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PHYSICS 1L03 (115)
Reza Nejat (43)
Lecture

# 1LO3_Forces.docx

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School
Department
Physics
Course
PHYSICS 1L03
Professor
Reza Nejat
Semester
Winter

Description
Types of Forces Types of Forces Fundamental Forces  There are only four types of forces in the Universe: Gravity, Electromagnetism, Weak Nuclear and Strong Nuclear.  Most physicists believe that all four of these forces are actually different manifestations are actually different manifestations of a single underlying force.  Electric forces and magnetic forces were not unified until the early part of the 20 century. Description of Forces  Gravity: the result of an object having mass. o Holds planets and stars together, makes things fall.  Electromagnetism: the force between charges. o Responsible for all familiar “everyday” forces (except gravity)  Weak Nuclear Forces: known for radioactive decay, where a neuron in an atom spontaneously decays into a proton, releasing a high-energy electron and an antineutrino. o Changes one flavour of quark into another.  Strong Nuclear Forces: responsible for holding atomic nuclei together. o Most of the elements on the periodic table have atomic nuclei with multiple protons. The protons are positively charged and are extremely close together.  Since like charges repel each other, and the strength of the repulsion increases as the separation decreases, one should expect that the nucleus of an atom will break apart. Relative Strengths Gravity  Relative Strenght:1  Range: Infinite Electromagnetism  Relative Strength: 106  Range: Infinite Weak Nuclear Forces  Relative Strength: 105  Range: 1/1000 the diameter of a typical atomic nucleus Strong Nuclear Forces  Relative Strength: 108  Range: Diameter of a typical atomic nucleus Gravitational Force  Occurs between any two objects that have mass. The more massive an object is, the more it will attract other massive objects.  Doubling the mass of one of the objects will increase the gravitational force by a factor of two. Whereas, increasing the separation by a factor of two will reduce the gravitational force by a factor of four. o Explains why the Sun, which is far more massive than the Earth, exerts far less gravitational force on us.  Since the force of gravity between two objects depends on the masses of both of these objects, the gravitational force that the Earth is pulling down on you with is equal to the gravitational force that you are pulling up on the earth with. o When you jump, the fact that you fall back towards the Earth more than it falls up towards you tells you not that the Earth exerts more force on you than you do on the Earth, but that the force is more effective at moving you because of your smaller inertial mass. o To quantitatively calculate the force of gravity, a constant of proportionality must be introduced. The symbol used for this constant of proportionality is a capital G, and it is referred to as the Gravitational Constant. It is a constant for all matter in the Universe, and it has been constant for a t least the last few billion years. 1 Types of Forces  Formula:  G is a constant of proportionalit1 m an2 m are the two masses.  R is the separation between the masses. Electrostatic Force  Electromagnetism includes forces generated by the motion of charged particles. However, the electrostatic force is the electromagnetic force that occurs between any two stationary, charged objects. o There is a significant similarity between the electrostatic force and the gravitational force, but the electrostatic force acts between charged objects. The more charge an object has, the more it will interact with other charged objects.  An important distinction between the gravitational and the electrostatic force becomes apparent at this point. Charge can be either negative (electron) or positive (proton).  When the charges are either bot positive or both negative, the resulting force is repulsive. But when one of the charges is negative and the other is positive, the resulting force is attractive. o This differs from the gravitational force, which always produces an attractive force (even with anti- matter).;  The inverse-squared relationship between the electrostatic force and separation is exactly equivalent to the inverse squared relationship between the gravitational force and the separation. o So if you double the distance between two charged objects, you reduce the electrostatic force by a factor of four.  Formula:  k is a constant of proportiona1ity q2and q are the two charges.  r is the separation between the objects. Forces of Convenience  Tension Force: String, rope, wire…  Spring Force: Elasticity returns an object to its original shape  Normal Force: Not that’s we’re calling other forces abnormal  Friction: When one surface moves with respect to another  Drag: Air resistance, fluid resistance. Tension Force  This type of tension isn’t responsible for your headache. This type of tension refers to a pulling force transferred from one end of a medium to the other end of the medium. o The medium can be a rope, a string, a steel I-beam or a human muscle.  Tension is a pull, not a push. A push would cause compression, which is fine for a steel I-beam, but a compression force cannot be transferred from one end of a string to the other.  Tension is caused by either elastic or inelastic deformation of molecular bonds. In this way, the electrostatic force is fundamentally responsible for this force. o The difference between elastic deformation and inelastic deformation comes down to a simple detail.  In the case of a pulling on a rope that doesn’t break, the
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