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Unformatted text preview: Lecture 8
P112 Feb 7, 2007 Labs
First lab begins this week
No prelab You must finish lab in class (No problem) No homework You must be registered for the lab you attend No scheduled makeup labs With instructor permission you may attend another lab section the week it is offered  subject to space  you should have a good excuse Missing lab = loose 1 grade (ex: B+ to B) Agenda for today
Forces
Fundamental Forces of Nature What is a Force How does it affect motion Newton s first and second laws Fundamental Forces
Gravitational Weak
Governs radioactive decays, elementary particle transformations Electromagnetic
Electric charge interactions Strong
binds protons and neutrons in the nucleus
Unification? In P112 we deal with the macroscopic manifestations..
Noncontact forces:
Gravity, magnetism ("field forces") All others are "contact forces"
Macroscopic effects of the microscale electromagnetic forces between molecules and atoms Solids: tension, compression, springs, normal, friction Fluids: pressure, buoyancy, drag,... What s a force?
Description of an interaction between two bodies or a body and its environment (a "push" or a "pull") Force is a vector Unit: Newton (N)
1N is the amount of force that gives an acceleration of 1m/s2 to a body with a mass of 1kg Forces involve two objects
Bob pushes on a crate FBob on crate
FBc
(4) Body providing the force (subscript) (3) Body on which the force acts Superposition of forces
Experiments show that the effect is the same: apply two forces = apply a force that corresponds to the vector sum of the two!
We ll use this all the time! Any force can be replaced by its component vectors
F Superposition of forces
Experiments show that the effect is the same: apply two forces = apply a force that corresponds to the vector sum of the two!
We ll use this all the time! Any force can be replaced by its component vectors
Fy F Fx Net Force
"Net force" (or resultant force) is the vector sum of all forces acting on a body r r r r R = F1 + F2 + F3 + ... = r F Quick problem:
What s the net force acting on this body? all angles 45o
y
F1=2N F2=1N F3=2N x Many ways to do this, e.g. by components:
y
Rx = Fx Rx = cos(45) 1N cos(45) 2N sin(45) 2N Rx = 0.7 3N = 2.1N Ry = Fy Ry = cos(45) 2N cos(45) 2N sin(45) 1N Ry = 0.7 1N = 0.7N F1=2N R=2.2N F3=2N F2=1N x Magnitude and direction:
R = Rx2 + Ry2 = (2.12 N 2 + 0.7 2 N 2 = 2.2N = arctan Ry Rx = arctan 0.7 = 18 0 2.1 How do forces affect motion?
What happens if the net force on a body is zero? How do forces affect motion?
What happens if the net force on a body is zero? 1. Body at rest: it will remain at rest 2. Body in motion: it will remain in motion (because of it s "inertia") Common misconception: force is needed to sustain motion Newton s laws:
From the 1687 edition of "Principia Mathematica" Newton s first law:
A body acted on by no net force moves with constant velocity (may be zero) and no acceleration
This is true if viewed from an inertial reference frame! Inertial Reference Frames:
You are standing on roller skates in a train (no friction). The train starts to accelerate forward. What happens? Inertial Reference Frame
You will start to move backward with respect to the train! Does this mean there is a net force acting on you? Inertial Reference Frame
Does this mean there is a net force acting on you? No, the net force remains zero! Doesn t Newton s first law say that you should remain at rest? Inertial Reference Frame
Doesn t Newton s first law say that you should remain at rest? The train s frame of reference is not an inertial reference frame so Newton s first law does not apply. The reference frame of the earth is an inertial reference frame! You are indeed remaining at rest with respect to the earth. Inertial reference frames (IRF)
An IRF is a reference frame that is not accelerating An IRF is a reference frame in which Newton s first law is valid! In this course we are only dealing with IRFs! Is Schwartz Auditorium an IRF?
Is Schwartz Auditorium accelerating? Yes! What is its acceleration? Acceleration of Schwartz due to rotation of the earth:
aI = 4
2 R T2 Radius of the earth: RE=6.4x106m Period T: 1day=8.64x104s
4 2 6.4 10 6 m m aI = = 0.03 2 4 2 2 s (8.64 10 ) s Small enough to neglect! Schwartz is a pretty good Inertial reference frame. Zero net force on body?
A (frictionless) box on the bed of a truck, the truck accelerates forward? A bird circling at a constant speed and radius? Zero net force on body?
A (frictionless) box on the bed of a truck, the truck accelerates forward? YES A bird circling at a constant speed and radius? No, there is a net force pointing toward the center of the circle What happens when the net force is NOT zero? What happens when the net force is NOT zero?
Experiment shows that:
the presence of a net force acting on a body causes the body to accelerate the acceleration is exactly proportional to the force!! r F = const For a given body, the ratio a The definition of Mass
(inertial) mass is defined as the ratio of the magnitude of the net force and the magnitude of the acceleration!
r F r a m Newton s second law of motion
If a net external force acts on a body, the body accelerates. The direction of acceleration is the same as the direction of the net force. The net force is equal to the mass of the body times the acceleration of the body. r r F=m a Weight, mass, force
What s the difference?
Force is a vector, unit: 1 N Mass is a scalar, unit: 1 kg Weight is a vector: it s the force of gravitational attraction that the earth exerts on your body. Weight of 1kg on earth: W= 1kg x 9.81m/s2 = 9.81 kgm/s2 = 9.81N Last question:
Does the spring balance measure weight or mass? Recap:
Net Force how to calculate a net force using vectors Newton s first law of motion inertial reference frames Newton s second law Next time: Gravitation! ...
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This note was uploaded on 03/27/2008 for the course PHYS 1112 taught by Professor Leclair,a during the Spring '07 term at Cornell.
 Spring '07
 LECLAIR,A
 mechanics, Work

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