Conceptual Physics Fundamentals

Conceptual Physics Fundamentals Chapter 3: EQUILIBRIUM AND LINEAR MOTION

This lecture will help you understand: • Aristotle on Motion • Galileo’s Concept of Inertia • Mass—A Measure of Inertia • Net Force • The Equilibrium Rule • Equilibrium of Moving Things • The Force of Friction • Speed and Velocity • Acceleration

Equilibrium and Linear Motion “When you’re over the hill, that’s when you pick up speed.” —Quincy Jones

Aristotle on Motion Aristotle’s classification of motion • natural motion • Every object in the universe has a proper place determined by a combination of four elements: earth, water, air, and fire • Any object not in its proper place will strive to get there. Example: stones fall; puffs of smoke rise

Aristotle on Motion • natural motion (continued) • Straight up or straight down for all things on Earth • Beyond Earth, motion is circular. Example: Sun and moon continually circle the Earth. • violent motion • produced by external pushes or pulls on objects Example: Wind imposes motion on ships.

Galileo’s Concept of Inertia Italian scientist Galileo demolished Aristotle’s assertions in early 1500s. Galileo’s discovery: • Objects of different weight fall to the ground at the same time in the absence of air resistance. • A moving object needs no force to keep it moving in the absence of friction.

Galileo’s Concept of Inertia Force • is a push or a pull Inertia • is a property of matter to resist changes in motion • depends on the amount of matter in an object (its mass)

Galileo’s Concept of Inertia CHECK YOUR NEIGHBOR The use of inclined planes for Galileo’s experiments helped him to _______. A. eliminate the acceleration of free fall • discover the concept of energy • discover the property called inertia • discover the concept of momentum

Galileo’s Concept of Inertia CHECK YOUR ANSWER The use of inclined planes for Galileo’s experiments helped him to _______. A. eliminate the acceleration of free fall • discover the concept of energy • discover the property called inertia • discover the concept of momentum Comment: Note that inertia is a property of matter, not a reason for the behavior of matter.

Mass—A Measure of Inertia Mass • a measure of the inertia of a material object • independent of gravity • greater inertia  greater mass • unit of measurement is the kilogram (kg) Weight • the force on an object due to gravity • scientific unit of force is the Newton (N) • unit is also the pound (lb)

Mass—A Measure of Inertia CHECK YOUR NEIGHBOR The concept of inertia mostly involves _______. A. mass • weight • volume • density

Mass—A Measure of Inertia CHECK YOUR ANSWER The concept of inertia mostly involves _______. A. mass • weight • volume • density Comment: Anybody get this wrong? Check the title of this slide! :-)

Mass—A Measure of Inertia CHECK YOUR NEIGHBOR If the mass of an object is halved, the weight of the object is _______. A. halved • doubled • depends on location • none of the above

Mass—A Measure of Inertia CHECK YOUR ANSWER If the mass of an object is halved, the weight of the object is _______. A. halved • doubled • depends on location • none of the above

Mass—A Measure of Inertia Mass and weight in everyday conversation are interchangeable. Mass, however, is different and more fundamental than weight. Mass versus weight • On Moon and Earth • Weight of an object on the Moon isless than on the Earth. • Mass of an object is the samein both locations.

Mass—A Measure of Inertia One Kilogram Weighs 9.8 Newtons. Relationship between kilograms and pounds • 1 kg = 2.2 lb = 9.8 N at Earth’s surface • 1 lb = 4.45 N

Mass—A Measure of Inertia CHECK YOUR NEIGHBOR When the string is pulled down slowly, the top string breaks, which best illustrates the _______. A. weight of the ball • mass of the ball • volume of the ball • density of the ball

Mass—A Measure of Inertia CHECK YOUR ANSWER When the string is pulled down slowly, the top string breaks, which best illustrates the _______. A. weight of the ball • mass of the ball • volume of the ball • density of the ball Explanation: Tension in the top string is the pulling tension plus the weight of the ball; both of which break the top string.

Mass—A Measure of Inertia CHECK YOUR NEIGHBOR When the string is pulled down quickly, the bottom string breaks, which best illustrates the _______. A. weight of the ball • mass of the ball • volume of the ball • density of the ball

Mass—A Measure of Inertia CHECK YOUR ANSWER When the string is pulled down quickly, the bottom string breaks, which best illustrates the _______. A. weight of the ball • mass of the ball • volume of the ball • density of the ball Explanation: It is the “laziness” of the ball that keeps it at rest, resulting in the breaking of the bottom string.

Net Force • Net force is the combination of all forces that change an object’s state of motion. Example: If you pull on a box with 10 N and a friend pulls oppositely with 5 N, the net force is 5 N in the direction you are pulling.

Net Force CHECK YOUR NEIGHBOR A cart is pushed to the right with a force of 15 N while being pulled to the left with a force of 20 N. The net force on the cart is _______. A. 5 N to the left • 5 N to the right • 25 N to the left • 25 N to the right

Net Force CHECK YOUR ANSWER A cart is pushed to the right with a force of 15 N while being pulled to the left with a force of 20 N. The net force on the cart is _______. A. 5 N to the left • 5 N to the right • 25 N to the left • 25 N to the right

Net Force Vector quantity • a quantity whose description requires both magnitude (how much) and direction (which way) • can be represented by arrows drawn to scale, called vectors • Length of arrow represents magnitude, and arrowhead shows direction Examples: force, velocity, acceleration

The Equilibrium Rule The equilibrium rule • the vector sum of forces acting on a non-accelerating object equals zero • in equation form: F = 0

The Equilibrium Rule Example: A string holding up a bag of flour Two forces act on the bag of flour: • tension force acts upward • weight acts downward The forces are equal in magnitude and opposite in direction; when the forces are added they cancel to zero and the bag of flour remains at rest.

