What is a Force?

The scientific definition for force is simply a push or a pull. For example, when you do homework you exert a force on your pen or pencil because you push and pull it across the paper.

Sometimes two forces act on something. Like if two people were pushing a shopping cart. If they were pushing in the same direction, you would add the two forces together. The sum of the forces is called the net force. In this case the net force is an unbalanced force. An unbalanced force is a force that changes an object’s motion or causes it to accelerate. The arrows show different forces and their direction, the wider the arrow the stronger the force.

Another way to look at this is you and your friend are riding in a car. It runs out of gas one mile from a gas station. You need to push the car to the gas station. If your friend is smart, he will push in the same direction you push, which is toward the gas station. If he does, your forces are added together. With this larger force, the car will move faster.

You can also have two forces acting in different directions. When the forces are equal and acting in different directions they balance each other out. When this happens, there is no net force because it is a balanced force. If one person was at the front of the shopping cart pushing backwards, and one person was pushing with equal force in the opposite direction, the forces would be balanced, and the shopping cart would not move. Back to the car running out of gas example, your friend might push the car in the opposite direction you are. If he does, your two equal forces in opposite directions balance out to zero net force. This means the car will not move.

Something else can happen when forces push or pull in opposite directions. If one force is more powerful than the other, they will not balance out to zero net force. This is because one force is stronger than the other, so the weak force is not strong enough to completely balance out the stronger force.  Suppose there is a person in the front of a shopping cart and one on the other end. They are pushing in opposite directions, but one of them is pushing with a much greater force. Because the forces are not balanced, the shopping cart is moving in the direction that the person with the stronger force is pushing. Back to the car running out of gas example again, it would be like if your friend was pushing in the opposite direction you were, but this time you weren’t pushing with equal forces. If your friend was pushing harder than you, the forces would not balance each other. The car would roll away from the gas station.

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All pictures courtesy of NASA unless otherwise noted