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Transcript
Section 12.1, Forces What Is a Force? A force is defined as a push or a pull that acts on an object. –Forces can cause resting objects to move, or accelerate moving objects by changing their speed or direction. –Sometimes forces are easily measured, like the force of gravity is measured by weight. Section 12.1, Forces What is Force? – Newtons are the units we use to measure force. –The abbreviation for the newton is N. –A newton is the amount of force it takes to accelerate one kilogram of mass at a rate of 1 meter per second each second or (1 m/s2) –The newton is named for Sir Isaac Newton. In this example we see how the force of gravity affects the scale. As more fruit is added to the scale, the force increases. Section 12.1, Forces What Is Force? –In many cases, arrows are used to represent both the magnitude and the direction of a force. Section 12.1, Forces Combining Forces –Forces moving in the same direction add together. Example: People pushing a car. The force of each person pushing adds up. –Forces moving in opposite directions subtract from one another. Example: People playing “tug of war” the forces of people pulling on the rope subtract. Section 12.1, Forces Combining Forces –Balanced Forces When the forces acting on an object are balanced, the net force is 0, and there is no change to the objects motion. Notice the arrows on the rope between the players. They are the same length and go in opposite directions. At this time they cancel each other and the net force is equal to 0. The forces are balanced. Section 12.1, Forces Combining Forces –Unbalanced Forces The net force of an unbalanced force is not 0. When an unbalanced force acts on an object, the object accelerates. Net force equals the size of the larger force minus the size of the smaller force. Section 12.1, Forces Friction Friction is a resistance to motion by objects that touch each other. There are four types of friction. –Static Friction – The friction force that acts on objects that are not moving. –Static friction always acts in the direction of the applied force. –The friction between the ground and Section 12.1, Forces Friction Friction is a resistance to motion by objects that touch each other. There are four types of friction. –Static Friction –Sliding Friction –Rolling Friction –Fluid Friction Section 12.1, Forces Static Friction –It acts on objects that are not moving. –Example – the friction that occurs between the ground and the sole of your shoe which keeps you from sliding each time you take a step. Section 12.1, Forces Sliding Friction –The force that opposes direction of motion of an object as it moves over a surface. –Sliding friction is less than static friction because once the object starts moving, it takes less force to keep it moving. –Example: Sliding an object across a table top. Section 12.1, Forces Rolling Friction –When a round object rolls across a flat surface, both the object and the surface are slightly bent out of shape. This change at the point of contact is called rolling friction. –Example: A bowling ball rolling down the lane of a bowling alley. Section 12.1, Forces Fluid Friction –The force of fluid friction opposes the movement of an object through a fluid substance. Remember that gases and liquids are both conisidered to be fluids. –Example: The resistance to motion of a submarine moving through the ocean. –Example: Aircraft experience fluid friction from Earth’s atmosphere. Section 12.1, Forces Gravity –Gravity is a universal attractive force that acts between any two masses. –Earth’s gravitational force attracts objects to its center. –It is important to understand that we know how gravity works and that it exists, but we do not know why it works. Section 12.1, Forces Notice how the supporting force of the bottom rock equals the force exerted on the top rock by gravity. The opposing arrows are the same length, so the net force is 0. Section 12.1, Forces Falling Objects –Air resistance and gravity are the two main forces that affect falling objects. Gravity causes objects to accelerate downward Air resistance acts in the direction opposite the downward motion and reduces the acceleration Section 12.1, Forces Falling Objects (continued) –Terminal velocity is the velocity a falling object reaches when it no longer continues to accelerate. At this point, velocity remains constant. The force of gravity and the force exerted by the air resistance become equal. Notice that the arrow above “Rocky”, representing the force of gravity is longer than the one below him, representing air resistance. What does this tell you about the direction “Rocky’s” flight is taking? Chapter 12 Quiz 1. 