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Chapter 8 Section 3 Notes Motion and Force
What is Force? force □A force is the cause of acceleration or change in an object’s velocity
Fundamental Forces 4 fundamental forces □There are 4 fundamental forces in nature: □ Gravity □ Electromagnetic force □ Strong nuclear force □ Weak nuclear force
Fundamental Forces □Gravity: very, very weak force; this force acts over a long distance; much weaker than the electromagnetic force; field force □Electromagnetic force: moderate force; acts over long distances; about 1/100 the strength of the strong force; field force
Fundamental Forces □Strong nuclear force: strongest of all forces, but acts over a very short distance; only acts over distances the size of the atomic nucleus □Holds together protons and neutrons in the nuclei of atoms □Weak nuclear force: associated with nuclear decay
Types of Forces □ Contact forces: caused when objects touch □Examples: when you push a shopping cart or catch a baseball and it stops moving □ Field forces: do not require that objects touch each other □Examples: attraction of gravity or repulsion between 2 north poles of a magnet □Both contact and field forces can cause an object to move or stop moving!
Balanced and Unbalanced Forces □Net force: the sum of all the forces acting on an object □Whenever there is a net force acting on an object, the object accelerates in the direction of the net force.
Balanced and Unbalanced Forces □An object will not accelerate if the net force is equal to zero. □Balanced forces: net force of zero □Do not cause a change in motion of an object.
Balanced and Unbalanced Forces □Example of Balanced Forces: □When 2 teams are playing tug of war and both teams are exerting equal amounts of pressure, there is no acceleration, and neither team moves.
Balanced and Unbalanced Forces □Unbalanced forces: occurs when forces acting on an object don’t cancel out; there is a greater force acting on 1 side of the object □Example: When 2 teams are playing tug of war and one team exerts a greater force and pulls the other team forward
Balanced and Unbalanced Forces □When an object is at rest, are there still forces acting on it? □Yes! It is just not moving because the forces are balanced. □What happens if forces act in different directions that are not opposite one another? □The combination of forces acts like a single force and cause acceleration in a combined direction □Example: You push your book east and your friend pushes the book north; the book will move northeast
Friction □Friction: force between 2 objects that opposes the motion of either object □Example: Imagine a car rolling on a flat, evenly paved street. Eventually, the car will stop due to the force of friction. □Friction depends on surfaces in contact: □Frictional forces are greater when both surfaces are rough □Example: new tennis shoes walking along a carpeted floor will keep you from falling down.
Friction □Two types of friction: □Static Friction: when 2 objects touch but do not move □Example: A filing cabinet sitting on the floor
Friction □Kinetic Friction: friction between moving surfaces. Two types of kinetic friction: □Sliding friction: when 1 object slides over another □Example: Sliding a box across the counter □Rolling friction: when 1 object rolls across another □Example: Rollerblading
Friction □The force necessary to make a stationary object start moving is usually more than the force necessary to keep it moving; therefore, static friction is usually greater than kinetic friction. □Rolling friction is usually less than sliding friction.
Friction and Motion □Friction is not all bad; it is actually required for many everyday tasks to work correctly. □Examples: □Cars could not move without friction; as a car’s wheels turn, they push against the road and as a reaction, the road pushes forward on the car. □You would not be able to write down these notes either!
Friction and Acceleration
Air Resistance □Air Resistance: a type of friction □Air Resistance is caused by the interaction between the surface of a moving object and the air molecules □Air Resistance depends on 3 factors: □Size of the object □Shape of the object □Speed of the object
Gravity □Gravity: force of attraction between 2 particles of matter due to their mass □Gravity is different from other forces because: gravity acts on an object even when the objects don’t touch □ All objects in the universe attract each other through the force of gravity!
Gravity □Gravity depends on two factors: □The mass of the objects: the greater the mass of the object, the larger the gravitational force □The distance between the objects: as the distance between 2 objects increases, the gravitational force decreases.
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