Forces are all around us. The force of gravity holds Earth in orbit around the sun. The force of magnetism makes bar magnets attract iron filings. And one known as the strong force glues together the building blocks of atoms. Forces affect every object in the universe — from the biggest galaxies to the smallest particles. All these forces have one thing in common: they cause objects to change their motion.
In the late 1600s, physicist Isaac Newton came up with a formula to describe this relationship: force = mass × acceleration. You may have seen it written as F = ma. Acceleration is a change in an object’s motion. This change could be speeding up or slowing down. It also could be a change in direction. Because force = mass × acceleration, a stronger force will cause a larger change in an object’s motion.
Scientists measure forces with a unit named after Newton. One newton is about how much you’d need to pick up an apple.
We experience many different types of forces in our daily lives. You apply a force to your backpack when you lift it up, or to your locker door when you push it shut. The forces of friction and air drag slow you down when you skate or bike around. But all these forces are actually different manifestations of four fundamental forces. And, when you get right down to it, these are the only forces at work in the entire cosmos.
Gravity is a force of attraction between any two objects. That attraction is stronger when the two objects are more massive. It’s also stronger when the objects are closer together. Earth’s gravity holds your feet on the ground. This gravitational tug is so strong because Earth is so massive and so close. But gravity acts over any distance. This means that gravity also pulls your body toward the sun, Jupiter and even distant galaxies. These objects are just so far away that their gravity is too weak to feel.
Electromagnetism, the second force, is exactly what it sounds like: electricity combined with magnetism. Unlike gravity, the electromagnetic force can attract or repel. Objects with opposite electric charges — positive and negative — attract each other. Objects with the same type of charge will repel each other.
The electric force between two objects is stronger when the objects are more charged. It weakens when the charged objects are farther apart. Sound familiar? In this sense, electric forces are very similar to gravity. But while gravity exists between any two objects, electric forces exist only between objects that are electrically charged.
Magnetic forces can also attract or repel. You may have felt this when bringing the ends, or poles, of two magnets together. Every magnet has a north and south pole. The north poles of magnets are attracted to south poles. The opposite is also true. Poles of the same type, however, push away from each other.
Electromagnetism is behind many kinds of pushes and pulls we experience in everyday life. That includes the push you exert on a car door and the friction that slows your bike. Those forces are interactions between objects due to the electromagnetic forces between atoms. How are those tiny forces so powerful? All atoms are mostly empty space surrounded by a cloud of electrons. When the electrons of one object come close to the electrons of another, they repel. This repelling force is so strong that the two objects move. In fact, the electromagnetic force is 10 million billion billion billion times stronger than gravity. (That’s a 1 followed by 36 zeroes.)
Gravity and electromagnetism are the two forces we can feel in our daily lives. The other two forces act inside atoms. We cannot directly feel their effects. But these forces are no less important. Without them, matter as we know it could not exist.
The weak force controls the interactions of tiny particles called quarks. Quarks are the fundamental bits of matter that make up protons and neutrons. Those are the particles that make up the cores of atoms. Quark interactions are complex. Sometimes, they release huge amounts of energy. One series of these reactions happens inside stars. Weak-force interactions cause some particles in the sun to transform into others. In the process, they release energy. So the weak force may sound wimpy, but it causes the sun and all other stars to shine.
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Historically, scientists have thought of electromagnetism and the weak force as different things. But recently, researchers have linked these forces together. Just as electricity and magnetism are two aspects of one force, electromagnetism and the weak force are related.
This raises an intriguing possibility. Could all four of the fundamental forces be connected? No one has proved this idea yet. But it is an exciting question on the frontiers of physics.
The strong force is the final fundamental force. It’s what keeps matter stable. Protons and neutrons make up the nucleus of every atom. Neutrons have no electric charge. But protons are positively charged. Remember, the electromagnetic force causes like charges to repel. So why don’t the protons in an atomic nucleus fly apart? The strong force holds them together. At the scale of an atomic nucleus, the strong force is 100 times stronger than the electromagnetic force that is attempting to push the protons apart. It is also strong enough to hold the quarks inside protons and neutrons together.
Feeling forces from afar
Notice that none of the four fundamental forces requires objects to touch. The sun’s gravity attracts Earth from afar. If you hold the opposite poles of two bar magnets near each other, they will pull on your hands. Newton called this “action-at-a-distance.” Today, scientists are still searching for some of the particles that “carry” forces from one object to another.
Light particles, or photons, are known to carry the electromagnetic force. Particles called gluons are responsible for the strong force — holding atomic nuclei together like glue. A complicated set of particles carries the weak force. But the particle responsible for gravity is still at large. Physicists think gravity is carried by particles called gravitons. But no gravitons have ever been observed.
Still, we don’t need to know everything about the four forces to appreciate their impacts. Next time you drop down the hill on a rollercoaster, thank gravity for the thrill. When your bike is able to brake at a stop light, remember the electromagnetic force made it possible. As the sunlight warms your face outdoors, appreciate the weak force. Finally, hold a book in your hand and consider that the strong force is what holds it — and you — together.