toilet paper tube, the forces acting upon it are balanced. The tube is pushing it up and gravity is pulling it down. When the tube is quickly removed, the only force acting on the egg is gravity, which causes its motion downward into the cup of water.

When Zoe starts her hovercraft, a cushion of air pushes the hovercraft up off the ground. At the same time, gravity pulls Zoe and the hovercraft down towards the Earth. Since the forces are balanced, the hovercraft... hovers! All motion for the hovercraft around the lab is related to forces acting on it sideways (due to the fire extinguisher).