Friction

• Is the force which opposes the motion of two objects or surfaces that are in contact with each other
• Fore example, a box sitting on a floor. The force holding the surfaces together is called the Normal force.
• The Normal force is defined as the force which acts perpendicular to the contact surface.

                                                   Fgravity = w = Fn

• The amount of frictional force is determined by the amount of force holding the two surfaces together. It is also determined by the roughness of the two surfaces.

Identifying forces: If the box moves across the floor at a constant speed, the forces acting on it are balanced.

• Frictional force = Ff, this force oppose the motion of the box
• Normal force = Fn, this hold the contact surface together. It is perpendicular to the contact surfaces.
• Applied force = Fa, is the force moving the box across the floor.

Coefficient of friction

• Is a constant for any two surfaces in contact with each other
• It is number determined by the roughness of the surfaces.
• The rougher the surfaces are, the higher the coefficient of friction.
• It is calculated using this formula: μ = Ff / Fn
• μ (mu) is the coefficient of friction, it is unitless
• Ff is the force of friction
• Fn is the normal forces

Example: A box of 15kg being pushed forward at a constant speed. The applied force is 50 N. What is the coefficient of friction?

     Fn = w = mg = 15kg (9.8 m/s^2) = 147 N

     Ff = Fa, μ = Ff/Fn = 50 N/ 147 N = 0.34

Newton's Third Law of Motion

For every action , there is an equal and opposite reactions.

Example 1: Rocket

Example 2: Walking forward

Example 2: Sitting on a chair

-----------------------------------------

Next related topic: Friction

Newton's Second Law of Motion

• F= ma (Force = mass * acceleration)
• A force is defined as a pushed or pull
• In the metric system, force is measured in Newtons (N).
• A newton is approximately equal to 1/4 lb (pound)
• In physics, mass is usually measured in kilograms
• Acceleration is measured in m/s^2

Example 1: A rocket of mas 1750 kg fires it engines in spaces. The engine exert of force of 4500 N. 

     a) At what rate will the rocket accelerate?

     F = ma => 4500 N = 1750 kg * a, solve for a,

     a = 4500/1750 = 2.57 N/g = 2.57 m/s^2

     b) How fast will the rocket be moving at the end of 25 s?

     

     Sf = si + at => Sf= 0 + (2.57 m/s^2) (25s) = 64.25 m/s

Example 2: Two forces act on a box. One force is 500 N and acts to the right. The second force is 840 N, and act to the left.

      a) What is the net force acting on the box?

     Fnet = Fa - Ff  = 840 -500 = 340 N left

    b) If the box mass is 120 kg, at what rate does it accelerate?

     F = ma => 340 N = (120 kg) a, a = 2.83 N/kg  or m/s^2

Weight:

• Weight refers to the force of gravity acting on an object
• The formula for calculating the weight of an object is a variation of Newton's 2nd Law.
• W= mg (Weight = mass * gravitational acceleration (9.8 m/s^2))
• Remember, mass is the amount of matter contained within an object. If you shoot a heavy object into space. Its weight will disappear, but its mass will be unchanged.

Example 1: An object has a mass of 320 kg. How much does this object weight?

     

     W = mg = 320 kg (9.8 m/s^2) = 3136 N

Example 2: A man weight 850 N. What is his mass?

     W = mg => 850 N = m (9.8 m/s^2), m = 86.7 kg

Example 3: 

 a) A man stands on a scale in an elevator. The elevator is not moving. The man's mass is 75 kg. What is the reading on the scale?

     W = mg = 75(9.8) = 735 N

b) If the elevator beginning to move upward 2 m/s^2. What is the reading on the scale now?

     F = ma = 75 kg ( 2 m/s^2) = 150 N

     Fnet = 735 N + 150 N = 885 N

c) The elevator come to stop and then start accelerating downward at 3 m/s^2. What is the reading on the scale now?

     F = ma = 75 (3) = 225 N

     Fnet = 735 - 225 = 510N

     

Newton's First Law of Motion

Newton's Laws of Motion

• Provide the foundation for all the root of physics
• There are 3 fundamental laws of motion

Newton's  first law:

• Also called the law of Inertia
• "A body at rest will stay at rest. If in motion will remain in motion unless acted upon by an unbalanced external force."
• In space where there is no friction, a body will continue to move indefinitely if it is already in motion.

 Inertia

• The resistance that a body demonstrates to a change in its state of motion.
• All object have inertia. 
• The amount of inertia possessed by a body is determined by its mass.  (Remember that mass is the amount of matter contained within an object)

Example: If a 150 lbs person run into a 150 lbs person, he could able to stop. If a 300 lbs person run into a 150 lbs person, he could not be able to stop.

• The more mass an object has, the greater its inertia. In other word, the harder it is to move it or slow it down.

Balanced vs Unbalanced forces: If a forces acts on a body, and there is no force to counter it, the force is said to be unbalanced.

• First box - Unbalanced force of 50 Newtons applied to the box
• Second box - The force are balanced. That is, they canceled out.