Gravity, Friction, and More Forces

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Gravity affects all masses, not just the force we see when objects fall to the ground. Gravity is never pushing. It always pulls.

The force of attraction between all things that have mass is gravity.

Gravity depends on the mass and the distance between masses. Larger masses have more gravity, and closer objects will always pull on you with more gravity.

Gravity is what keeps the Earth in orbit around the sun because the sun has so much mass that it exert a gravitation force on our entire solar system, keeping all of the planets in orbit.

gravitation force in solar system


A measure of gravitation force is weight and weight depending on the force of gravity and the object's mass.

Example: Assume we weigh two objects on a scale. You will see that more weight on the object with more mass.

Weight depends on location because gravitational force can change depending on the location.

Example: The force of gravity is more on the Earth than on the moon. Moon has less mass than the Earth. Since the moon has smaller gravitation, some objects will be weightless on the moon than on the Earth.

Gravity pulls you down towards the Earth because there is a constant acceleration towards the ground. The approximate force of gravity on the Earth is 9.8 m/s2.

Example: Assume you throw something up into the air. It goes up and gets slower until it stops in mid-air and then again comes back down toward the Earth. It is accelerating down until it crashes into the ground. - This is negative acceleration.


It's the force that opposes movement between touching surfaces and always acts against the direction of motion.

Example: Try sliding a notebook across a table. It slows down and finally stops. Friction is the net force that affecting the notebook. It belongs to Newton's first law that an object in motion will remain in motion unless acted on by a net force.

Have you felt that the wheels of the skateboard slow down when you ride it? That because of the friction with the sidewalk and with the bearing in the wheels.

Rougher surfaces have more friction.

Example: Sandpaper is harder to slide than regular paper because sandpaper has a rougher surface. That means more friction.

We can minimize the force of friction by greasing surfaces.

Example: We have fluid in our knees to reduce joint friction.

There is friction with air and water too. Air Resistance is the technical term for friction with the air.

Example: Assume you drop a feather. It floats from side to side because air resistance opposes the downward moment.

Static Friction - No Motion

The result of molecules on one surface adhering to the other surface is called static friction.

Sliding Friction - Sliding Motion

The force of friction that affects surfaces in motion is sliding friction. It is also known as Kinetic Friction

Example: Assume you are pushing a box, the force of friction that resists the motion is sliding friction. The sliding friction is less powerful than static friction because the surfaces aren't always bonded like they are in static friction.

Rolling Friction - Rolling Motion

When an object like a wheel or ball rolling freely over a surface, there is friction between the surface and wheel or ball. That called rolling friction. Rolling friction is weaker than sliding friction. That's why it is much easier to move something on wheels.

Example: The friction between the wheels of a skateboard and the sidewalk is rolling friction.



Two forces are acting on it when an object is falling to the ground. Those are the force of gravity and the air resistance. Air resistance is opposing its motion. There is no net force, and the forces are balanced on the object when the air resistance equals the force of gravity. If there is no net force, the object stops accelerating, and it continues falling at a constant speed. The speed at which the force of gravity equals air resistance is called terminal velocity. Many factors such as surface area, mass, how the object is oriented, and even the thickness of air are caused by this speed.


Magnets either an attractive or repelling force. Magnet has a negative and positive end. Like charges repel, and opposite charges attract. Magnet forces pull together when we place a positive with a negative end, but if we try to touch a negative end with another negative or positive with a positive, they push each other away. (Repulsive Force)

Macnatic Force


Electric forces, also like magnetic forces, are caused by positive and negative changes. Electric forces are created by moving charges, while magnetic faces are creating by unmoving changes. Both electric and magnetic forces are related to the amount of charge and the distance between charges.

When the charge is stronger and closer together, the electric and magnetic forces increase. The interaction of electric forces and magnetic fields is called electromagnetism. Any electric charge in motion has a magnetic region around it, and a magnetic field surrounds the wire that carries an electric current. By wrapping a current-carrying wire around an iron, the core can make an electromagnet.

Electric and Macnatic Force


CENTRIPETAL FORCE is the force affecting an object in a circular motion, and it always points toward the center of the circular path of motion.

Example: The moon moves in a circular orbit around the Earth and is affected by the centripetal force of gravity. Assume you swing a yo-yo around in a circle. Tension from the string is the centripetal force that keeps the yo-yo in a circular motion.


The buoyant force is the force that keeps a rubber duck floating. It is an upward force exerted by a fluid onto an object that is immersed in the fluid. The density of the fluid and the amount of fluid that the object displaces cause the buoyant force. The buoyant force is stronger if the dense fluid and less fluid displaced.

The weight of the fluid that has been displaced and the buoyant force is equal. It is a principle, and that called ARCHIMEDES' PRINCIPLE.

Buoyant Force

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