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Chapter 6 Forces

Chapter 6 Forces. Quiz 6. Objectives Chapter 6. Define a force and differentiate between contact forces and long-range forces Recognize the significance of Newton's second law of motion and use it to solve motion problems

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Chapter 6 Forces

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  1. Chapter 6 Forces Quiz 6

  2. Objectives Chapter 6 • Define a force and differentiate between contact forces and long-range forces • Recognize the significance of Newton's second law of motion and use it to solve motion problems • Explain the meaning of Newton's first law and describe an object in equilibrium

  3. Objectives Chapter 6 • Describe how the weight and the mass of an object are related • Differentiate between the gravitational force weight and what is experienced as apparent weight • Define the friction force and distinguish between static and kinetic friction • Describe simple harmonic motion and explain how the acceleration due to gravity influences such motion

  4. Objectives Chapter 6 • Explain the meaning of interaction pairs of forces and how they are related by Newton's third law • List the four fundamental forces and illustrate the environment in which each can be observed • Explain the tension in ropes and strings in terms of Newton's third law

  5. Following Slides Just for Fun “May the Force be with you young Physics Student”

  6. Historical Perspective • Aristotle: Motion into 2 categories, Natural and Violent • Natural: Objects seek their natural motion. Either up or down. A ball comes to rest because it is its natural motion. • Violent: Imposed motion, via force. Responsible for things moving against • The earth’s natural motion: Not moving, no force big enough to move it.

  7. Galileo and Copernicus • Copernicus said that the Earth revolved around the sun (Contrary to Aristotle who thought the Earth was too big to move) • Galileo countered that it doesn’t take a force for an object to keep moving • Changed the notion that it takes a force to keep something moving

  8. Galileo • Galileo: College prof Galileo, not really aware of politics, first in Venice. One of his Benefactors, a cardinal, good friend who gave lots of money, become pope. Galileo writes his book about Aristotle verse Galileo. Different than normal science books, written like a play. 3 main characters, Simplisico (Greek), Salvatory (Galileo), and Segacio (Impartial judge). Debate. Ultimate problem, the pope had a preference to the greek way. Galileo discovered new things. So into Simplisico’s mouth, he doesn’t speek well, argument not well constructed, his name means simple (Idiot or moron). Idiot says! Sage, the wisemen is won over to salvatory. Salvatory does a good job with argument. So idio says what pope thinks, and hero says what Galileo thinks. Galileo gets in trouble for mocking pope. They were friends, and Galileo makes fun of pope. Pope asks him not to publish it, Galileo does it anyway. Gets put under house arrest after inquisition, doesn’t really matter, Galileo is old!

  9. Terminology • Force: Any push or Pull • Net Force: Combination of all forces acting on object. If not zero the object will change speed • Friction: Force that acts between objects as they move past each other. • Tension/Compression: Pulling apart of atoms or pushing together of atoms

  10. TensionionalvsCompressional • Tension is going to test the strength of the atoms to hold onto one another by being pulled apart • Compression is going to test the ability of the atoms to compress together (form a closer bond) without sliding past one another (too close and repulsion gets too big leading to break)

  11. Isaac Newton • Three Laws he discovered • 1st Law: Every object continues in a state of rest, or of motion in a straight line at a constant speed, unless it is compelled to change that state by forces exerted upon it. • Law of Inertia

  12. 1st Law • Unless acted upon by an unbalanced force, this golf ball would sit on the tee forever.

  13. Newtons’s 1st Law and You Don’t let this be you. Wear seat belts. Because of inertia, objects (including you) resist changes in their motion. When the car going 80 km/hour is stopped by the brick wall, your body keeps moving at 80 km/hour.

  14. Isaac Newton • 2nd Law: Forces cause acceleration • The net force of an object is equal to the product of its mass and acceleration, or F=ma. • Only forces which act on that object affect the acceleration of the object.

  15. Newton’s 2nd Lawshows that different masses accelerate to the earth at the same rate, but with different forces. • We know that objects with different masses accelerate to the ground at the same rate. • However, because of the 2nd Law we know that they don’t hit the ground with the same force. F = ma 98 N = 10 kg x 9.8 m/s/s F = ma 9.8 N = 1 kg x 9.8 m/s/s

  16. If mass remains constant, doubling the acceleration, doubles the force. If force remains constant, doubling the mass, halves the acceleration.

  17. Isaac Newton • 3rd Law • For every force, there is an equal and opposite force • Only true in certain situations

  18. Examples of Newton’s Third Law • The baseball forces the bat to the left (an action); the bat forces the ball to the right (the reaction).

  19. 3rd Law • Consider the motion of a car on the way to school. A car is equipped with wheels which spin backwards. As the wheels spin backwards, they grip the road and push the road backwards.

  20. 3rd Law The reaction of a rocket is an application of the third law of motion. Various fuels are burned in the engine, producing hot gases. The hot gases push against the inside tube of the rocket and escape out the bottom of the tube. As the gases move downward, the rocket moves in the opposite direction.

