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A brief Introduction to Physics

 We express the forces on an object with a Free Body Force Diagram.  The vectors representing forces on an object always come from the center of mass.

 What is the force diagram of a box that is being pushed (neglecting friction)?  What is the force diagram of a box being dropped (neglecting air resistance)?  What is the force diagram of a box being pushed across the floor with friction

 Balance Video!  http://youtu.be/6aB8I63JFB8 http://youtu.be/6aB8I63JFB8

 Equilibrium is a nice way of saying that all the forces are balanced. So lets look around the room and see what we can see where the forces are balanced?  What does this mean about moving objects at a constant speed?  What does this mean about accelerating objects?

 Friction is a force which opposes motion.  Friction is an important force in our lives. We rely on friction in many ways. An athlete usually wears shoes which provide him or her with a greater friction between the shoe and the surface. We rely on friction as an important aspect of our motion. In what other ways does friction play a role in our everyday lives?

From Glencoe Physical Science

 There are two factors which affect friction between two surfaces:  Kind of surfaces in contact (rough or smooth)  Amount of force pressing the surfaces together. The rougher the surface and the stronger the force between the surfaces, the greater the amount of friction.

 When objects fall they accelerate toward the ground.  Using Newton’s 2 nd Law we can see this relationship: F = ma  The force caused by gravity acts upon a certain mass to cause it to accelerate toward the ground at a constant rate.  Acceleration due to gravity is 9.8 m/s 2

 The weight of an object is caused by the force of attraction between the earth and objects on the surface of the earth. Gravitation force is equal to weight, therefore…

Weight = mass X 9.8 m/s 2 Weight = 70.0 kg X 9.8 m/s 2 Weight = 686 N Remember that 1 N = (1 kg) (1m/s 2 )

 Mass is the amount of matter in an object. This does not change in an object as it moves away from the earth.  Weight is caused by gravity. Therefore, the greater the distance, the less the force of attraction. The less the distance the greater the force of attraction.  Your weight is dependent upon the mass of the earth. If you were able to go to the planet, Jupiter, your weight would be 27 time greater, because the mass of Jupiter is 27 times greater.

 Even in the space shuttle, there is a force of attraction exerted by the earth on the shuttle and its contents.  When the space shuttle temporarily “falls” toward the earth, the contents of the space shuttle appear to be weightless, but in fact they are falling with the space shuttle. This is called “freefall”.

This Can be expressed as an equation too.

a

Fa

Moving

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