2. 3. 4. 5. A _________ is defined as a push or a pull that acts on an object. The units we use to measure force are __________. __________ is a universal attractive force that acts between two masses. A resistance to motion by objects that touch each other is called ________. Name the four types of friction we have studied in this section. Sec. 12.2, Newton’s 1st and 2nd Laws of Motion Aristotle, Galileo, and Newton –These three scientists lived in a time period that covered over 2000 years, but each one was instrumental in developing our modern concepts of force and motion. Sec. 12.2, Newton’s 1st and 2nd Laws of Motion Aristotle, Galileo, and Newton –Aristotle (384 B.C. – 322 B.C.) Incorrectly proposed that force is required to keep an object at a constant speed. Though it held back the progress of motion studies, it gave man something to disprove, which is far better than having nothing at all. Sec. 12.2, Newton’s 1st and 2nd Laws of Motion Aristotle, Galileo, and Newton –Galileo (1564 – 1642) Studied how gravity produces constant acceleration. He concluded that without friction, moving objects would continue to move indefinitely (forever). Sec. 12.2, Newton’s 1st and 2nd Laws of Motion Aristotle, Galileo, and Newton –Sir Isaac Newton (1642 – 1727) Newton is often referred to as England’s greatest scientist. His work included research in gravity, motion, forces, light, and telescopes. Newton even served for several terms in England’s Parliament. Sir Isaac Newton 1642 - 1727 Sec. 12.2, Newton’s 1st and 2nd Laws of Motion Sir Isaac Newton – Newton’s First Law –Newton’s first law of motion: “The state of motion of an object does not change as long as the net force acting on the object is 0.” –This law is sometimes called “The Law of Inertia”. –Put simply, objects at rest tend to stay at rest, while moving objects continue to move with the same speed and direction. Sec. 12.2, Newton’s 1st and 2nd Laws of Motion Sir Isaac Newton - Newton’s 2nd Law –Newton’s second law of motion states that the acceleration of an object is equal to the force acting on it divided by the objects mass. –Acceleration = Net force or a = F mass m Newton’s 2nd law also applies when a net force acts in the opposite direction of the object’s motion, causing deceleration. Sec. 12.2, Newton’s 1st and 2nd Laws of Motion Weight and Mass Sometimes we try to use the terms weight and mass interchangeably, but this is incorrect. –The mass of an object is defined as the amount of matter it contains. –Weight is the force gravity exerts on an object (a mass). W = mg –Or, Weight = Mass x Acc. due to gravity Notice that in the picture below, the weight of the object decreases, the farther it moves from the Earth’s center, but its mass stays the same. Sec. 12.2, Newton’s 1st and 2nd Laws of Motion Weight and Mass –It would also be correct to state that “Mass is a measure of the inertia of an object and weight is a measure of the force of gravity acting on an object. Section 12.2 Quiz 1. Newton gave us _______ (a number) laws of motion. 2. The motion of an object does not change as long as the net force acting on the object is ____ (a number). 3. The acceleration of an object is equal to the net force acting on it divided by its ________. 4. The force of gravity acting on an object (matter) is its _________. Section 12.2 Quiz 5. An object has a mass of 10 grams and is accelerating at a rate of 9.8 cm/s2. What is its weight? Getting Started Quiz #5 1. Perform the indicated operations in scientific notation. 9.5 x 1020 + 5 x 1021 (6 x 1015)(3 x 105) Getting Started Quiz #5 2. A gas at a temperature of 27 oC has a volume of 6 L. The volume decreases to 3 L. What is the new temperature? Balance the following equations and name the type of reaction taking Place. –NaOH + CO2 NaCO3 + H20 –MgCl2 + AgNO3 Mg(NO3)2 + 2AgCl Section 12.3, Newton’s Third Law Newton’s Third Law –Newton’s third law involves motion and momentum. –Newton’s third law states that whenever one object exerts a force on a second object, the second object exerts an equal and opposite force on the first. Section 12.3, Newton’s Third Law Newton’s Third Law –Action and Reaction Forces When you press your hand against the wall, you exert a force against it. The