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.
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/s^{2}. 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!
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:
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.
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:
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.