# PHYSICS 231 INTRODUCTORY PHYSICS I Lecture 5 Main

- Slides: 35

PHYSICS 231 INTRODUCTORY PHYSICS I Lecture 5

Main points of last lecture • Range (yi=yf): • Relative velocity: • Newton’s Laws: vbe = vbr + vre 1. If 2. 3. S F = 0, velocity doesn’t change.

Newton’s Third Law Force on “ 1” due to “ 2” • Single isolated force cannot exist • For every action there is an equal and opposite reaction • Action and Reaction Forces act on different objects

Free-body Diagram • Draw arrows for all forces acting ON object • If the object is in “equilibrium” (at rest or constant v) • Otherwise, find acceleration by Ignore rotational motion for now. Treat object as a particle.

Example 4. 1 Two blocks sit on a frictionless table. The masses are M 1=2 kg and M 2=3 Kg. A horizontal force F=5 N is applied to Block 1. What is the acceleration of the blocks? 2. What is the force of block 1 on block 2? F 1. a = 1 m/s 2 2. F 21= 3 N M 1 M 2

Mechanical Forces • • • Gravity: w=mg (down) Normal forces Strings, ropes and Pulleys Friction Springs (later)

Rules for Ropes and Pulleys • Force from rope points AWAY from object • (Rope can only pull) • Magnitude of the force is Tension • Tension is same everywhere in the rope • Tension does not change when going over pulley Approximations: Neglect mass of rope and pulley, neglect friction in pulley

Example 4. 2 I pull a 5 kg mass up with a rope, so that it accelerates 2 m/s 2. What is the tension in the rope? T = 59 N

Example 4. 3 - Atwood Machine a) Find acceleration b) Find T, the tension in the string c) Find force ceiling must exert on pulley 5 kg a) a = g/6 = 1. 635 m/s 2 b) T = 57. 2 N c) Fpulley=2 T = 114. 5 N 7 kg

Example 4. 4 a Which statements are correct? Assume the objects are in static equilibrium. T 1 is _____ T 2 A) Less than B) Equal to C) Greater than cos(10 o)=0. 985 sin(10 o)=0. 173

Example 4. 4 b Which statements are correct? Assume the objects are static. T 2 is ______ T 3 A) Less than B) Equal to C) Greater than cos(10 o)=0. 985 sin(10 o)=0. 173

Example 4. 4 c Which statements are correct? Assume the objects are static. T 3 is ______ Mg A) Less than B) Equal to C) Greater than cos(10 o)=0. 985 sin(10 o)=0. 173

Example 4. 4 d Which statements are correct? Assume the objects are static. T 1+T 2 is ______ Mg A) Less than B) Equal to C) Greater than cos(10 o)=0. 985 sin(10 o)=0. 173

Example 4. 4 Given that Mlight = 25 kg, find all three tensions T 3 = 245. 3 N, T 1 = 147. 4 N, T 2 = 195. 7 N

Cable Pull Demo

Inclined Planes • Choose x along the incline and y perpendicular to incline • Replace force of gravity with its components 30. 0

Example 4. 5 Find the acceleration and the tension a = 4. 43 m/s 2, T= 53. 7 N

Example 4. 6 M Find M such that the box slides at constant v M=15. 6 kg

Frictional Forces • RESISTIVE force between object and neighbors or the medium • Examples: • Sliding a box • Air resistance • Rolling resistance

Sliding Friction • Parallel to surface, opposing direction of motion • ~ independent of the area of contact • Depends on the surfaces in contact • Object at rest: Static friction • Object in motion: Kinetic friction

Static Friction, ƒs • Just enough force to keep object at rest. • ms is coefficient of static friction • N is the normal force f F

Kinetic Friction, ƒk • mk is coefficient of kinetic friction • Friction force opposes direction of motion • N is the normal force f F

Coefficients of Friction

Example 4. 7 The man pushes/pulls with a force of 200 N. The child and sled combo has a mass of 30 kg and the coefficient of kinetic friction is 0. 15. For each case: What is the frictional force opposing his efforts? What is the acceleration of the child? f=59 N, a=3. 80 m/s 2 / f=29. 1 N, a=4. 8 m/s 2

Example 4. 8 Given m 1 = 10 kg and m 2 = 5 kg: a) What value of ms would stop the block from sliding? b) If the box is sliding and mk = 0. 2, what is the acceleration? c) What is the tension of the rope? a) ms = 0. 5 b) a=1. 96 m/s 2 c) 39. 25 N

Example 4. 9 What is the minimum ms required to prevent a sled from slipping down a hill of slope 30 degrees? ms = 0. 577

Other kinds of friction • Air resistance, F ~ Area v 2 • Rolling resistance, F ~ v Terminal velocity:

Coffee Filter Demo

Example 4. 9 An elevator falls with acceleration a = 8. 0 m/s 2. If a 200 -lb person stood on a bathroom scale during the fall, what would the scale read? 36. 9 lbs

Accelerating Reference Frames • Equivalent to “Fictitious” gravitational force

Fictitious Force: Derivation Eq. of motion in fixed frame F-maf looks like force in new frame, maf acts like fake gravitational force!

Example 4. 10 You are calibrating an accelerometer so that you can measure the steady horizontal acceleration of a car by measuring the angle a ball swings backwards. If M = 2. 5 kg and the acceleration, a = 3. 0 m/s 2: a) At what angle does the ball swing backwards? b) What is the tension in the string? q q = 17 deg T= 25. 6 N

Example 4. 11 a A fisherman catches a 20 lb trout (mass=9. 072 kg), and takes the trout in an elevator to the 78 th floor to impress his girl friend, who is the CEO of a large accounting firm. The fish is hanging on a scale, which reads 20 lb. s while the fisherman is stationary. Later, he returns via the elevator to the ground floor with the fish still hanging from the scale. In the instant just after the elevator begins to move upward, the reading on the scale will be _______ 20 lbs. a) Greater than b) Less than c) Equal to

Example 4. 11 b A fisherman catches a 20 lb trout (mass=9. 072 kg), and takes the trout in an elevator to the 78 th floor to impress his girl friend, who is the CEO of a large accounting firm. The fish is hanging on a scale, which reads 20 lb. s while the fisherman is stationary. Later, he returns via the elevator to the ground floor with the fish still hanging from the scale. On the way back down, while descending at constant velocity, the reading on the scale will be ________ 20 lbs. a) Greater than b) Less than c) Equal to

Example 4. 11 c A fisherman catches a 20 lb trout (mass=9. 072 kg), and takes the trout in an elevator to the 78 th floor to impress his girl friend, who is the CEO of a large accounting firm. The fish is hanging on a scale, which reads 20 lb. s while the fisherman is stationary. Later, he returns via the elevator to the ground floor with the fish still hanging from the scale. In the instant just before the elevator comes to a stop on the 78 th floor, the mass of the fish will be _______ 9. 072 kg. a) Greater than b) Less than c) Equal to

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