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Forces. History. Archimedes was the first to write about forces. Galileo envisioned a force as a push or a pull. Newton developed equations dealing with forces and motion. History. Modern physics includes the study of extremes in gravity, speed, and size. Force.
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History • Archimedes was the first to write about forces. • Galileo envisioned a force as a push or a pull. • Newton developed equations dealing with forces and motion.
History • Modern physics includes the study of extremes in gravity, speed, and size.
Force • is best described as a push or a pull • acts in a single direction with a certain size or magnitude
Vectors are labeled with a boldface F or F. Force • is a vector quantity since it has size and direction
Force • The SI unit for force is the newton (N). • In this course, force is always outside the object (external).
Two Categories of Force • Contact • Field
Contact Forces (mechanical forces) a force between systems that acts only when one system touches another
Compression Compression Tension Tension Torsion Torsion Shear Shear
Field Forces (action-at-a-distance forces) a force which acts between objects that are not touching
Magnetic Magnetic Gravitational Gravitational Electric Electric
Field Forces Although gravitational force is exerted by all objects, it only becomes significant if the object is large like an asteroid, moon, or planet.
Field Forces • Do not confuse these field forces with the four fundamental forces: • strong nuclear • electromagnetic • weak nuclear • gravitational
Fundamental Forces Strong Nuclear Forces p n p n
Fundamental Forces Electromagnetic Forces
Fundamental Forces Weak Nuclear Forces Quarks U - “up” D - “down” proton
Fundamental Forces Gravitational Force
Balanced Forces • occur when the pushes and pulls cancel each other out • result in a total net force of zero
Balanced Forces FB FA no acceleration
Unbalanced Forces • occur when one force is larger than the rest • act in the direction of the larger force • result in a change in motion
Unbalanced Forces Fnet = FB - FA Fnet FA FB acceleration
Weight • is the result of the earth’s gravitational attraction on an object • is directly proportional to mass
Directly Proportional When one value goes up, the other does also.
250 200 150 Weight (N) 100 50 0 0 5 10 15 20 Mass (kg) Slope: 9.81 N/kg
weight(in N) mass (in kg) proportionality constant Weight • is measured in newtons since it is a force w = k m The earth’s proportionality constant is 9.81 N/kg.
Sample Problem 1 If a car has a mass of 2123 kg, what is its weight in newtons? w = km w = 9.81 N/kg × 2123 kg
Sample Problem 1 Remember in elementary school, you solved: 3 × 4 = 12 Now solve: 9.81 N/kg × 2123 kg = 20826.63 N
Sample Problem 1 If a car has a mass of 2123 kg, what is its weight in newtons? w = km w = 9.81 N/kg × 2123 kg w = 20826.63 N w = 20,800 N 3 Sig Digs
The tallest building in New York has a weight of 3,248,311,039 N. What is its mass in kg? • 31,865,931,292.59 kg • 3,248,311.039 kg • 331,122,430.1 kg • 331,122.4301 kg
Weight is directly proportional to _____ • mass. • the proportionality constant. • balanced forces. • magnitude.
Who envisioned a force as a push or a pull? • Archimedes • Einstein • Newton • Galileo
Which of the following is out of place? • Shear • Compression • Magnetic • Torsion
Early Thoughts on Motion • From the time of the ancient Greeks to the Medieval age, scientists believed that things tended to stop if left to themselves. This is true, but does it mean that objects stop by themselves?
Early Thoughts on Motion • Galileo concluded that objects have no natural tendency to stop. • In order for an object to stop, it must be affected by some outside influence. • He called this property inertia.
Newton’s Laws of Motion • Newton had clocks that were accurate enough to make time measurements. • He extended Galileo’s studies and formulated his laws of motion.
First Law—Inertia • Objects at rest remain at rest. • Objects in motion continue in that motion. • Both of these statements change if a net unbalanced external force acts on the object.
First Law—Inertia • We cannot measure inertia directly. • We measure it indirectly by measuring the mass of an object.
First Law—Inertia • If no net force acts on an object, it is in equilibrium. • If any unbalanced force acts on an object, it will change its motion. • The force can be a contact force or a field force or a combination of both.
Summary: Summary: First Law—Inertia • Inertia is the property of all objects to have an unwilling-ness to change motion. • The more mass an object possesses, the more inertia it has.
Second Law—Accelerated Motion This law states the relationship between force, mass, and acceleration in the form of an equation. F = ma
Second Law—Accelerated Motion If both sides are divided by m F = ma m m F we get . a = m
Second Law—Accelerated Motion If both sides are divided by a F = ma a a F we get . m = a
increases increases Second Law—Accelerated Motion F = m a • If the force increases on an object (constant mass), what happens to the acceleration? Acceleration increases. This is a direct relationship.
increases increases Second Law—Accelerated Motion F = m a This is why a more powerful engine makes a vehicle go faster.
increases decreases Second Law—Accelerated Motion F = m a • If the mass of the object increases (constant force), then the acceleration … decreases. This is an inverse relationship.
increases decreases Second Law—Accelerated Motion F = m a An inverse relationship occurs when one variable going up causes another variable to go down.
increases decreases Second Law—Accelerated Motion F = m a This is why a truck with a load accelerates more slowly than when it is empty.
Second Law—Accelerated Motion F = m a If there is no net (unbalanced) force on the object, it does not accelerate.