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SI Unit: Newtons (N)

What Are Forces?

A force is an influence, such as a push or pull, that makes an object accelerate. In physics, an object is said to accelerate when it changes either its speed or direction. Forces can either be contact forces, such as an object pushing another object, or non-contact forces, such as gravity or magnetism, which act invisibly over a distance. The SI unit used to measure the magnitude, or strength, of a force is called the newton (N), named after Isaac Newton.

Understanding Newtons and What They Mean

The newton is a unit of measurement made up of other units, something scientists call a "derived" unit. Units such as kilograms, meters, and seconds, on the other hand, cannot be split apart any further. Physicists define one newton as the force required to accelerate a 1 kg object at a rate of 1 m/s (meters per second), each second. Written out, this looks like 1 kg * m/s2. Don't worry if this is super confusing (it's only college-level physics, after all), we promise it'll make sense soon!

First, let's imagine you have a shopping cart full of bananas. The cart and all the bananas together have a mass of exactly 1 kg, and the cart isn't moving at all. When you push the cart, you apply a force to make it accelerate and start to move. The strength of that force can be measured in newtons.

If you keep pushing this 1 kg cart with a constant force of 1 N, after one second, the cart will be moving at a speed of 1 m/s. After 2 seconds, it will be moving at a speed of 2 m/s. And after 3 seconds... 3 m/s. Now that's some healthy fast food!

The Different Types of Forces

To make it easier to describe them, scientists separate forces into different types. Here are some forces used to describe objects and forces in everyday life:

  • Gravitational force: The gravitational force is the force that pulls objects towards each other. It's what makes things fall to the ground when you drop them. The Earth has a gravitational force that pulls everything towards its center. This force keeps us on the ground and gives weight to objects.
  • Normal force: The normal force is the force that objects exert on each other when they are in contact. For example, when you're sitting on a chair, the chair pushes back on you with a force called the normal force. It's called "normal" because it acts perpendicular (at a right angle) to the surface of contact.
  • Frictional force: Friction is the force that opposes motion when two surfaces rub against each other. It's what makes it harder to slide a heavy box on the floor compared to a light one. Friction helps us walk, run, and grip things without slipping. Different surfaces can have more or less friction.
  • Tension force: Tension force is the force that happens when something is being pulled or stretched. For example, when you pull a rubber band, there is tension force in the rubber band that tries to bring it back to its original shape.
  • Applied force: Applied force is the force that you apply to an object when you push or pull it. When you kick a soccer ball or push a swing, you're using an applied force to make them move.
  • Spring force: Spring force is the force that occurs when a spring is compressed or stretched. Springs have a natural tendency to go back to their original shape, so when you stretch or compress a spring, it exerts a force in the opposite direction.
  • Magnetic force: Magnetic force is the force between magnets or between a magnet and certain objects. Magnets have invisible magnetic fields around them, and when they come close to each other or certain materials, they can either attract (pull together) or repel (push away) each other.
  • Centrifugal force: Centrifugal force is a bit tricky to understand. When you spin something around, like a merry-go-round, you might feel like you're getting pushed away from the center. That feeling is called centrifugal force, but it's not exactly a real force. It's just the way your body naturally wants to keep moving in a straight line, even though you're going in a circle.

Top Facts About Force for Kids

  • 1. Forces can add together or cancel each other out.

    When two equal forces going the exact opposite direction meet, they cancel each other out perfectly. For example, if you and a friend each try to push a box with the same force, the box will not change its speed or direction. If, however, both forces act in the same direction, the forces get added together. So if you and a friend both push the same box from the same side this time, the box will accelerate twice as if you push it alone.

  • 2. There are 4 basic forces in the universe that make up all other forces.

    Physicists have discovered 4 basic, or fundamental, forces at work in the universe. Other forces, like friction, buoyancy, or even pushing a box, can all be explained in terms of these 4 forces:

    • Gravity, which is what makes things fall to the ground and keeps the planets in orbit around the sun.
    • Electromagnetism, the forces between particles that have electrical charge. This force is what makes magnets work and powers electronics like computers and TVs.
    • The strong nuclear force, which is what holds the protons and neutrons in the nucleus of an atom together.
    • The weak nuclear force, which is responsible for certain kinds of radioactive decay and is important for how the sun produces energy.

  • 3. Weight is actually a force—mass is not.

    While mass is a measurement of the amount of matter in an object, weight is actually a force. When you step on the scale to measure your weight, what you're actually measuring is the force of gravity from Earth pulling you down to its surface.