# Chapter 10: Forces

## The Nature of Force

##### Force

• In science, the word force has a simple and specific meaning.
• Force is a push or a pull
• When one object pushes or pulls another object, you say that the first object exerts a force on the second object.
• Force is a vector quantity.
• Described by magnitude and direction.
• An arrow represents the direction and strength of a force and the longer the arrow, the greater the force.
• The SI unit for force is a Newton.

##### Combining Forces

• Often, more than a single force acts on an object at one time.
• Net Force is the combination of all forces acting on an object.

##### Balanced Forces

• Balanced Forces are equal forces acting on one object in opposite directions.

• When equal forces are exerted in opposite directions there is no net force.
• Balanced forces acting on an object do not change the object's velocity.

## Friction, Gravity and Elastic Forces

##### Friction

• Friction is a force that two surfaces exert on each other when they rub against each other.
• Friction acts in the direction opposite to motion.
• Without friction or other unbalanced forces, an object will not stop until it hits another object.

##### Causes of Friction

• Smooth surface has less friction than rough surface.
• Strength of force of friction depends on types of surfaces involved and how hard the surfaces push together. (i.e. rubbing hands together)

###### 4 Types of Friction
• Static Friction - Friction that acts on objects that are not moving
• Example: moving a piece of furniture across room.
• Must use extra force to start friction of stationary objects.
• Sliding Friction - Friction where two solid surfaces slide over each other. When an object is pulled at a constant velocity across a level surface, the pulling force equals the sliding friction.
• Rolling friction - Friction where an object rolls across a surface.
• Rolling friction is less than sliding friction.
• Fluid friction - Friction where a solid object moves through a fluid.
• Less than sliding friction.
• Parts of machines bathed in oil.

###### Gravity
• Gravity is a force that pulls objects toward each other.
• Newton realized gravity acts everywhere in universe.
• Law of Universal Gravitation states that the force of gravity acts between all objects in the universe.
• Any two objects in the universe, without exception, attract each other.

###### Mass and Distance
• Force of gravity between object increases with greater mass and decreases with greater distance.
• The further from Earth it is, the less gravitational force.
• Mass is the measure of the amount of matter in an object. (Kg)

###### Gravity & Weight
• Weight is gravitational force exerted on a person or object at the surface of a planet.
• Stepping on scale (weight) shows the gravitational force Earth exerting on you.
• Objects with greater mass have greater weight.
• Weight = Mass x Acceleration due to gravity
• Weight – Newtons (N).
• Mass – Kilograms (KG).
• Acceleration due to gravity = 9.8 meters per second per second (m/s²) (at Earth's surface)
• Weight varies with strength of gravitational force – mass doesn't.
• Strength of gravitational force exerted on an object or person by the moon is 1/6 of the force exerted by Earth.

###### Gravity and Motion
• Free fall – when the only force acting on an object is gravity.
• In free fall, the force of gravity alone causes an object to accelerate in the downward direction.
• All objects in free fall accelerate at the same rate.

###### Air Resistance
• Air resistance is a type of fluid friction.
• Air resistance is an upward force exerted on falling objects.
• Objects with greater surface area experience more air resistance as they fall. (Flat piece of paper vs. crumpled piece of paper)

###### Projectile Motion
• A projectile is an object that is thrown.
• A ball thrown horizontally falls at the same rate as ball dropped.

###### Elastic Forces
• Matter is considered elastic if it returns to its original shape after is squeezed or stretched.
• Compression is an elastic force that squeezes or pushes matter together.
• Tension an elastic force that stretches or pulls matter.

## Newton's Laws

###### Isaac Newton
• In the early 1600s Galileo said, that when an object is in motion, a force is needed to change the motion of an object.
• In the late 1600s, Isaac Newton built upon Galileo's idea and proposed three basic laws of motion.

###### First Law of Motion
• An object will remain at rest or moving at a constant velocity unless it is acted upon by an unbalanced force.
• An unbalanced force will cause an object to speed up, slow down or change direction.

###### Inertia
• Inertia is the tendency of an object to resist a change in motion.
• Newton's first law is also called the law of inertia.
• The greater the mass of an object - the greater its inertia - the greater the force required to change its motion.

###### Second Law of Motion
• Acceleration depends on the net force acting on the object and on the object's mass.
• Acceleration = Net Force / Mass (Acceleration is the rate at which velocity changes)

###### Third Law of Motion
• 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.

• Action and reaction forces do not cancel out because they are acting on different objects.

###### Momentum
• Momentum is a quantity of motion.
• Characteristic of a moving object that depends on both the mass and velocity of the object.
• Momentum = Mass x Velocity Measured in kilogram-meters per second (kg-m/s)

###### Conservation of Momentum
• In science, conservation refers to the conditions before and after an event.
• The Law of Conservation of Momentum states that, in the absence of outside forces, the total momentum of objects that interact does not change.
• The total momentum of any group of objects remains the same, or is conserved, unless outside forces act on the objects.

## Rockets and Satellites

###### How Do Rockets Lift Off?
• Rockets and space shuttles lift into space using Newton’s third law of motion.
• They burn fuel and push the exhaust gases downward at a high velocity as they lift off. In turn, the gases push upward on the rocket.

• A rocket can rise into the air because the gases it expels with a downward action force exert an equal but opposite reaction force on the rocket.
• As long as this upward force, called thrust, is greater than the downward pull of gravity, there is an unbalanced force in the upward direction that causes a change in the rocket's velocity. As a result, the rocket accelerates upward into space.

###### What is a Satellite?
• Satellite any object that orbits another object in space. (e.g. Moon or Artificial Satellite)

###### Circular Motion
• Artificial satellites travel around Earth in an almost circular path.
• An object traveling in a circle is constantly changing direction, so it is accelerating.
• If an object is accelerating, an unbalanced force must be acting on it.
• Centripetal Force is any force that causes an object to move in a circular path. (Center seeking, and acts perpendicular to the direction the object is moving)

###### Satellite Motion
• Satellites in orbit around Earth continuously fall toward Earth, but because Earth is curved they travel around it.
• The speed with which an object must be thrown in order to orbit Earth is 7,900 meters per second.

###### Satellite Location
• Some travel in low orbits. (Mapping, or Observation) These satellites complete a rotation around the Earth in a couple of hours.
• Others in higher orbits travel more slowly. (Communications) These travel 36,000 kilometers above Earth's surface.

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