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

     

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