Chapter 4 Forces and Newtons Laws of Motion

Chapter 4 Forces and Newton’s Laws of Motion MFMc. Graw PHY 1401 - Ch 04 - Revised: 2/3/2010 1

Forces and Newton’s Laws of Motion • Forces • Newton’s Three Laws of Motion • The Gravitational Force • Contact Forces (normal, friction, tension) • Application of Newton’s Second Law • Apparent Weight MFMc. Graw PHY 1401 - Ch 04 - Revised: 2/3/2010 2

Net Force The net force is the vector sum of all the forces acting on a body. The net force is the resultant of this vector addition. Bold letters represent vectors. The units of Force are Newtons, or the abbreviation N, which represent the SI units: kg-m/s 2 MFMc. Graw PHY 1401 - Ch 04 - Revised: 2/3/2010 3

Free Body Diagrams The free body diagram (FBD) is a simplified representation of an object, and the forces acting on it. It is called free because the diagram will show the object without its surroundings; i. e. the body is “free” of its environment. We will consider only the forces acting on our object of interest. The object is depicted as not connected to any other object – it is “free”. Label the forces appropriately. Do not include the forces that this body exerts on any other body. The best way to explain the free body diagram is to describe the steps required to construct one. Follow the procedure given below. (1) Isolate the body of interest. Draw a dotted circle around the object that separates our object from its surroundings. (2) Draw all external force vectors acting on that body. (3) You may indicate the body’s assumed direction of motion. This does not represent a separate force acting on the body. (4) Choose a convenient coordinate system. MFMc. Graw PHY 1401 - Ch 04 - Revised: 2/3/2010 4

Free Body Diagram y N 1 The force directions are as indicated in the diagram. The magnitudes should be in proportion if possible. F T x w 1 MFMc. Graw PHY 1401 - Ch 04 - Revised: 2/3/2010 5

Newton’s First Law of Motion: Inertia and Equilibrium Newton’s 1 st Law (The Law of Inertia): If no force acts on an object, then the speed and direction of itsmotion do not change. Inertia is a measure of an object’s resistance to changes in its motion. It is represented by the inertial mass. MFMc. Graw PHY 1401 - Ch 04 - Revised: 2/3/2010 6

Newton’s First Law of Motion If the object is at rest, it remains at rest (velocity = 0). If the object is in motion, it continues to move in a straight line with the same velocity. No force is required to keep a body in straight line motion when effects such as friction are negligible. An object is in translational equilibrium if the net force on it is zero and vice versa. Translational Equilibrium MFMc. Graw PHY 1401 - Ch 04 - Revised: 2/3/2010 7

Newton’s Second Law of Motion Net Force, Mass, and Acceleration Newton’s 2 nd Law: The acceleration of a body is directly proportional to the net force acting on the body and inversely proportional to the body’s mass. Mathematically: This is the workhorse of mechanics MFMc. Graw PHY 1401 - Ch 04 - Revised: 2/3/2010 8

Newton’s Second Law of Motion An object’s mass is a measure of its inertia. The more mass, the more force is required to obtain a given acceleration. The net force is just the vector sum of all of the forces acting on the body, often written as F. If a = 0, then F = 0. This body can have: Velocity = 0 which is called static equilibrium, or Velocity 0, but constant, which is called dynamic equilibrium. MFMc. Graw PHY 1401 - Ch 04 - Revised: 2/3/2010 9

Newton’s Third Law of Motion Interaction Pairs Newton’s 3 rd Law: When 2 bodies interact, the forces on the bodies, due to each other, are always equal in magnitude and opposite in direction. In other words, forces come in pairs. Mathematically: designates the force on object 2 due to object 1. MFMc. Graw PHY 1401 - Ch 04 - Revised: 2/3/2010 10

Types of Forces Contact forces: Normal Force & Friction Tension Gravitational Force MFMc. Graw PHY 1401 - Ch 04 - Revised: 2/3/2010 11

Contact Forces Contact forces: these are forces that arise due to of an interaction between the atoms in the surfaces of the bodies in contact. MFMc. Graw PHY 1401 - Ch 04 - Revised: 2/3/2010 12

Normal Forces Normal force: this force acts in the direction perpendicular to the contact surface. N Normal force of the ground on the box N w Normal force of the ramp on the box w MFMc. Graw PHY 1401 - Ch 04 - Revised: 2/3/2010 13

Normal Forces y Example: Consider a box on a table. N FBD for box x w Apply Newton’s 2 nd law This just says the magnitude of the normal force equals the magnitude of the weight; they are not Newton’s third law interaction partners. MFMc. Graw PHY 1401 - Ch 04 - Revised: 2/3/2010 14

Frictional Forces Friction: a contact force parallel to the contact surfaces. Static friction acts to prevent objects from sliding. Kinetic friction acts to make sliding objects slow down. Sometimes called Dynamic friction. MFMc. Graw PHY 1401 - Ch 04 - Revised: 2/3/2010 15

Frictional Forces MFMc. Graw PHY 1401 - Ch 04 - Revised: 2/3/2010 16

Tension This is the force transmitted through a “rope” from one end to the other. An ideal cord has zero mass, does not stretch, and the tension is the same throughout the cord. MFMc. Graw PHY 1401 - Ch 04 - Revised: 2/3/2010 17