  21. 3rd Law • Only applicable if objects act on each other • If objects act on an intermediate object, forces do not have to be balanced • Tug of War: Force on rope not balanced on each side

  22. Mass vs Weight • Mass is how much matter you have • Your mass does not change when you change location • Weight is a measurement of the forces acting on you • Your weight can change when you change location

  23. Weightlessness • Apparent weight is the weight you actually feel • In an elevator that is accelerating, your apparent weight changes • When you accelerate against gravity, you feel heavier • When you accelerate with gravity, you feel lighter • On different surfaces, your weight changes • Weigh yourself on carpet vs hardwood vs cushion

  24. Forces Present • Four major Forces to be aware of that act in this world (from strongest to weakest) • 1) The Strong Nuclear Force: Holds the nucleus of atoms together. Small range (nucleus). Only effective at ranges less than 1-15meters. Without this force, the electromagnetic forces between the protons would not allow for the nucleus to exist at all.

  25. Forces Present • 2) Electromagnetic: Causes attraction or repulsion, depending on charge of objects. This force is roughly 140times stronger than gravity. This force has infinite range

  26. Forces Present • 3) The Weak Nuclear Force: Is responsible for nuclear decay. This force, with a slightly small range of 1-17meters, cause emissions to occur from the nucleus.

  27. Forces Present • 4) Gravity: Very weak force, but like electromagnetic, it works over infinite distances. The only difference is it can only attract, not repel like the previous.

  28. Long Range Forces • Electromagnetic and Gravity are long range forces, they exert their force over the entire universe. The objects do not need to be in contact for the force to exist • Example: Objects falling to earth • Electrons being repelled by one another

  29. Contact Forces • When objects are in direct contact with one another (again, this is kind of weird because the objects really don’t touch), that is a contact force • Pushing a wheelbarrow • Skidding/sliding on a surface and slowing down due to friction (a contact force)

  30. Friction • A contact force which always opposes the motion of the object. Friction can be caused by gravity (object is being pulled to earth and that causes the objects to rub past one another) or by two objects being forced past one another • At the molecular level, friction is an electromagnetic force

  31. Normal Force • This is the force which acts perpendicular to the weight of the object and supports the weight of the object • Normal Force: Normal means at right angles to.

  32. How Large? • On a flat surface, the Normal force is equal to the weight of the object • Weight = mg (mass x gravity) • A person who has a mass of 50 kg has a weight (on earth) of 490 N

  33. Question Check • What is the effect of friction on a moving object? • Slows motion, always opposes the direction of motion • Galileo found that a ball rolling down one incline will pick up enough speed to roll up another. How high up will it roll compared to the initial incline?

  34. Question Check • The law of inertia states that no force is required to maintain motion. Why then do you have to keep pedaling a bike to keep it going? • Friction • What is the weight of 2.0 kg of yogurt? • 19.6 N

  35. Question Check • An elephant and a mouse would both have zero weight in gravity-free space. If they were moving toward you with the same speed, would they bump into you with the same effect? Explain • No, the elephant has much more inertia

  36. Question Check • A massive ball is suspended on a string and slowly pulled by another string attached to it from below

  37. Question Check • Is the string tension greater in the upper or the lower string? Which string is more likely to break if pulled slowly from below? • What if the string on the bottom is snapped quickly (pulled quickly)?

  38. Friction in Greater Detail • Coefficient of Static Friction: Us • Us  How hard is it to slide the object past another (how much force) starting from rest? • It is a % of the total weight of the object • Us(N)=F(static) • N is weight of object, Us is some number usually between 0 and 1, but numbers as high as 5 are seen in drag racing

  39. Coefficient of Static Friction • This value corresponds to the atoms falling into grooves with one another, and the force required to pull them past one another • A small Us value is associated with objects that slide easily from rest (object on Ice) to very large values (examples will vary based off two surfaces in contact)

  40. Question • An object with a mass of 15 kg is resting on a table top. If it takes 22 N to get the object to start moving, what is the value of Us between the table top and the object? • If I place another 15 kg mass on top of the object, will the force required to start the object increase? Will the Us value change?

  41. Coefficient of Kinetic Friction • When an object is moving (Relative to the surface that it is in contact with) the friction is said to be Kinetic. • This value is generally much smaller than the Static Friction value because the molecules jump from mountain to mountain

  42. Practice Questions • An object has a weight of 88 N and has a Us value of 0.3. What force is required to start moving the object (from rest) along the surface (pushing horizontally)? • That object has another object placed on top, what happens to the • Us value? • Static Friction value

  43. Practice Questions • An object at rest has a weight of 800 N and requires 200 N of horizontal push to overcome static friction. What is the Us Value between the object and the floor? • If the Uk value is half the value of the Us, what acceleration does the object undergo when it starts to move?

  44. Uk • In the same token, Uk(N) = Kinetic Friction • Question: An object that has a weight of 50 N has a Us value of 0.20 and a Uk value of 0.10. The object is at rest on a table. • A) How much force is required to start moving the Object? • B) What is the net force acting on the object as soon as it starts moving?

  45. Question • An elevator with mass of 1200 kg is supported by a cable. If three people enter the elevator with a total mass of 190 kg, what is the tension in the cable in N?

  46. The Quiz • Questions concerning Frictional Forces (Us/Uk) calculations, thinking about the force of friction • Remember, NET forces cause acceleration, balanced forces cause constant speed • Questions over types of forces • Some weight and mass questions • Some questions concerning Newton’s 3 Laws

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