Newton’s Laws The Study of Dynamics
Isaac Newton Arguably the greatest physical genius
ever. Came up with 3 Laws of Motion to
explain the observations and analyses of Galileo and Johannes Kepler.
Invented Calculus. Published his Laws in 1687 in the book
Mathematical Principles of Natural Philosophy. (the Principia)
What is Force? A force is a push or pull on an
object. Forces cause an object to
accelerate… To speed up To slow down To change direction
Newton’s First Law The Law of Inertia. A body in motion stays in motion at
constant velocity and a body at rest stays at rest unless acted upon by an external force.
(note:)This law is commonly applied to the horizontal component of velocity, which is assumed not to change during the flight of a projectile.
The First Law is Counterintuitive
Aristotle firmly believed this.But Kell Physics students know better!
Newton’s Second Law A body accelerates when acted upon
by a net external force. Net F = ma The acceleration is proportional to
the net force and is in the direction which the net force acts.
In projectiles, this law is commonly applied to the vertical component of velocity.
Newton’s Second Law ∑Fvectors = ma
where ∑F is the net force measured in Newtons (N)
m is mass (kg) a is acceleration (m/s2)
Newton (SI system) 1 N = 1 kg m /s2
1 N is the force required to accelerate a 1 kg mass at a rate of 1 m/s2
Pound (British system) 1 lb = 1 slug ft /s2
Units of force
Newton’s Third Law For every action there exists
an equal and opposite reaction.
If object A exerts a force F on B, then B exerts a force of -F back on A.
This is rocket science.
2-Dimensional problem
Larry pushes a 20 kg block on a frictionless floor at a 45o angle below the horizontal with a force of 150 N while Moe pulls the same block horizontally with a force of 120 N.a) What is the acceleration?b) What is the normal force?
20 kgFL FM
FG
N
Step 1: Draw the problem
Larry pushes a 20 kg block on a frictionless floor at a 45o angle below the horizontal with a force of 150 N while Moe pulls the same block horizontally with a force of 120 N. What is acceleration?
Working a Newton’s 2nd Law Problem
20 kgFL FM
Step 2: Diagram
Force diagram
Working a Newton’s 2nd Law Problem
Free Body diagram
20 kgFL FM
FG
N
FL
FM
FG
N
Step 3: Set up equationsF = maFx = max
Fy = may
Working a Newton’s 2nd Law Problem
Always resolve two-dimensional problems into two one-dimensional problems.
Step 4: Substitute Make a list of givens from the word problem.
Substitute in what you know.
Working a Newton’s 2nd Law Problem
Step 5: Solve Plug-n-chug. Calculate your unknowns. Sometimes you’ll need to do
kimematic calculations following the Newton’s 2nd law calculations.
Working a Newton’s 2nd Law Problem
Gravity as an accelerating force
A very commonly used accelerating force is gravity. Here is gravity in action. The acceleration is g.
Gravity as an accelerating force
In the absence of air resistance, gravity acts upon all objects by causing the same acceleration…g.
The pulley lets us use gravity as our accelerating force… but a lot slower than free fall. Acceleration here is a lot lower than g.
Gravity as an accelerating force
The problem of weight
Are weight and mass the same thing?
No. Weight can be defined as the force due to gravitation attraction.
W = mg
Friction The force that opposes a sliding motion.
Enables us to walk, drive a car, etc.
Due to microscopic irregularities in even the smoothest of surfaces.
There are two types of friction
Static frictionexists before sliding occurs
Kinetic frictionexists after sliding occurs
In general fk <= fs
Friction and the Normal Force
The frictional force which exists between two surfaces is directly proportional to the normal force.
That’s why friction on a sloping surface is less than friction on a flat surface.
Static Friction fs sN
fs : static frictional force (N) s: coefficient of static friction
N: normal force (N) Static friction increases as the
force trying to push an object increases… up to a point!
A force diagram illustrating Static Friction
Bigger Frictional Force
Normal Force
Gravity
Bigger Applied Force
A force diagram illustrating Static Friction
Frictional Force
Normal Force
GravityEven Bigger Applied Force
The forces on the book are now UNBALANCED!
Static friction cannot get any larger, and can no longer completely oppose the applied force.