Physics- using a small number of basic concepts, equations, and assumptions to describe the physical world

Transcription

1 pd 3 concepts Projectile Trajectory Projectile Motion- the curved path that an object follows when launched, or otherwise projected near the earth s surface. Projectiles follow parabolic trajectories Projectile motion is free fall with an initial horizontal velocity Formulas (when falling from rest) Vyf = ay t Vyf^2 = 2ay y y= ½ay( t)² x = vx t When an object reaches its highest point in a projectile or trajectory its vertical velocity is zero Projectiles launched at an angle The projectile s initial velocity will have a vertical and a horizontal component First thing you do is find the components! Formulas Vix= vi cosθ Viy= vi sinθ x=(vix) t t= x/(vix) Vyf= Viy + ay t y= (Viy) t +½ay( t)² Physics- using a small number of basic concepts, equations, and assumptions to describe the physical world The Seven Areas of physics: 1. Mechanics- motion and its causes, interactions between objects Ex: falling objects, friction, weight, spinning objects 2. Thermodynamics- heat and temperature. Ex: melting and freezing processes, engines, refrigerators 3. Vibrations and Wave Phenomena- specific types of repetitive motions Ex: springs, pendulums, sound 4. Optics- light Ex: mirrors, lenses, color, astronomy 5. Electromagnetism- electricity, magnetism, and light Ex: electrical charge, circuitry, permanent magnets, electromagnets Precision- the degree of exactness of a measurement Accuracy- a description of how close a measurement is to the correct or accepted value of the quantity measured

2 Significant figures- a common convention used in science to indicate precision zeros between other nonzero digits are significant Zeros in front of nonzero digits are not significant. Zeros that are at the end of a number and also to the right of the decimal are significant. Zeros at the end of a number but to the left of a decimal are significant if they have been measured or are the first estimated digit; otherwise, they are not significant. Rules for Calculating with Sig Figs: Addition or Subtraction- Given that addition and subtraction take place in columns, round the final answer to the first column from the left containing an estimated digit Multiplication or division- The final answer has the same number of significant figures as the measurement having the smallest number of significant figures. Rounding sig figs: Round down- whenever the digit following the last significant figure is a 0, 1,2,3,or 4 if the last significant figure is an even number and the next digit is a 5, with no other nonzero digits Round up- whenever the digit following the last significant figure is a 6,7,8,or 9 if the digit following the last significant figure is a 5 followed by a nonzero digit if the last significant figure is an odd number and the next digit is a 5, with no other nonzero digits SI is the standard measurement system for science Base units: m, kg, s Derived units Force= mass x acceleration N= kg x m/s^2

3 giga G mega M kilo k hecto h deca da deci d centi c milli m micro µ nano n pico p femto f Two Cables with the Same Angle: Sign Concepts *T1y + T2y = Fg, so each y component is half of Fg *T1x = T2x because there is no acceleration of the object in either direction Tension with One Straight Cable: *T1y = Fg, because there is only one y-component

4 *T1x = T2x because there is no acceleration of the object in either direction Different Angles: *T1x = T2x because there is no acceleration in either direction * T1y + T2y = Fg, but each is not exactly half of Fg Friction Concepts *Friction is proportional to: Surfaces (smoother surface, less friction) Weight (heavier object, more friction) o Fn (Fnormal) is how much an object pushes on the object and increases the amount of friction The coefficient of friction u (mu) affects the size of friction (larger coefficient, increased friction) Surface area does not affect friction *Equation for force of friction: Ff = Uk * Fn- kinetic friction Ff = Us * Fn- static friction *Static U values are larger on an object because, when stationary, the surfaces somewhat stick together at contact points. This adhesion is caused by electrostatic forces between molecules of the two surfaces 2-D Force & Rocket Problem 2-D Force The normal force is the same as the force of gravity on a flat surface. The normal force = the force of gravity when the y acceleration is 0. The normal force is perpendicular to the surface. The weight of an object is the mass multiplied by 9.8. Free body diagrams use vectors to show the magnitudes of forces acting on an

5 object. How To Solve a 2-D Force Problem 1. Draw a free body diagram. 2. Use vector addition to add or subtract the vectors. (If force of gravity and normal force are the same, they cancel each other out.) Rocket Problem When no forces are acting upon an object, it is in a state of equilibrium. When thrust force down is exerted on an object that is already moving horizontally, the object will move downward, but it will also continue to move horizontally as well. Newton s First Law: an object at rest, remains at rest, and an object in motion continues in motion with constant velocity unless the object experiences a net external force.

6 Hockey Puck Problem: A hockey puck has been hit with a force and now is traveling on the ice. What will happen to the puck? 1. Draw Free Body Diagram 2. According to Newton s First Law, inertia is defined to be the resistance an object has to a change in its state of motion. For this reason, there is no forward force in the diagram. 3. The force of friction acts in the opposite direction of the original force applied to the hockey puck. 4. The force of friction causes the acceleration to be negative and therefore slows the hockey puck until it stops. Solving the Equation for Distance Given: Vi, mass Solving for acceleration Fnet=ma -Ff=ma -Ffm=a Solving for Friction Fnet=ma (a=0 therefore ma=0) Fg-FN=0 FN=Fg Ff=ukFN Solving For Time Vf=Vi+at (Vf equal 0) -Via=t Solving the Distance Plug in all the values previously found into: d=vit+12at2

7 1d Force Newton s 2nd Law: Fnet = ma Concepts of 2nd Law: * Greater mass = smaller acceleration * Smaller mass = greater acceleration 2 Equations: Fp - Ff = ma Horizontal Fn - Fg = ma Vertical * When velocity is constant, a = 0 m/s^2 * Units of force are Newtons (N) or kg/m/s^2 Inclined Plane Concepts: * Fg = ma *Fnormal = Fperpendicular -cos(theta) X Fg *Fparallel -sin(theta) X Fg *If angle is less than 45, then Fparallel is smaller than Fperpendicular