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LPH 105 W15 Ch4 final thoughts

Newton's Laws

Richard Datwyler

on 6 May 2016

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Transcript of LPH 105 W15 Ch4 final thoughts

Newton's Laws of Motion
Today we begin Dynamics
Force of:
4 types
Strong/Weak Nuclear
Newton's First Law of Motion
Every object continues in its state of rest, or
uniform velocity in a straight line, as long as
no net force acts on it.
Newton's First law is also known as
'the law of inertia'
What is inertia?
Say we are sitting in a vehicle, and somehow
'magically' I am pulled towards the cute
opposite gender counterpart next to me.

What happened?

Did some mysterious force pull me towards this, lovely individual?
Inertial Reference frames
An inertial reference frame is one where Newton's laws are valid. There must be no acceleration, or you could say there must be a constant velocity.
What is Mass?
Mass is the measure of inertia.

The more mass something has
the more force is required to give it
some motion.

SI units : Kilogram , kg.
Is mass the same as weight?
Newton's Second Law

Stated: The acceleration of an object is directly
proportional to the net force acting on it, and
inversely proportional to its mass.
A Force is an action that can accelerate an object

Its SI units are Newtons , N

The base unit of a N comes from this 2nd law of motion

F =ma [N] = [kg][m/s ]
What net force would be required to bring a 2000 kg truck from rest up to 25 m/s in a time of 50 seconds?
Notice it is a vector equation
Thus it can be broken into components.

Newtons Third Law
Stated: Whenever one object exerts a Force on a second
object, the second exerts an equal force in the opposite
direction on the first.
If a 2000 kg tow truck crashes into a 1250 kg car, which feels the greater magnitude of Force?

A. Truck
B. Car
C. Same
Which of these has a greater magnitude of acceleration?
Push or a pull

contact or
long range
Free body Diagrams
Key parts:
Particle representation
Axis, coordinate system
Net Force = F
Normal = N
Force of Friction = F
F =
Force of Gravity=Weight
F =
Normal Force
Gravity pulls down, what keeps us from falling through the earth?

What is normal about the normal force?
Takes the value of what it needs to
Perpendicular to any surface
The proportionality constant is a coefficient. The coefficient of friction.

Friction: is proportional to the normal force.
Two types we will study:
Kinetic friction

Static friction

Applied Force
Frictional force
Inclined planes

Only real trick here is changing the
x and y axes.

Then do same steps.

2 list: Concept
1. Sum of all forces. F = ma
2. Gravity F = mg : always straight down.
3. Normal force: Always normal, and what it needs to be.
5. Newton's 3: Two FBD, substitution.
6. Friction: Links y direction to x direction, substitution.
7. Tension: Same everywhere, force in direction of rope.
8. Pulley: Turn force, count the ropes (tension and acceleration.
Hang a mass, what is tension?
Loop rope over branch?
Pull up on box with 2 ropes?
Add up the ropes.
List: What to do
1. Draw picture
2. FBD (axis, force - all of them)
3. Components
a. On axis (list x and y, separately)

b. Off axis (put on both lists, x & y)
4. F = ma ( components, concepts)
5. Algebra
6. Answer problem (kinematics)
Also notice the sum, this means we have to
ADD all the different vectors together.
This is most effectively done in their individual components.
It is constant
Kinetic = motion
this is moving friction

No motion
ranges from 0 to
Maximum value
it can be bigger than kinetic friction
31) a 20.0 kg block is on a smooth 30 degree surface, connected by a thin cord over a pulley to a 5.00 kg block which hangs vertically. (a) What is the tension in the cord? (b) What is the acceleration of the big block
20.0 kg
5.00 kg
List: What to do
1. Draw picture
2. FBD (axis, force - all of them)
3. Components
a. On axis (list x and y, separately)

b. Off axis (put on both lists, x & y)
4. F = ma ( components, concepts)
5. Algebra
6. Answer problem (kinematics)
"When would static friction=0?"
"I don't understand what normal force is or how it applies to the equations..."
"I'm not sure I understand why finding forces must always come in pairs"
"I don't understand what the difference between the force of gravity and normal force?"
T = 14.7 N
v = 3.83 m/s
5 kg
10 kg
What coefficient of static friction is needed to keep it from sliding.
let the coefficient of kinetic friction be .35 what is the tension and the acceleration?
"I'm having trouble gathering all the correct information (including positive or negative) from the free body diagrams. "
"In following example 4-18 How big would the coefficient of friction need to be to keep the box from sliding at all? (assume all other quantities are the same, answer should look like '0.54') ( 10 kg, 40 N at 30 d"
"Why isn't kinetic friction always greater than static friction, if kinetic friction takes over if enough force is applied?"
"What does the coefficient of the force of friction mean? What does it represent with friction?"
4. Inclined planes: Tilt axis, F swings of - y axis
(C) give it a coefficient of friction of .25, find (a,b) again.
.263 m/s^2

1.96 m/s^2
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