Chapter 6
Force and Motion
An object that experiences a push or a pull
has a FORCE exerted on it.
The object is called the SYSTEM.
The world around the object that exerts
forces on it is called the ENVIRONMENT.
Forces have both magnitude and direction
and are therefore _____________.
Contact vs. Long-Range Forces
Contact forces – Act on an object only by
touching it.
Long-Range Forces – Exerted without
contact. Ex. The force of gravity.
Each force has a specific, identifiable,
immediate cause called the Agent.
Ex. Gravity – Earth’s mass
VECTORS.
Newton’s Second Law of Motion
Fnet = ma
or ∑ F = ma
Or a = Fnet/m
Recall: vectors are added and subtracted
in one direction at a time only.
Acceleration is a vector quantity. Mass is
a scalar quantity.
Newton’s First Law
Known as the Law of Inertia.
Equilibrium – Net forces are zero!
Free-Body Diagrams
– Net Forces = sum of all forces
Constructing Free-Body Diagrams
1. Sketch the Problem.
2. Choose a coordinate system.
3. Locate EVERY point where the
environment touches the system.
4. Draw a motion diagram including the
velocity and acceleration vectors.
5. Draw the Free-Body diagram.
6. Check your answer. –vector addition!
Review Ch. 6 Sec. 1
For the following ten items, Identify as:
a. Contact Force
b. Long-Range Force
c. Not a Force
Yes this is a quiz!
1. Weight
2. Mass
3. Inertia
4. Push of a hand
5. Air drag
6. Spring force
7. Acceleration
8. Friction
9. Tension
10. Mass times acceleration
6.2 Using Newton’s Laws
Newton’s second law gives us a connection
between the net force exerted on an object and
its acceleration. The law identifies the cause of
a change in velocity and the resulting
displacement.
Aristotle’s followers believed that the heavier an
object the faster the fall. Galileo hypothesized
that all objects, no matter their weight, gain
speed at the same rate.
What is the weight force Fg, exerted
on an object of mass m?
Fg = mg
Scales – what do they measure?
Apparent Weight:
What would happen to the
readings on the bathroom scale if
you took readings in an elevator?
What would happen to the
readings if the cable holding the
elevator were to break?
Does this mean you have no
weight?
Friction:
Imagine pushing on a crate and it not
moving. You push harder and it still
does not move across the floor.
Describe the forces acting on the
crate.
The force opposing you moving the
crate from rest is called STATIC
FRICTION.
Static friction:
– http://www.physclips.unsw.edu.au/jw/weight_a
nd_friction.htm#coefficients
Kinetic friction:
– http://www.physclips.unsw.edu.au/jw/weight_a
nd_friction.htm#coefficients
Ff kinetic = μk FN
0 ≤ Ff static ≤ μs FN
Some common coefficients are in table 6-3, pg 131
Causes of friction:
When two surfaces touch, they temporarily
bond. In order to move the object one
must break this bond. When objects are
moving past each other, there is still an
electrostatic attraction at the atomic level
and this is the weaker kinetic friction.
6.3 Interaction Forces
Identifying Interaction Forces.
System and Environment. Consider two
systems whose motions you want to study.
Recall that the environment is all the other
systems whose motions are not being studied.
Ex. – Catcher catching a baseball. There are
external forces acting on both systems and also
interactions between the two systems.
F hand on ball F ball on hand
OR
FA on B
FB on A
Question?
Does this mean the ball causes the hand
to exert a force?
The two forces either exist together or not
at all.
Newton’s Third Law
An interaction pair is two forces that are
equal in magnitude but opposite in
direction.
*ALL FORCES COME IN PAIRS.
Fundamental Forces
Four Fundamental Forces.
Gravity
Electromagnetic Ex. Static cling, molecular bonds.
Strong Nuclear – acts between protons and
neutrons.
Weak Nuclear – some kinds of radioactive decay.
Goal: To show that at some level, all four
interactions are really one!
Forces on Ropes and Strings
Identify the Force Pairs.
If the rope breaks, the bucket will fall, so
there must be a force holding the rope
together. The force that the top part of the
rope exerts on the bottom part is
F TOP ON BOTTOM
Newton’s Third Law states that this is part
of an interaction pair. So….
F BOTTOM ON TOP
would be the other half.
These forces are equal in magnitude but
opposite in direction.
If the bucket is in equilibrium, the net force
must be zero! So the tension on the top of
the rope must be the weight of the bucket.
Tug of war example.
6.4 Pulleys
Examine the following diagram and label
the forces.
Frictionless
pulley
m1
m2
If m1 = 0.5 kg and m2 = 0.2 kg what
happens to the system?
What does the pulley do?
Frictionless
pulley
m1
m2
PSS
1.
2.
3.
4.
5.
6.
Separate the system from the environment.
Draw a sketch of the problem with a coordinate
system.
Identify all forces on the system and add them
to your sketch
Draw a free-body diagram
Use Newton’s Second Law to equate the
forces
Solve the problem
Online practice using FBD
http://www.glenbrook.k12.il.us/gbssci/phys
/shwave/fbd.html
http://www.physicsclassroom.com/Class/n
ewtlaws/newtltoc.html