• [Ended]Physics Tutoring Session #9 "Motion" on Wed. 2nd March, 2016 @ 3:00 - 4:00pm

    Physics

    Chima Sunday (Tutor)
    02-03-2016 13:12:00 +0000

    Today we are going to take a new dimension. Basically, we will be discussing on Motion, but our priority will be on solving motion related problems often tested in JAMB and other exams. So sit tight and have a nice tutoring hour. Don't forget to come along with your questions, and were you need more explanations, and i promise to do the best within my power to make the time worth.
    Welcome and enjoy your time.
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    motion
    If you had to think consciously in order to move your body, you would be severely disabled. Even walking, which we consider to be no great feat, requires an intricate series of motions that your cerebrum would be utterly incapable of coordinating. The task of putting one foot in front of the other is controlled by the more prim- itive parts of your brain, the ones that have not changed much since the mammals and reptiles went their separate evolutionary ways. The thinking part of your brain limits itself to general directives such as “walk faster,” or “don’t step on her toes,” rather than mi- cromanaging every contraction and relaxation of the hundred or so muscles of your hips, legs, and feet.
    Physics is all about the conscious understanding of motion, but we’re obviously not immediately prepared to understand the most complicated types of motion. Instead, we’ll use the divide-and- conquer technique. We’ll first classify the various types of motion, and then begin our campaign with an attack on the simplest cases. To make it clear what we are and are not ready to consider, we need to examine and define carefully what types of motion can exist.
    Motion involves a change of position of a body with time. The study of motion without involving the cause of it is call Kinematics

    Types of Motion
    a) Random Motion: Here body moves in a haphazard or disorderly manner. Example is the Brownian motion, motion of smoke particles etc.
    b) Translational Motion: here body moves from one point to another without rotating. E.g is the motion of a boy running from one goal post to another, a ball rolling vertically upward. etc
    c) Rotational Motion: It involves motion of a circle about a center or axis. E.g rotation of the fan blade, motion of the wheel of a moving car. etc
    d) Vibratory Motion: In this kind of motion the path of a moving body is repeated at a succesive equal intervals of time. E.g is the motion of devil as described in the christain bible, motion of a simple pendulum. etc.

    Parameters of Motion
    i) Displacement is defined as the distance travelled in a specified direction
    ii)Speed v is the rate of change of distance with time. v= s/t. Speed is said to be uniform if s/t is constant through out the journey.
    Velocity is speed calculated in a given direction[recall that distance is for speed and rate of change of displacement is for velocity]
    iii)Acceleration a is the rate of change of velocity increase with time While De acceleration is the rate of change of velocity decrease with time. a = v/t.
    A motion is said to be a uniform acceleration if v/t is constant through out the journey e.g is the acceleration due to gravity.

    Equations of Motion
    If a body starts with initial velocity u, accelerates uniformly along a straight line with acceleration a and covers a distance s in a time t when it velocity reaches a final velocity v. then
    s = (v+u/2)t ...............................(1)
    a = v-u/t; v = u + at ......................(2)
    Eliminating t in (1) and (2) to obtain
    v2 = u2 + 2as ........................... (3)
    Eliminating v in (1) and (2) to have
    s = ut + (1/2)at2
    These four equations are used in solving problems associated with uniformly accelerated motion.

    Examples
    (Jamb 2015 Question 4) What force has to be exerted on a mass 60kg to give it an acceleration of 10ms-2 vertically upwards? [g =10ms-2]
    Option A 600N
    Option B 1200N
    Option C 400N
    Option D 300N
    Solution
    Here we need the force that to find the resultant R
    let the upward force be F, and down ward force be W(weight of the body i.e mg). So since the two are opposite, then, m = 60kg, a = 10m/s^2 and g = 10m/s^2
    R = F - W
    so F = R + W = ma + mg = m(a + g) = 60(10 + 10) = 1200N.

    (Jamb 2015 Question 5) A satellite revolving around the earth is kept on its orbit by
    Option A centrifugal forces only
    Option B centripetal forces only
    Option C centripetal and frictional forces
    Option D centripetal and centrifungi forces
    Solution
    When a body is undergoing a rotational motion, two forces are invited for a play, one is centripetal force, this force is trying to keep this body in motion in its circulating orbit while the other force centrifugal, just like its name "fugal" it is trying to force the body out from its orbit. But since this are equal and opposite. its resultant effect is zero. Note the question ask for the one keeping the body in its orbit, so correct answer is Option B.

    Question 46 A train with an initial velocity of 20ms-1 is subjected to a uniform deceleration of 2ms-2 the time required to bring the train to a complete halt is
    Option A 20s
    Option B 40s
    Option C 10s
    Option D 5s
    Solution recall from equations of motion that v = u + at, were symbols have there usual meaning.
    so t = (v - u )/a. since a is decelerating a = -a, final velocity v = 0
    then t = -u/-a = u/a = 20/2 = 10s
    So Answer is Option C

    (Jamb 2013 Question 7) Calculate the apparent weight loss of a man weighing 70kg in an elevator moving downwards with an acceleration of 1.5ms-2
    Option A 686N
    Option B 595N
    Option C 581N
    Option D 105N
    Solution
    When a body is in an elevator moving downward its resultant force R is given below
    R = F + W
    , F = W - R = mg + ma = m(g - a).
    a = 1.5m/s^2, g = 10m/s^2
    F = 70(10 - 1.5) = 70(8.5) = 595N
    So apparent loss in weight = the man weight in air - his weight in the elavator = 700 - 595 = 105N
    Answer Option D



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