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Physics YEA Assessment

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Transcript of Physics YEA Assessment

Measurements Kinematics Physics Instruments of Measurements Metre Rule Accuracy of measurement Vernier Callipers Micrometre Screwgauge 0.001m/0.1cm/1mm 10 division, 0.0001m/0.01cm/0.1mm
20 division, 0.00005m/0.005m/0.05mm
n division,[(1/n)/1000]m/[(1/n)/10]cm/(1/n)mm 0.001mm Practicals Tables
1. Headings should be apt
2. Measurements must be written to the
accuracy of the instrument
3. Calculated values
a. For addition and subtraction, least d.p.
b. Other than - and +, least s.f.
4. Collect at least 8 sets of data
5. No correction tape
6. No pencil
7. Calculate row by row(not column by column)
so that the calculations do not use the truncated values.
8. For average, values to least s.f. Graphs
1.Quantity and the unit is shown when labelling axes.
2. The range must be correctly chosen so that the graph covers >50% of the graph paper and the error is reduced.
3. Odd scales are not used
4. All data points are plotted correctly
5. A best fit graph is drawn. Best fit graph has equal number of scatter on either side.
6. Large gradient triangle is drawn .
7. Coordinates of the 2 points in the gradient triangle is written.(Must not be plotted point)
8. When reading off graph accuracy must be to half-square.(i.e. if your scale is 1:1, your accuracy should be to 0.5)
9. Anomaly points(outliers) are circled.(Must not have more than 3 outliers)
10.Only pencil can be used to draw graph.
11. Calculating gradient by using the 2 coordinates can only be done on the main paper and not the graph paper. 1. Able to answer questions posed based on
results from the graph.
2. Uncertainties cannot be generic (i.e. happens
to every experiment) and must display an understanding in the conditions in the current
experiment.(e.g. When we are doing free fall experiment, we must say "force may have been applied at the start" instead of "human reaction time is uncertain".
3. Uncertainty statement must consider what is the uncertainty and how it affects the measured value ,and how it affects the experiment.
4. Solutions must be provided to each uncertainty listed. Must explain and be clear on how the solution will improve the measured results and in turn improve the experiment.(e.g. For the free fall experiment a possible solution for the above problem of force being applied by the person who drops the ball, we can say "I should use a retort stand and hold the ball stationary and let go of it by reducing the tightness." Prefixes:
Tera-10^12 Base Units:
Length - m
Mass - kg
Time - s
Electric current - A
Temperatur - K
Amount of substance - mol Conversions What is kinematics? Kinematics is how we describe motion. Instantaneous Velocity:
Velocity is Displacement/Time.
Instantaneous Velocity is the velocity
of an object when the time tends towards
zero. Therefore we say that instantaneous velocity
is the velocity at an instant. Change in Velocity:
The change in velocity(v1 - v2) of an
object over a period of time is acceleration.
Therefore acceleration is (v1 - v2)/(t1 - t2). Back into concepts of adding and subtracting
vectors. You cannot subtract a vector physically
(i.e. you cannot draw a diagram) however what
you can do is add the first vector the opposite of the
second vector. E.G. V1 - V2 V1, 10ms-1 V2, 5ms-1 - = + V1, 10ms-1 -V2, -5ms-1 = -V2, -5ms-1 V1, 10ms-1 = V1 + (-V2) = 10 + (-5) = 5 ms-1 Finding final velocity(Vf) with initial velocity(Vi) and acceleration(a) for t seconds.
a = (Vf - Vi) / t
Vf - Vi = (a)(t)
Vf = a*t + Vi

