CHAPTER 2 p. 36 m Section 1 the nature of force

 Force: is a push or a pull.  When an object pushes or pulls on another object then you say the 1 st object has exerts a force on the second object.  Example: while you were writing this you pushed on your pencil.  Like velocity and acceleration, a force is described by its strength and by the direction in which it acts.  If you push a car you exert a different force then if you where to pull the car.

 We measure force in Newton's. One Newton is about the force it takes to pick up a lemon. This comes from the formula kg. m/s squared forced is measured in kilograms and acceleration is measured in meters per second.  So a Newton is the unit we use to measure how much force is being exerted.

 When more than one force acts upon an object you call the total amount of force NET FORCE  Net force: the overall force on an object when all of the individual forces are added together.  When two different forces act in the same direction then you add them together.  When two forces act on an object and one is acting in a different direction then the other then you still add them but one will be a negative and one will be a positive.

 Whenever there is a net force acting on an object then it is called an unbalanced force.  Unbalanced force: forces that produce a nonzero net force, which changes an objects motion.  What does it mean to have a “net force” ?  This means to have something left over. You ended up with something that didn’t balance.  You could have a force exerted on an object but the motion didn’t change. This is a balanced force.  Balanced force: Equal forces acting on one object in opposite directions.

 Who can define friction?  Friction: the force that two surfaces exert on each other when they rub against each other.  Somebody give me an example of friction.  Sliding down the a hill  Carpet burns  Skiing water and snow  And flight  What is the cause of friction?  There are two things that determine the cause of friction.  1 st how the two surfaces are pushed together.  2 nd the type of surfaces involved.

 Static friction: this is friction that act on objects that are not in motion.  Sliding friction: when two solid surfaces slide over each other.  Rolling friction: when an object rolls across a surface,  Fluid friction: friction that occurs when a solid object moves through a fluid.

 What is gravity? Who in here can tell me what gravity is and how it works?  Gravity : a force that pulls objects towards each other.  Who was the man that came up with the idea of gravity?  Isaac Newton

 What keep the moon orbiting around the Earth?  What keeps the Earth orbiting around the sun along with the other planets?  Universal Gravity: the law of universal gravity states that the force of gravity acts between all objects in the universe. This means that ANY two objects in the universe, without exception will attract each other.  Here size matters the larger the object the more gravity it has.

 There are two things that affect gravity between two objects.  Can anybody tell me what they are?  Mass  Distance  Mass is a measure of the amount of matter in an object.

 We know what mass is, it’s the amount of matter in an object.  Who can tell me what weight is?  Weight is :the force of gravity on an object at the surface of a planet.  When you weigh yourself here the Earths gravity is pulling on you. When you weigh yourself on the moon would you weigh more or less?  Less,  Why? Because the moons gravity is a fraction of the Earths.

 What way do objects act when gravity is acting on them?  They fall. Or go toward the center of the object of gravity.  What is free fall?  This is when the only force acting on an object is gravity.  When in free fall all objects regardless of mass fall at exactly the same rate.

 Who can tell me the #1 factor that affects thing falling?  Air resistance. In the last slide you learned that all things fall at the same rate regardless of mass. Is this correct?  No. in a free fall they fall at the same rate and what is the definition of a free fall?  So that means that if there is air then you can have different rates of things falling.  Feather /stone.  Leaf / acorn.

 What is air resistance?  Air Resistance : the fluid friction experienced by objects moving through the air.  Friction is always in the opposite direction of the motion.  This type of friction isn't the same for all objects. what are some of the factors that can affect Air Resistance?  Shape.  Surface  Type of material.

WWhen thing fall do they keep going faster and faster or do they only reach a certain speed and no more? WWhen an object falls it falls at a constant speed. Here on Earth objects fall at about 9.8 m/s so if you were to fall off the top of the school and it took you 3 second to reach the ground how fast were you going? 229.4 m/s now if there are meters in a mile how many mph were you going? AAbout 65 to 66 mph.

 So we know what happens if you hit bottom after 3 seconds. But what if there isn't anything to hit what will happen?  Remember that everything in nature wants to be balanced. When an object is falling it will increase in speed until the air resistance pushing against the object falling reaches the same as the weight of the object. at this point its velocity remains constant. When an object reaches this point we call it  Terminal velocity: the greatest velocity a falling object can achieve

 When a person goes skydiving they will fall at an increasing rate until what?  Until terminal velocity is reached. And what is terminal velocity, this is when the force of air resistance equals the weight of the object.