The Equilibrium Rule CHECK YOUR NEIGHBOR The equilibrium rule, F = 0, applies to _______. A. vector quantities • scalar quantities • both of the above • neither of the above

The Equilibrium Rule CHECK YOUR ANSWER The equilibrium rule, F = 0, applies to _______. A. vector quantities • scalar quantities • both of the above • neither of the above Explanation: Vector addition takes into account + and - quantities that can cancel to zero. Two forces (vectors) can add to zero, but there is no way that two masses (scalars) can add to zero.

Support Force Support force (normal force) is an upward force on an object that is opposite to the force of gravity. Example: A book on a table compresses atoms in the table, and the compressed atoms produce the support force.

The Support Force CHECK YOUR NEIGHBOR When you stand on two bathroom scales with one foot on each scale, and with your weight evenly distributed, each scale will read _______. A. your weight • half your weight • zero • more than your weight

The Support Force CHECK YOUR ANSWER When you stand on two bathroom scales, with one foot on each scale and with your weight evenly distributed, each scale will read _______. A. your weight • half your weight • zero • more than your weight Explanation: You are at rest on the scales, so F = 0. The sum of the two upward support forces is equal to your weight.

Equilibrium of Moving Things Equilibrium • a state of no change with no net force acting • static equilibrium Example: Hockey puck at rest on slippery ice • dynamic equilibrium Example: Hockey puck sliding at constant speed on slippery ice

Equilibrium of Moving Things Equilibrium test • whether something undergoes changes in motion Example: A refrigerator at rest is in static equilibrium. If it is moved at a steady speed across a floor, it is in dynamic equilibrium.

Equilibrium of Moving Things CHECK YOUR NEIGHBOR A bowling ball is in equilibrium when it _______. A. is at rest • moves steadily in a straight-line path • Both of the above • None of the above

Equilibrium of Moving Things CHECK YOUR ANSWER A bowling ball is in equilibrium when it _______. A. is at rest • moves steadily in a straight-line path • Both of the above • None of the above

The Force of Friction Friction • occurs when objects rub against one another • applies to solids, liquids, and gases • acts in a direction to oppose motion Example: When an object falls down through air, the force of friction (air resistance) acts upward.

The Force of Friction • depends on the kinds of material and how much they are pressed together • is due to tiny surface bumps and to “stickiness” of the atoms on a material’s surface Example: Friction between a crate on a smooth wooden floor is less than that on a rough floor.

The Force of Friction CHECK YOUR NEIGHBOR The force of friction can occur _______. A. with sliding objects • in water • in air • All of the above

The Force of Friction CHECK YOUR ANSWER The force of friction can occur _______. A. with sliding objects • in water • in air • All of the above Comment: Friction can also occur for objects at rest. If you push horizontally on your book and it doesn’t move, then friction between the book and the table is equal and opposite to your push.

The Force of Friction CHECK YOUR NEIGHBOR When Josh pushes a refrigerator across a kitchen floor at a constant speed, the force of friction between the refrigerator and the floor is _______. A. less than Josh’s push • equal to Josh’s push • equal and opposite to Josh’s push • more than Josh’s push

The Force of Friction CHECK YOUR ANSWER When Josh pushes a refrigerator across a kitchen floor at a constant speed, the force of friction between the refrigerator and the floor is _______. A. less than Josh’s push • equal to Josh’s push • equal and opposite to Josh’s push • more than Josh’s push

The Force of Friction CHECK YOUR NEIGHBOR When Josh pushes a refrigerator across a kitchen floor at an increasing speed, the amount of friction between the refrigerator and the floor is _______. A. less than Josh’s push • equal to Josh’s push • equal and opposite to Josh’s push • more than Josh’s push

The Force of Friction CHECK YOUR ANSWER When Josh pushes a refrigerator across a kitchen floor at an increasing speed, the amount of friction between the refrigerator and the floor is _______. A. less than Josh’s push • equal to Josh’s push • equal and opposite to Josh’s push • more than Josh’s push Explanation: The increasing speed indicates a net force greater than zero. The refrigerator is not in equilibrium.

Speed and Velocity Speed • defined as the distance covered per amount of travel time • units are meters per second • in equation form Example: A girl runs 6 meters in 1 second. Her speed is 6 m/s.

Speed and Velocity Average speed • the entire distance covered divided by the total travel time • doesn’t indicate various instantaneous speeds along the way • in equation form: Example: Drive a distance of 80 km in 1 hour and your average speed is 80 km/h.

Speed and Velocity Instantaneous speed is the speed at any instant. Velocity • a description of how fast and in what direction • a vector quantity

Speed and Velocity CHECK YOUR NEIGHBOR The average speed of driving 30 km in 1 hour is the same average speed as driving _______. A. 30 km in one-half hour • 30 km in two hours • 60 km in one-half hour • 60 km in two hours

Speed and Velocity CHECK YOUR ANSWER The average speed of driving 30 km in 1 hour is the same average speed as driving _______. A. 30 km in one-half hour • 30 km in two hours • 60 km in one-half hour • 60 km in two hours

Speed and Velocity • Constant speed is steady speed, neither speeding up nor slowing down. • Constant velocity is constant speed and constant direction (straight-line path with no acceleration). • Motion is relative to Earth, unless otherwise stated.

Acceleration Galileo first formulated the concept of acceleration in his experiments with inclined planes.

Conceptual Physics Fundamentals