Example (text problem 4. 77): A pulley is hung from the ceiling by a rope. A block of mass M is suspended by another rope that passes over the pulley and is attached to the wall. The rope fastened to the wall makes a right angle with the wall. Neglect the masses of the rope and the pulley. Find the tension in the rope from which the pulley hangs and the angle . y T FBD for the mass M x w Apply Newton’s 2 nd Law to the mass M. MFMc. Graw PHY 1401 - Ch 04 - Revised: 2/3/2010 18

Example continued: Apply Newton’s 2 nd Law: FBD for the pulley: y T F x This statement is true only when = 45 and T MFMc. Graw PHY 1401 - Ch 04 - Revised: 2/3/2010 19

Gravitational Forces Gravity is the force between two masses. Gravity is a long-range force. No contact is needed between the bodies. The force of gravity is always attractive! r is the distance between the two masses M 1 and M 2 and G = 6. 67 10 11 Nm 2/kg 2. M 1 F 21 F 12 M 2 r MFMc. Graw PHY 1401 - Ch 04 - Revised: 2/3/2010 20

Gravitational Forces Let M 1 = ME = mass of the Earth. Here F = the force the Earth exerts on mass M 2. This is the force known as weight, w. Near the surface of the Earth MFMc. Graw PHY 1401 - Ch 04 - Revised: 2/3/2010 21

Gravitational Forces Note that is the gravitational force per unit mass. This is called the gravitational field strength. It is also referred to as the acceleration due to gravity. What is the direction of g? What is the direction of w? MFMc. Graw PHY 1401 - Ch 04 - Revised: 2/3/2010 22

Gravitational Forces Example: What is the weight of a 100 kg astronaut on the surface of the Earth (force of the Earth on the astronaut)? How about in low Earth orbit? This is an orbit about 300 km above the surface of the Earth. On Earth: In low Earth orbit: The weight is reduced by about 10%. The astronaut is NOT weightless! MFMc. Graw PHY 1401 - Ch 04 - Revised: 2/3/2010 23

Applying Newton’s Second Law The one equation everyone remembers! Sumof the forces acting on the objects in the system “m” is the System Mass “a” is the System Response This equation is just the tip of the “iceberg” of the mechanics problem. The student will need to anlyze the forces in the problem and sum the force vector components to build the left hand side of the equation. MFMc. Graw PHY 1401 - Ch 04 - Revised: 2/3/2010 24

Applying Newton’s Second Law Example: A force of 10. 0 N is applied to the right on block 1. Assume a frictionless surface. The masses are m 1 = 3. 00 kg and m 2 = 1. 00 kg. Find the tension in the cord connecting the two blocks as shown. block 2 block 1 F Assume that the rope stays taut so that both blocks have the same acceleration. MFMc. Graw PHY 1401 - Ch 04 - Revised: 2/3/2010 25

FBD for block 2: FBD for block 1: y y N 1 N 2 F T T x x w 1 w 2 Apply Newton’s 2 nd Law to each block: MFMc. Graw PHY 1401 - Ch 04 - Revised: 2/3/2010 26

Example continued: (1) (2) These two equations contain the unknowns: a and T. To solve for T, a must be eliminated. Solve for a in (2) and substitute in (1). MFMc. Graw PHY 1401 - Ch 04 - Revised: 2/3/2010 27

Pick Your System Carefully y y N 1 N 2 T F T x x w 2 w 1 Include both objects in the system. Now when you sum the x-components of the forces the tensions cancel. In addition, since there is no friction, y-components do not contribute to the motion. MFMc. Graw PHY 1401 - Ch 04 - Revised: 2/3/2010 28

Apparent Weight Stand on a bathroom scale. y FBD for the person: N Apply Newton’s 2 nd Law: x w MFMc. Graw PHY 1401 - Ch 04 - Revised: 2/3/2010 29

Apparent Weight The normal force is the force the scale exerts on you. By Newton’s 3 rd Law this is also the force (magnitude only) you exert on the scale. A scale will read the normal force. is what the scale reads. When ay = 0, N = mg. The scale reads your true weight. When ay 0, N > mg or N < mg. In free fall ay = -g and N = 0. The person is weightless. MFMc. Graw PHY 1401 - Ch 04 - Revised: 2/3/2010 30

Apparent Weight Example (text problem 4. 128): A woman of mass 51 kg is standing in an elevator. The elevator pushes up on her feet with 408 newtons of force. What is the acceleration of the elevator? FBD for woman: y Apply Newton’s 2 nd Law: (1) N x w MFMc. Graw PHY 1401 - Ch 04 - Revised: 2/3/2010 31

Apparent Weight Example continued: Given: N = 408 newtons, m = 51 kg, g = 9. 8 m/s 2 Unknown: ay Solving (1) for ay: The elevator could be (1) traveling upward with decreasing speed, or (2) traveling downward with increasing speed. The change in velocity is DOWNWARD. MFMc. Graw PHY 1401 - Ch 04 - Revised: 2/3/2010 32

Force Summary Friction Tension Opposes motion No mass Proportional to normal force No stretching Non-conservative Static & dynamic Pulleys Massless No friction (bearing) Normal Forces Perpendicular to surface at point of contact. Tension in rope continuously changes direction Magnitude needed to maintain equilibrium MFMc. Graw PHY 1401 - Ch 04 - Revised: 2/3/2010 33

Summary • Newton’s Three Laws of Motion • Drawing free body diagrams • Contact forces versus long-range forces • Different forces (gravity, friction, normal, tension, air resistance) • Application of Newton’s Second Law MFMc. Graw PHY 1401 - Ch 04 - Revised: 2/3/2010 34