Download Lecture 8: Forces & The Laws of Motion

Lecture 9:
Forces & Laws of Motion
Questions of Yesterday
You must apply a force F to push your physics book across your desk at a
constant velocity.
1a) The net force acting on the book is…
a) F
b) between 0 and F
c) greater than F
d) 0
1b) Are other forces acting on the book in the horizontal direction?
a) YES
b) NO
c) not enough information to know
3) A large crate is at rest in the bed of a truck. As the truck accelerates the
crate remains at rest relative to the truck. In what direction is the net force
on the crate?
a) the same direction as the truck’s acceleration
b) opposite the direction of the truck’s acceleration
c) the net force is zero
Newton’s First Law
An object in motion will remain in its original state of
motion unless acted on by an external force.
But is that really what we observe??
If objects in motion actually often come to a stop…
and Newton’s first law really is true…
What must be happening?
Friction
An object moving on a surface or through a viscous medium
encounters resistance to motion from the surface or medium
FRICTION
The resistive force that acts between two surfaces
to oppose motion
motion
FA
Friction = f
Friction ALWAYS acts in a direction opposite the motion of the
object
Friction
FRICTION
The resistive force that acts between two surfaces
to oppose motion
N
Friction = f
FA
F
g
Static Friction
You apply a horizontal force FA to try to move the block across the
table but it doesn’t move…
what forces are acting on the block?
N
FA
fs
F
g
Static Friction
Static Friction
Friction force that acts on object at rest in a direction opposite the
applied force to keep it stationary
N
FA
fs
F
g
How do the magnitudes of FA and fs compare?
Static Friction
FA = -fs
Object is at rest -> NO Acceleration, NO net force
N
FA
fs
F
g
What if the block is moving?
Kinetic Friction
Kinetic Friction
Friction force that acts on a moving object in a direction
opposite to the objects motion
motion
N
FA
fk
F
g
How do FA and fk compare?
Kinetic Friction
constant velocity
motion from FA
constant acceleration
motion from FA
N
N
FA
fk
F
FA =
g
-fk
FA
fk
F
g
FA > -fk
Friction & the Normal Force
Which box would you rather push across the floor?
Why?
N
N
10 kg
FA
FA
100
kg
Fg
F
g
Fg and N shouldn’t affect horizontal
motion!
Friction force is only other force acting in horizontal direction!
Friction & the Normal Force
Friction Force is proportional to Normal Force acting on an
object….
which typically depends on the weight of the object
fk
N
10 kg
Fg
N
FA
FA
fk
100
kg
F
g
Friction & the Normal Force
mk, ms = coefficients of kinetic, static friction
mk, ms are experimentally determined values that depend on the
nature of the surface
fk
N
10 kg
Fg
N
FA
FA
fk
100
kg
fs ≤ msN
fk = mkN
F
g
Friction & the Normal Force
You apply a force of magnitude FA to move a box at constant
velocity, what is the force of friction resisting the box’s motion in
each case (in terms of m, g, mk)?
FA
FA
M
30o
30o
M
fs ≤ msN
fk = mkN
M
FA
Free Body Diagrams
M
q
If the block is sliding down the inclined plane at a constant
velocity what is the coefficient of kinetic friction between
the block and the plane (in terms of m, g and q)?
Friction: Practice Problem
5 kg
ms = 0.5
mk = 0.3
10 kg
What will the acceleration of the system be if…
it is released from rest?
it is set in motion with the 10.0 kg moving downward?
Friction: Practice Problems
A boy coasts down a hill on a sled, reaching a level
surface at the bottom with a speed of 7.0 m/s. If the
coefficient of friction between the sled’s runners and
the snow is 0.050 and the boy and sled together
weigh 600 N…
How far does the boy travel on the level surface before
coming to a stop?
Friction: Practice Problems
A firefighter of mass 80 kg slides down a vertical pole
with an acceleration of 4 m/s2.
What is the friction force that acts on the firefighter?
Questions of the Day
1) A student pushes her physics book across a flat table.
Another student pushes his book up a 30o inclined plane.
Assuming the coefficient of kinetic friction is the same in both
cases, in which case is the force of friction acting on the book
greater?
a) the book on the flat table
b) the book on the inclined plane
c) the force of friction is the same in both cases
2) If you hold your physics book up against the chalkboard, in
what direction is the force of friction directed?
a) upwards
b) downwards
c) away from the chalkboard
d) into the chalkboard