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Newton'a Laws of Motion

The provided notes give concepts related to Newton's 3 Laws of Motion

SPS 8 Force, Motion, and Work

Domain 5 Teacher Notes – SPS8abcde

I.  Motion

       A.  Newton’s Laws of Motion

         1.  The Law of Inertia or Newton’s First Law: An

               object at rest tends to stay at rest and an object

               in motion tends to remain in motion in a

               straight-line path unless acted on by an

               unbalanced force.

               a.  Inertia is another word for mass.  The more

                    mass an object has, the greater its tendency

                    to maintain its current state.


       Click here for video clip


               b.  Applications:

                    i.  People are often thrown from automobiles

                        in wrecks because the car comes to a

                        sudden stop, but the person has a

                        tendency to stay in motion.

                   ii.  The ride is much smoother on a cruise ship

                        than a fishing boat, because the cruise ship

                        is more massive and is not affected as

                        much by the waves.


        2.  Newton’s Second Law: The acceleration of an

             object is directly proportional to the applied force

             and inversely proportional to its mass.

                                 F=ma               F= force
                                                     m = mass

                                                         a = acceleration


                          Click here for video clip















             Sample Problem: What is the force exerted by a

             2 kg mass that accelerates at 3 m/sec/sec?


 mass=2 kg                          F=ma

 acceleration=3 m/sec/sec   F =2 kg x 3 m/sec/sec

                                             F=6 kilogram×meter/sec/sec


      Check the SCIENCE FACTS AND FORMULAS sheet

                          1 newton = 1 kilogram×meter/second/second


                So the correct answer is 6 newtons.


        3.  Newton’s Third Law: For every action there is an

             equal and opposite reaction.

      a.  If object A exerts a force on object B, then

           object B exerts an equal force on object A in

           the opposite direction.


             Click here for video clip


      b.  Consequences: Forces always exist in pairs.  It

                   is impossible for you to push on something

                   without it pushing back.  Newton’s Third law

                   can be used to explain the motion of rockets

                   and balloons.  As the gases exit the balloon or

                   rocket, they push it in the opposite direction.


 B.  Motion depends on the observer’s frame of



C.  Speed

  1.  A measure of how fast something is moving

  2.  The distance traveled in a given amount of time

  3.  Formula speed = distance






   4.  Sample Problem:  A bicyclist rides for 1.5 hours

        from Snellville to downtown Atlanta. He travels 21

        miles.  What is his average speed?


        d=21 miles                     speed=distance

         t=1.5 hours                                   time

                                              speed=  21 miles

                                                           1.5 hours

                                                      Speed = 14 mi/hr

D.  Velocity

  1.  Speed in a particular direction

  2.  formula:  velocity = distance and direction



The formula from the SCIENCE FACTS AND FORMULAS sheet is  Velocity (V) = V0 + at, where V0 = Initial Velocity, a = Acceleration, and t = Time


          3.  Sample Problem:  What is the average velocity of

               a commercial jet that travels west from New York

               to Los Angeles (4800 km) in 6.00 hours?

                Velocity = distance



                             =  4800 km

                                6.00 hours


                             =  800 km/hr west


E.  Acceleration

  1.  The rate at which velocity changes

  2.  formula:                       final velocity-initial velocity

                   acceleration =                   time


     or        acceleration =


Acceleration = Change in Velocity/Time Elapsed       

3.  Acceleration occurs if either of these two

     conditions exist.

     a.  The speed of an object is changing.  It can be

           increasing or decreasing.

The direction of the movement is changing.

                Click here for Video Clip (4 min)

        4.  Sample Problem:  If a car accelerates from 5 m/s to

             15 m/s in 2 seconds, what is the car's average



               V = 15 m/s                     

              Vo =  5 m/s                            a= 15 m/s - 5 m/s

                t =  2 sec                                        2 sec

                                                            a= 5


II.  Forces


The force of gravity between any two objects increases as the mass of either object increases.  The force of gravity decreases as the distance between the objects increases.


The force of gravity experienced by something is also known as its weight.  Weight can be calculated multiplying mass by the acceleration of gravity (g).




Weight depends on both mass and the acceleration of gravity.  Mass depends only on the amount of matter in an object.  Mass does not change when the location of an object changes.


Click here for video clip

The force and weight are both measured in Newtons.

Mass is measured in kilograms.  Acceleration is

measured in m/s/s (m/s2).


          3.  Free Fall

               a.  A free-falling object is an object which is falling

                    under the sole influence of gravity.

        b.  Free-falling objects do not encounter air


        c.  All free-falling objects (on Earth) accelerate

             downwards at a rate of approximately 10 m/s/s

             (to be more exact, 9.8 m/s/s). This quantity

             known as the acceleration of  gravity has a

             special symbol to denote it - the symbol g.