Therefore if the question says that an object's instantaneous velocity when t=1 is x ms^-1 and it accelerates at y ms^-2 for z seconds, and asks for the instantaneous velocity at (t+z) seconds, the answer will be (y * z) + x. If y = 3, z= 2 and x = 4, the answer will be (3 * 2) + 4 = 10 ms^-1. Free Body Diagrams -Isolate object.
-Shows all forces acting on the object
-shows where the force starts from
-Shows the type of force Book 1 resting on Book 2 Book 1 Book 2 Free body diagram of Book 1 W1(starts from centre) N1 Free body diagram of Book 2 W2 N2 N3 Explanation:
Book 1 has weight(W1) that
is exerted by the earth. We know'
that weight is from our center of
gravity, so W1 starts from the centre
of Book 1. Since Book 1 is in contact with Book 2, there
will be a normal contact force which will push it up
(Newton's 3rd law). So, this force, N1 will start
from the point of contact, the bottom of the book. Book 2 has weight(W2) that is exerted by the earth.
We know that weight is from the center of gravity so
W1 starts from the center. The weight from Book 1 is acting on Book 2 so,
we have a force, N2 which is directly equal to and opposite
N1. The book now has a normal force pushing it up(N3) stopping the book from sinking. So, N3 = N2 + W2. Action-Reaction Pairs N2 and N1 are action reaction pairs.
N1 = W1 since Book 1 is stationary.
N3 = W2 + N2 since Book 2 is stationary. Logic:
As long as some object is stationary, it means
that the forces in it cancel each other out. Equations of motion The equations of motion we need to know: v = u + at
v^2 = u^2 + 2as
s = 1/2(v+u)t Derived equations of motion: s = ut +1/2a/(t^2)
Some more... forgot... i dun know.... Graphical Approach to solving problems Relations: Area bounded by a-t graph and the t-axis is equal to change in velocity.
Area bounded by v-t graph and the t-axis is equal to change in displacement. Gradient of v-t graph is equal to the instantaneous acceleration. Gradient of x-t graph equals to instantaneous velocity. v-t graph a-t graph x-t graph area bound area bound gradient gradient Newton's 3rd Law Newton's 3rd Law states that each action has an
equal and opposite reaction. This action and reaction
is called a action-reaction pair. Characteristics of action-reaction pair An action-reaction pair must have the same nature meaning
they must both be contact forces or non-contact forces. Contact forces Slapping,
Normal Non-contact force Gravity,
Magnetic An action-reaction pair must be equal and opposite. An action-reaction pair must be acting on different objects. Example When I punch the door and the door does not move
.There is a force from my hand pushing the door. But, the door pushes back at me naturally. The force from my hand is acting on the door and the force from the door is acting on me. Since the door did not move, the force from my hand is equal to the force from the door. Both of these forces arise due to contact(are contact forces). So, the 2 forces are a action reaction pair. Newton's 3 laws of motion Newton's Second Law Newton's first law Inertia Tendency to be stationary Tendency to continue moving
on a straight line. Newton's First law Equal and opposite Acting on different
bodies Same
nature Free Fall A state where the only force acting on you is gravity.
Acceleration due to free fall is constant: 9.81 ms^-2. On Earth, not achievable. Because we have forces like Air Resistance. Viscous force due to air. Governed by 2 things: velocity and surface area.
Greater velocity, Greater air resistance.
Greater surface area, Greater air resistance.

When a person falls from the sky and pulls his parachute half way through his journey:

At first, his velocity = 0 therefore, air resistance = 0 and resultant force = Weight. Therefore acceleration is 9.81 ms^-2. As his velocity increases, the air resistance increases and thus resultant force is weight - air resistance and acceleration reduces until a point where air resistance = Weight and thus by Newton's second law, F = ma, F = 0, m > 0. Therefore a = 0. However, this velocity is still too high. So, the person pulls his parachute and there is a sudden increase in surface thus suddenly increasing air resistance. This might cause the person to "jump" a bit. After that, the acceleration is negative and the velocity decreases. Because of the decreasing velocity, air resistance decreases to a point where air resistance = weight and by Newton's Second Law acceleration = 0 and the person reaches terminal velocity. This is a lower velocity and is safe for the person to land. Moment : Turning effect of a Force Mathematical Expression:
Moment = F x d where F
is the force and d is the perpendicular
distance from the pivot. Unit : Nm Center of Gravity How to measure?
Pick any 3 points on the object. Hang the object through the 3 points and draw a vertical line down from the object. Where the 3 line intersect is the CG lies. Stability As long as the CG is directly above
the base, the object is stable. If the CG is below the pivot, the object is able to "self-stabalize". Work , Energy , Power Mathematical Expression of Work done:
W = F x d. Where W is work done, F is the force and d is the distance travelled by the force.
Work done = Sum of Energy gained and lost Energy GPE = mgh KE = 1/2mv^2 Elastic Potential Energy = 1/2kx^2 Power = Work done per s Unit : J Unit : W Efficiency = Power input / Useful Power output x 100% Resolution of Forces If a force, F, is at an angle of x degrees, it can be resolved into a vertical and a horizontal force as such F x degrees Horizontal force: Fcosx Vertical Force: Fsinx Once resolved the force, the Vertical and Horizontal forces work independently. The vertical force does not affect the horizontal in any way. If the sum of vertical forces = 0, the object is not moving vertically. If the sum of the horizontal forces = 0, the object is not moving horizontally.
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