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.

Metric System

Metric System Prefixes

Metric System

Based on multiples of 10. Prefixed are place in front of the base unit to represent larger or smaller quantities.

Important prefixes:

     kilo (K)      = 1,000
     hecto (H)   = 100
     deca (D)   = 10
     Base         = 1 (meter, gram, liter, joule)
     deci (d)     = .1 (1/10)
     centi (c)    = .01 (1/100)
     milli (m)    = .001 (1/1,000)

Other prefixes:
     mega (M)  = 1000,000
     giga (G)     = 100,0000,000
     micro (µ)   = 1/1000,000

Base Units:

• meter (m) is the base unit for length
• gram (g)     "        mass
• liter (l)        "       volume
• joule (J)      "       energy

Examples:

   1) Convert 30 mL to kL

         a) 30.0, move the decimal to the left six places,  = .000030 kL

          OR 

         b)

    2) How many grams are there in 5 kilogram (kg) ?

         a) 5.0, move the decimal to the right three places, = 5000. g

OR

         b)

    3) Convert 350 Dg to mg.

          a) 350.0, move the decimal to the right four places, = 3500000. mg

          OR

          b)

      4) Convert 20 cm to m.

          a) 20.0, move the decimal to the left two places, = .20 m

          OR

          b) 

Measurement of matter

     Mass: the amount of matter within an object
     
     Weight: the force of gravity of mater acting on an object (9.8 m/s^2)

     Volume: the amount of space taken up by an object.
                  
                   Volume = A x h, A = area, h = height