             The distance traveled by a falling object is

             calculated using the formula, d = ½ gt2



        d.  Sample problem:  A rock is dropped from the

              top of a cliff and strikes the ground 6.5

              seconds later. How high is the cliff in meters?


   g=9.8 m/sec2           d = ½ gt2

      t = 6.5 sec                  = ½ (9.8 m/sec2)  (6.5 sec)2

                                       =  ½ (9.8 m/sec2)  (42.25 sec2)

                                       =  ½ (414.05 m)

                                       =  207 m


 Electromagnetic Forces – Like charges repel each

     other, opposite charges attract.

 1.  As the distance between the charges increase, the

      magnitude of the force decreases.  The same holds

      true for magnets.

 2.  Magnets will always have a North Pole and a South

      Pole.  Just like with electrical charges, opposite

      poles attract.  While it is possible to separate

      positive and negative charges, it is impossible

      to separate north and south magnetic poles.


        3.  Electric and Magnetic Field Lines- The lines always

              go from positive charges to negative charges and

              from north poles to south poles.  The closer the

Magnetic field is strongest where the lines are closer together

              lines, the stronger the field.



1.List a use for electromagnets.


Lift and move cars in a junk yard




2.  How can you make a stronger


     a.  More coils of wire around the nail

     b.  More batteries


Click here for video clip



  two conditions must be met for work to occur

the object must move through a distance

a force must act upon the object in the direction the object moves


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  SI unit for work is the joule, J.  (Newton-meter)

  Formula:  work = force X distance   W=F X d

  When an object is lifted to a new location or pushed

      up a ramp, the work equals the potential energy


5.  Sample Problem:  What work is done if Hernando

     uses 88 N of force to pull a table 12 meters?


      F = 88N          Use the formula  W=F X d

      d = 12 m        from the              W=88N X 12 m

          SCIENCE FACTS AND FORMULAS sheet       W= 1056 N-m or

                                                                 1056 J



the rate of work

SI unit is the watt, W (joule/second)

formula:  power = work/time  (work divided by time)

      4. Sample Problem:  When doing a chin-up, a physics

student lifts her 40-kg body [which has a force (weight) of 400 N] a distance of 0.25 meters in 2 seconds. What is the power delivered by the student's biceps?

You must first calculate the work done to lift her body

W = F X d

                      = (400 N) (0.25 m)

W = 100 J

To calculate power

                        Power= work



                         Power=100 J

                                      2 sec

                        Power = 50 Watts


III.   Energy

 Law of conservation of energy.

 Energy can neither be created nor destroyed.

 It can be changed into other forms.

Two kinds of energy

Click here for video clip

Potential energy is stored energy or energy of position.  Examples: Magnetic, gravitational, chemical, elastic, and nuclear

                   PE = mgh where m= mass

                                              g = acceleration due to gravity

                                             h = height

Kinetic energy is energy of motion or energy in action.  Examples: a moving baseball or a roller coaster going downhill

KE = ½ mv2

Forms of energy

mechanical energy

 Any object which possesses mechanical energy -   whether it be in the form of potential energy or kinetic energy - is able to do work. That is, its mechanical energy enables that object to apply a force to another object in order to cause it to be   displaced.

    b. Simple machines help us lift, pull, increase

        elevation of heavy things, change the direction of

        the force, increase the force, split things, fasten

        things, and cut things


   i.  inclined plane - Ramp, stairs

             ii.  wedge – two inclined planes back to back –

                  screwdriver, knife, axe

             iii.  screw – inclined plane wrapped around a


              iv.  pulley – rope revolves around a fixed point;

                    more pulleys make work easier

               v.  lever – has a fulcrum – see-saw

               vi.  wheel and axle – bicycle, car, doorknob,

                     screwdriver in use

          Click here for Simple Machines video clip


mechanical advantage-the number of times a machine multiplies an effort force. 
Formulas from formula sheet:

            Actual Mechanical Advantage:  

    where FR is Force due to resistance and FE is Force due to effort


IMA=   Effort Length

                              Resistance Length


Sample Problem:  In a pulley set-up, an effort force of 50 Newtons lifts a resistance force of 100 Newtons.  What is the MA of the pulley system?


  IMA=     Effort Length

         Resistance Length

Think about which formula to use


AMA= Fr            M.A. =   100N

           Fe                                        50N

                         M.A  =     2


Heat, Temperature and Internal Energy

The temperature of an object is directly proportional to the average kinetic energy of its particles.  As temperature goes up, particles move faster.

The Internal Energy of a substance is total of the potential and kinetic energy of all its particles.

Energy moving from one location to another is known as heat.  Objects do not contain heat.  Instead they contain internal energy.

Chemical energy relates to potential energy stored in the bonds between atoms in a compound.

                     Click for Video Clip