A force is a push or pull that makes something happen. When you kick a ball, your foot supplies the force that makes the ball fly into the air. Forces are usually invisible. Gravity is the invisible force that pins us to the ground, while magnetism is another hidden force that makes a compass spin. Simple forces can change an object’s shape, alter its direction, or change its speed.
Changing shape: If you bend something, you push and pull on the atoms inside it and alter its shape.
Changing direction: A tennis racquet applies force to the ball to make it move in a different direction.
Changing speed: The harder you hit a golf ball, the more force you use, and the further the ball travels.
If two equal forces act in opposite directions, they cancel each other out and there's no movement or change of any kind. Architects design buildings so that the downward pull of gravity is balanced by upward forces inside the buildings.
Levers, wheels, pulleys, and gears all magnify forces and make jobs easier to do. These force magnifiers are called simple machines. If you sit on a seesaw, you can lift someone heavier by moving further from the balancing point. The seesaw works as a lever and multiplies your lifting force. You press down with a small force and the lever makes a bigger lifting force at the other end.
Machines such as cranes are built from many simple machines working together.
An object's speed in a certain direction is its velocity. If it speeds up or changes direction, its velocity changes. A change in velocity is called an acceleration. Force is needed to accelerate an object.
Increasing velocity: When a car speeds up, its engine produces more force to make the vehicle accelerate.
Changing direction: As a car turns, it accelerates, even if it keeps the same speed, since the change in direction changes the car's velocity.
Decreasing velocity: When the driver brakes, force is applied to the wheels, slowing the car. This is called deceleration, or negative acceleration.
The difference in speed between two moving objects is called their relative velocity. Because velocity is speed in a particular direction, you have to take account of the direction in which the objects are moving when you calculate their relative velocity.
Relative velocity zero: When two people run at the same speed in the same direction, their relative velocity is zero.
Catching up: When one person runs faster than the other, their relative velocity is the difference in their speeds.
Heading for collision: If two people run equally fast in opposite directions, their relative velocity is the sum of their speeds.
The force of gravity holds everything in the Universe together like a mysterious, invisible spider's web. Gravity is also the force of attraction between your body and Earth that, quite literally, keeps your feet on the ground.
The pull of gravity
Everything in the Universe that is made of matter (and so has mass) pulls on everything else with gravitational force. The bigger and closer the objects are, the greater the pull of gravity between them. Earth’s gravity is the reason that we do not float off into space, and why objects fall down to the ground. Without air resistance to slow them down, all objects - from feathers to pool balls - would fall at the same rate.
Gravity in a vacuum
A ball usually falls faster than a feather, because its shape creates less air resistance. In a vacuum, with no air resistance, the ball and feather fall at the same speed.
Gravity and relativity
The gravity from a massive object, such as the Sun, still pulls at vast distances and never disappears entirely. It affects light as well as physical objects. According to a theory called relativity developed by German-born physicist Albert Einstein (1879-1955), the gravity of a body like the Sun actually bends space and time, rather like a heavy weight setting on a rubber sheet.
Because space-time is bent, things moving near the Sun will curve toward it as though being pulled inward.
Friction is a dragging force that occurs when one object moves over another. You may not realize it, but you use this force whenever you walk down the street. Friction helps your shoes grip the pavement so that you don’t slip.
A rough force
When one object moves over another, the rough surfaces of the two objects catch against one another and stick. This sticking process makes it harder to move the objects, producing a force between them that we call friction.
In general, the rougher a surface is, the more friction it produces. But even polished surfaces that look and feel smooth will have microscopic bumps and ridges that produce some friction. One way to reduce friction is to make objects as smooth as possible, so they slide past one another more freely. Another way is to use lubrication.
It’s harder to move through water than through air, because water is a thick liquid. Water pulls against the bow (front) of a boat, slowing it down with what’s called drag. Boats solve this problem with curved front edges that plane (rise out of the water) as they move along, reducing drag.
The same force can have very different effects. If you stand on snow in ordinary shoes, the force of your weight makes you sink. But if you wear skis, your weight is spread over a larger area. You don’t sink because you produce less pressure. Pressure depends on the size of the force and the area it acts over. The bigger the force or the smaller the area, the greater the pressure.
Pressure = Force / Area
Larger area, lower pressure: You can’t push a thick bar into a wall. If you press with a certain force on the far end, the same force travels down the bar but can’t penetrate the wall.
Smaller area, higher pressure: Sharpening the end of the bar focuses the force into a smaller area, increasing the pressure so the bar pierces the wall. This is why nails taper to a point.
Water exerts pressure on anything submerged in it. The deeper you go, the more water pushing down on you, and the greater the water pressure.
A magnet is surrounded by an invisible field of magnetic force that is strongest at the north and south poles of the magnet. Magnets will push and pull on other objects with magnetism.
Earth’s iron core is a huge magnet with its own magnetic field. A compass has a magnetic pointer that is attracted to Earth’s magnetic poles.