wall, in turn, exerts an equal and opposite force against your hand. Your hand does not go through or move the wall and the wall does not change the position of your hand. Section 12.3, Newton’s Third Law Newton’s Third Law –Action-Reaction Forces and Motion All action-reaction forces do not cancel. Some examples are: –The thrust of a jet engine pushing against the air and moving the plane forward. –An oar from a rowboat pushing against the water and moving the boat forward. In these examples acceleration occurs and the net force is not 0. Section 12.3, Newton’s Third Law Newton’s Third Law –Momentum Momentum is an objects mass multiplied by its velocity. Momentum = Mass x Velocity Momentum is expressed in kg•m/s –Kg = the mass –m/s = the velocity Section 12.3, Newton’s Third Law Newton’s Third Law –Conservation of Momentum If we view momentum through a closed system, momentum is conserved. –A closed system is one in which the only active forces are with the colliding cars. Forces such as gravity, friction, wind resistance, etc. are not a part of the system, so aren’t considered in the calculations. Section 12.3, Newton’s Third Law Newton’s Third Law –The Law of Conservation of Momentum In a closed system, the loss of momentum of one object equals the gain in momentum of another object, therefore momentum is conserved. Take friction and wind resistance out of the system and think of a billiard ball striking another billiard ball. Section 12.3, Newton’s Third Law Newton’s Third Law Section 12.4, Universal Forces Electromagnetic Forces Electric and magnetic forces are the only forces that can both attract and repel. Section 12.4, Universal Forces Electric Forces –Two positively charged particles will repel each other. –Two negatively charged particles will repel each other. –A positively charged and a negatively charged particle will attract each other. Section 12.4, Universal Forces Magnetic Forces –Magnetic forces act on certain metals, on the poles of magnets and on moving charges. –Two like poles on a magnet repel. N N or S S –Two opposite poles of a magnet attract. N S Earth’s Magnetic Field In Space Section 12.4, Universal Forces Nuclear Forces –Strong Nuclear Force A strong nuclear force that acts only on protons and neutrons in the nucleus It holds the protons and neutrons together in the nucleus. It has a very short range of only about 10-15 of a meter. Section 12.4, Universal Forces Nuclear Forces –Weak Nuclear Force Weaker in strength than the strong nuclear force. Also an attractive force that acts over a short range (about 10-18M). Section 12.4, Universal Forces Gravitational Force Gravitational force is a universal attractive force that acts between any two masses. Newton’s law of universal gravitation states that every object in the universe is attracted to every other object. Notice that the strength of the gravitational force depends on the mass and distance between objects. Section 12.4, Universal Forces Gravitational Force –Gravity Acts Over Large Distances Gravitational force between two objects is proportional to their masses and decreases rapidly with distance. –As the distance between objects doubles, their gravitational forces become ¼ as strong. –Gravity is the weakest Universal Force, but the most effective over long distances. Section 12.4, Universal Forces Gravitational Force –Earth, Moon, and Tides Earth’s moon has the same general gravitational relationship to Earth as Earth does to the sun. The same gravity/inertia relationship exists between Earth and moon as exists between Earth and Sun. Section 12.4, Universal Forces Gravitational Force Earth, Moon, and Tides –The Moon and Sun, both exert a gravitational force on Earth causing tides to form. –How great the “tidal range” (difference in height between high and low tide) is depends on how Earth, Moon, and Sun are alligned. When the Moon and Sun are at a 90o angle relative to Earth, their gravitational forces act against each other producing the lowest tidal ranges. The Sun and Moon both affect Earth’s tides. When they are aligned, their combined gravitational forces produce the widest tidal ranges.. Section 12.4, Universal Forces Gravitational Forces Satellites –The same principles of gravity and inertia work to keep satellites in orbit around our planet. –There are many manmade satellites around our planet used for purposes such as weather tracking, defense communications, mapping, and many others. Section 12.4, Universal Forces