Transcript Newton’s 3 Laws and Free Body Diagrams

J. Pulickeel
SPH4U1
February 2010
Newton’s Three Laws
1.
An object in motion tends to stay in motion and an
object at rest tends to stay at rest, unless acted upon
by an external force
2.
3. For every action, there is an equal and opposite
reaction. Equal in magnitude and opposite in
direction.
Explain what’s Happening using Newton’s Laws….
The ladder undergoes the same
accelerated and decelerated motion
that the truck experiences. Once
the strap is no longer present, the
ladder is more likely to maintain its
state of motion.
If the truck were to abruptly stop
the ladder in motion would continue
in motion. Assuming a negligible
amount of friction between the truck
and the ladder, the ladder would
slide off the top of the truck and be
hurled into the air. Once it leaves
the roof of the truck, it becomes a
projectile and continues in
projectile-like motion.
In the picture below, Kent Budgett is pulling upon a rope which is attached to a
wall. In the bottom picture, the Kent is pulling upon a rope which is attached to
an elephant. In each case, the force scale reads 500 N. Kent is pulling ...
a. with more force when the rope is attached to the wall.
b. with more force when the rope is attached to the elephant.
c. the same force in each case.
Kent is pulling with 500 N of force in each case. The rope
transmits the force from Kent to the wall (or to the elephant) and
vice versa. Since the force of Kent pulling on the wall and the
wall pulling on Kent are action-reaction force pairs, they must
have equal magnitudes. Inanimate objects such as walls can
push and pull.
While driving down the road, a firefly strikes the
windshield of a bus and makes a quite obvious mess
in front of the face of the driver. This is a clear case of
Newton's third law of motion. The firefly hit the bus
and the bus hits the firefly. Which of the two forces is
greater: the force on the firefly or the force on the
bus?
Trick Question! Each force is the same size. For every
action, there is an equal ... (equal!). The fact that the
firefly splatters only means that with its smaller mass, it
is less able to withstand the larger acceleration resulting
from the interaction. Besides, fireflies have guts and bug
guts have a tendency to be splatterable. Windshields
don't have guts. There you have it.
3. Many people are familiar with the fact that a rifle recoils when fired. This recoil
is the result of action-reaction force pairs. A gunpowder explosion creates hot
gases which expand outward allowing the rifle to push forward on the bullet.
Consistent with Newton's third law of motion, the bullet pushes backwards upon
the rifle. The acceleration of the recoiling rifle is ...
a. greater than the acceleration of the bullet.
b. smaller than the acceleration of the bullet.
c. the same size as the acceleration of the bullet.
The force on the rifle equals the force on the bullet. Yet,
acceleration depends on both force and mass. The bullet has a
greater acceleration due to the fact that it has a smaller mass.
Remember: acceleration and mass are inversely proportional.
Free Body Diagrams
List all the forces acting on the hockey puck
The Normal Force: The force acting
perpendicular to the surface of contact
The Force of Gravity: The force pulling an
object towards the centre of the earth
Force of Friction: the force resisting the
motion of solid surfaces.
- Static Friction: friction between two solid objects that are not
moving relative to each other
- Kinetic Friction: occurs when two objects are moving relative to
each other and rub together.


Fs   S FN


Fk   K FN
The Applied Force: The force applied.
Calculating the Forces...
The Normal Force
IN THIS CASE, the normal force has the same magnitude as
the Force of Gravity.
The Coefficient of Friction (µ)
The coefficient of Friction a dimensionless scalar value which describes
the ratio of the force of friction between two bodies and the force pressing
them together.
The coefficient of friction is an empirical measurement – it has to be
measured experimentally, and cannot be found through calculations


Fs   S FN


Fk   K FN
This value will always be given to you!
FBD: Level Planes vs. Ramps
The block is not moving forward because there
is no horizontal applied force.
What would the FBD look like if a force was
applied, but the block was not moving?
What would the FBD look like if a force was
applied, but the block was moving?
FBD: Tension
What would the FBD for the following setup look
like?
+
y
FB
FA
+
x
FC
9.8N
Is the object in motion or at rest?
=
=
=
=
=
=
=
FBD: Tension
The sign below hangs outside the physics
classroom, advertising the most important truth
to be found inside. The sign is supported by a
diagonal cable and a rigid horizontal bar. If the
sign has a mass of 50 kg, then determine the
tension in the diagonal cable which supports its
weight.
Since the mass is 50 kg, the weight is 490 N. Since
there is only one "upward-pulling" cable, it must supply
all the upward force. This cable pulls upwards with
approximately 490 N of force.
Thus, sin(30°) = (490 N ) / (FT).
The tension is 980 N.
FT
Fg
FBD: Level Planes vs. Ramps
An object placed on a inclined surface will often slide down the surface. The
rate at which the object slides down the surface is dependent upon how
inclined the surface is; the greater the incline, the faster the rate at which the
object will slide down it.
You will note that in both cases, the box not only fell down, but it also
travelled horizontally. That means there is an x and y Force acting on the box!
FBD: Level Planes vs. Ramps
+
y
In this situation, no external force is being
applied, but the block slid down the ramp.
+x
This is because some of the force of gravity is
pulling the block into the plank of wood
(opposite to the Normal Force)
Some of the Force of Gravity is overcoming
friction to cause the block to move forward .
This means that Fg is the sum of FN and Fx
PROBLEM 1: A board is inclined at 35°. The forces acting on the block are
illustrated. Find the net external force on the block and determine whether it will
move.
11 N
18 N
+
y
y
+x
11 N
18 N
35°
22 N
Fg,y
35°
35°
22 N
Fg,
x
x
PROBLEM 1: A board is inclined at 35°. The forces acting on the block are
illustrated. Find the net external force on the block and determine whether it will
move.
+
y
y
+x
11 N
18 N
35°
13 N
18 N 35°
22 N
x
13 N
Therefore the net force is 2 N in the positive x direction.