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.

 

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.

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 Physics B students know better!

A force diagram illustrating no net force

A force diagram illustrating no net force

A force diagram illustrating no net force

A force diagram illustrating no net force

Another example illustrating no net force

Newton’s Second Law

  

A body accelerates when acted upon by a net external force.

The acceleration is proportional to the net force and is in the direction which the net force acts.

This law is commonly applied to the vertical component of velocity.

Newton’s Second Law

 ∑F = ma  where ∑F is the net force measured in Newtons (N)   m is mass (kg) a is acceleration (m/s 2 )

Units of force

   Newton (SI system)  1 N = 1 kg m /s 2 1 N is the force required to accelerate a 1 kg mass at a rate of 1 m/s 2 Pound (British system)  1 lb = 1 slug ft /s 2

Newton’s Third Law  

For every action there exists an equal and opposite reaction.

If A exerts a force F on B, then B exerts a force of -F on A.

Working a Newton’s 2 nd Law Problem

Step 1: Draw the problem

Larry pushes a 20 kg block on a frictionless floor at a 45 same block o angle below the horizontal with a force of 150 N while Moe pulls the horizontally with a force of 120 N. What is acceleration?

F L

20 kg

F M

F L

Working a Newton’s 2 nd

Step 2: Diagram

Law Problem

 Force diagram

N

20 kg

F M F G N F

M

F

G

F

L  Free Body diagram

Working a Newton’s 2 nd Law Problem

Step 3: Set up equations

 

F = ma

  F x F y = ma x = ma y

Always resolve two-dimensional problems into two one-dimensional problems.

Working a Newton’s 2 nd Law Problem

Step 4: Substitute

  Make a list of givens from the word problem.

Substitute in what you know.

Working a Newton’s 2 nd

Step 5: Solve

Law Problem

  

Plug-n-chug.

Calculate your unknowns.

Sometimes you’ll need to do kimematic calculations following the Newton’s 2 nd law calculations.

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.

Gravity as an accelerating force 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.

2-Dimensional problem

Larry pushes a 20 kg block on a frictionless floor at a 45 acceleration?

force?

o 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 b) What is the normal

F L N

20 kg

F G F M

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

Flat surfaces – 1 D

N

N = mg for objects resting on horizontal surfaces.

mg

Ramps – 2 D

N = mgcos 

N

mgsin  The normal force is perpendicular to angled ramps as well. It’s always equal to the component of weight perpendicular to the surface.

mgcos 

mg

 

Ramps – 2 D

N = mgcos 

N

mgsin  How long will it take a 1.0 kg block to slide down a frictionless 20 m long ramp that is at a 15 o angle with the horizontal?

mg

 mgcos  

Applied forces affect normal force.

applied force normal friction weight N = applied force

V = 0 A = 0 Normal feeling N N V > 0 A > 0 V > 0 A = 0 V > 0 A < 0 Normal feeling N Light feeling N mg Ground floor mg mg Just starting up Between floors mg Arriving at top floor

V = 0 A = 0 V < 0 A < 0 V < 0 A = 0 V < 0 A > 0 Normal feeling N Light feeling N Normal feeling N N mg Top floor mg Beginning descent mg mg Between floors Arriving at Ground floor

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 friction

 exists before sliding occurs  

Kinetic friction

 exists after sliding occurs

In general f

k

<= f

s

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

 f s     s N f s : static frictional force (N)  s : coefficient of static   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

Frictional Force Normal Force Applied Force Gravity

A force diagram illustrating

Static Friction

Bigger Frictional Force Normal Force Bigger Applied Force Gravity

A force diagram illustrating

Static Friction

The forces on the book are now UNBALANCED!

Normal Force Frictional Force Gravity Static friction cannot get any larger, and can no longer completely oppose the applied force.

Even Bigger Applied Force

  f k   

Kinetic Friction

=  k N f k : kinetic frictional force (N)  k : coefficient of kinetic friction N: normal force (N) Kinetic friction (sliding friction) is generally less than static friction (motionless friction) for most surfaces.

Determination of the Coefficients of Friction

Coefficient of Static Friction

1) Set a block of one material on an incline plane made of the other material.

2) 3) Slowly increase angle of plane until the block just begins to move. Record this angle.

Calculate  s = tan  .

Determination of the Coefficients of Friction

Coefficient of Kinetic Friction

1) Set a block of one material on an incline plane made of the other material.

2) 3) Slowly increase angle of plane until the block just begins to move at constant speed after giving it a slight tap. Record this angle.

Calculate  k = tan  .

Magic Pulleys

N mg T -x m 1 m 2 x T mg

Pulley problem Mass 1 (10 kg) rests on a frictionless table connected by a string to Mass 2 (5 kg). Find (a) the acceleration of each block and, (b) the tension in the connecting string.

m 1 m 2

Pulley problem Mass 1 (10 kg) rests on a table connected by a string to Mass 2 (5 kg) as shown. What must the minimum coefficient of static friction be to keep Mass 1 from slipping?

m 1 m 2

Pulley problem Mass 1 (10 kg) rests on a table connected by a string to Mass 2 (5 kg). If  s = 0.3 and  k = 0.2, what is tension in the string?

m 1 m 2