Forces

Acceleration

Acceleration is the rate at which velocity changes.
Acceleration refers to increasing speed, decreasing speed, or change in direction.
Velocity describes both the speed and direction of an object.
Whenever an objects speed changes, the object accelerates. Just as objects can speed up, they can also slow down. This change in speed is sometimes called deceleration or negative acceleration.
An object traveling at a constant velocity can be accelerating. Acceleration can be a change in direction as well as a change of speed.
If an object is not changing direction, you can describe its acceleration as the rate at which its speed changes.
To determine acceleration, you calculate the change in sleep per unit of time.
Acceleration= (final speed-initial speed)/time

Force

A force is a push or pull. When one object pushes or pulls another object, the first object exerts a force on the second object.
Like velocity and acceleration, a force is described by its strength and by the direction in which it acts.
The direction and strength of a force can be represented by an arrow. The length of the arrow tells you the strength of the force - the longer the arrow, the greater the force.
The strength of a force is measured in Newtons.
The combinations of all the forces on an object is called the net force. The net force determines if and how an object will accelerate.
A non-zero net force causes a change in the object’s motion.

Friction

The force that two surfaces exert on each other when they rub against each other is called friction.
Two factors that affect the force of friction are the type of surfaces involved and how hard the surfaces are pushed together.
In general, smooth surfaces produce less friction than rough ones.
Friction increases when surfaces push harder against each other.
Friction acts in a direction opposite to the direction of the objects motion. Without friction, a moving object will not stop until it strikes another object.

Gravity

Gravity is a force that pulls objects toward each other.
Gravity acts everywhere in the universe, not just Earth. The force of gravity acts between all objects in the universe that has mass.
The gravitational force between some objects is stronger than the force between others.
Two factors that affect the gravitational attraction between objects: mass and distance.
The more mass an objects has, the greater the gravitational force between it and other objects.

As distance increased, gravitational force decreases.

Weight and mass

Mass is a measure of the amount of matter in an object. Weight is a measure of the force of gravity on an object.
Your mass is the same on Earth as it would be on any planet but weight varies with the strength of gravitational forces.

Newton’s laws of motion

Newton’s first law of motion states than an object at rest will remain at rest unless acted upon by a non-zero net force. An object moving at a constant velocity will continue moving at a constant velocity unless acted upon a non-zero net force.
All objects, moving or not, resist changes in motion. Resistance to change in motion is called inertia.
Newton’s first law of motion is also called the law of inertia.
Inertia depends on mass. The greater the mass of an object, the greater it’s inertia, and the greater the force required to change its motion.
Newton’s second law of motion states than an objects acceleration depends on it’s mass and on the net force acting on it.
Acceleration is measured in meters per second per second. Mass is measured in kilograms,
Force is measured in kilograms times meters per second per second. This unit is also called Newton.
One newton is the force required to give a 1-kg mass a acceleration of 1 meters per second per second.
Newton’s third law of motion states that if one object exerts a force on another object, then the second object exerts a force of equal strength in the opposite direction on the first object. Another way to state Newton’s third law is that for every action, there is a equal but opposite reaction.
When you walk, the ground pushes you forward on your feet with an equal but opposite force. You can only walk because the ground pushes you forward.
If mass increases and forces stays the same, acceleration decreases.
Action and reaction force do not cancel each other out because they are acting on different things.

Momentum

Momentum is a characteristic of a moving object that is related to the mass and the velocity of the object. The momentum of a moving object can be determined by multiplying the object’s mass by it’s velocity.
The unit for momentum is kilograms times meters per second. Like velocity, acceleration, and force, momentum is described by both direction and a strength. The momentum of an object is in the same direction as its velocity.

The law of conservation of momentum states that in the absence of outside forces like friction, the total momentum of objects that interact does not change..

Free fall

When the only force acting on an object is gravity, they object is said to be in free fall.
The force of gravity causes the object to accelerate. Free fall is motion where the acceleration is caused by gravity.
When something falls on earth, there is fluid friction from the air around it. This friction acts against gravity, reducing the acceleration of falling objects. Air friction increases as an object falls.

If an object falls for long enough, increased air friction will reduce it’s acceleration to zero. The object will continue to fall, but it will fall at a constant velocity.

Satellite motion and centripetal force

Satellites in orbit around Earth continuously fall toward Earth, but because Earth is curved they travel around it.
Satellites do not need fuel due to it’s inertia continuing it’s movement.
Centripetal force is a force that causes an object to move in a circular path.
Centripetal forces always point toward the center of the circle an object is moving in. If you could turn off centripetal force, inertia would cause the object to fly off in straight line.