This preview shows pages 1–3. Sign up to view the full content.
Lab 3  Newton's Second Law
Introduction
Sir Isaac Newton
put forth many important ideas in his famous book
The Principia
. His three
laws of motion are the best known of these. The first law seems to be at odds with our everyday
experience. Newton's first law states that any object at rest that is not acted upon by outside
forces will remain at rest, and that any object in motion not acted upon by outside forces will
continue its motion in a straight line at a constant velocity. If we roll a ball across the floor, we
know that it will eventually come to a stop, seemingly contradicting the First Law. Our
experience seems to agree with
Aristotle's
idea, that the
"impetus"
given to the ball is used up as
it rolls. But Aristotle was wrong, as is our first impression of the ball's motion. The key is that
the ball
does
experience an outside force, i.e., friction, as it rolls across the floor. This force
causes the ball to decelerate (that is, it has a "negative" acceleration). According to Newton's
second law
an object will accelerate in the direction of the net force
. Since the force of friction is
opposite to the direction of travel, this acceleration causes the object to slow its forward motion,
and eventually stop. The purpose of this laboratory exercise is to verify Newton's second law.
Discussion of Principles
Newton's second law in vector form is
( 1 )
F
=
ma
or
F
net
=
ma
This force causes the ball rolling on the floor to decelerate (that is, it has a "negative"
acceleration). According to Newton's second law an object will accelerate in the direction of the
net force. If
F
is the magnitude of the net force, and if
m
is the mass of the object, then the acceleration is given by
( 2 )
a
=
F
m
Since the force of friction is in the opposite direction to the direction of motion, this acceleration
causes the object to slow its forward motion, and eventually stop. Notice that
Eq. (1)
and
Eq. (2)
are written in vector form. This means that Newton's second law holds true in all directions. You
This preview has intentionally blurred sections. Sign up to view the full version.
View Full Document can always break up the forces and the resultant acceleration into their respective components in
the
x
,
y
, and
z
directions.
( 3 )
F
net,x
=
ma
x
( 4 )
F
net,y
=
ma
y
( 5 )
F
net,z
=
ma
z
Consider a cart on a lowfriction track as shown in Fig. 1. A light string is attached to the cart
and passes over a pulley at the end of the track and a second mass is attached to the end of this
string. The weight of the hanging mass provides tension in the string, which helps to accelerate
the cart along the track. A small frictional force will resist this motion. We assume that the string
is massless (or of negligible mass) and there is no friction between the string and the pulley.
Therefore the tension in the string will be the same at all points along the string. This results in
This is the end of the preview. Sign up
to
access the rest of the document.
This note was uploaded on 08/21/2011 for the course PY 211 taught by Professor Owen during the Spring '07 term at N.C. State.
 Spring '07
 OWEN

Click to edit the document details