Mechanics Topic 2.1 Kinematics. Kinematic Concepts: Displacement Is a measured distance in a given...
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Transcript of Mechanics Topic 2.1 Kinematics. Kinematic Concepts: Displacement Is a measured distance in a given...
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Mechanics
Topic 2.1 Kinematics
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Kinematic Concepts:Displacement
Is a measured distance in a given directionIt is a vector quantityIt tells us not only the distance of the object from a particular reference point but also the direction from that reference pointTypically, it is measured from the origin of a Cartesian co-ordinate system
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Kinematic Concepts:Speed
Is the rate of change of distanceOr is the distance covered per unit timeIt is a scalar quantity…it has magnitude onlySpeed is the total distance (d) covered in total time (t)Speed (s) = total distance (d)
total time (t)
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Kinematic Concepts:Velocity
Is the rate of change of displacementIs a measured speed in a given directionIt is a vector quantity…It tells us not only the speed of the object but also the direction
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Average Velocity
Defined as the total displacement (s) of the object in the total time (t)
Velocity (vav) = total displacement (s)total time (t)
vav = s twhere indicates a change in the value
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Instantaneous Velocity
Is the velocity at any one instant
v = s t
*Where t is tending towards zero
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Kinematic Concepts:Acceleration
Is the rate of change of velocity in a given directiona = v / t (where v = v – u)It is a vector quantityAcceleration in the same direction as motion results in an increase in speedAcceleration in the opposite direction as motion results in a decrease in speedAcceleration perpendicular to the direction of motion results in a change in direction
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Graphical Representation of Motion
These come in 5 forms…1. Distance-time graphs2. Displacement-time graphs3. Speed-time graphs4. Velocity-time graphs5. Acceleration-time graphs
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Gradiants of Graphs
Gradiant of a Displacement-time graph is the velocity
Gradiant of a Velocity-time graph is the acceleration
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Areas Under Graphs
Area under a Velocity-time graph is the displacementArea under a Acceleration-time graph is the velocityAreas can be calculated by the addition of geometric shapes
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The Equations of Uniformly Accelerated Motion
There are 4 equations which we use when dealing with constant acceleration problems
You need to be able to derive them
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The 4 Equations
Supposing the velocity of a body increases from u to v in time t, then the uniform acceleration, a is given bya = change of velocity
time takena = v – u
t v = u + at - equation (1)
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Since the velocity is increasing steadily, the average velocity is the mean of the initial and final velocities, i.e.Average velocity = u + v
2If s is the displacement of the body in time t, then since average velocity = displacement/time = s/tWe can say s = u + v
t 2 s = ½ (u + v) t - equation (2)
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But v = u + at s = ½ (u + u + at) t s = ut + ½at2 - equation (3)
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If we eliminate t from (3) by substituting in t = (v – u)/a from (1), we get on simplifyingv2 = u2 +2as - equation (4)
Knowing any three of s, u, v, a, t, and the others can be found
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Experiments show that at a particular place all bodies falling freely under gravity, in a vacuum or where air resistance is negligible, have the same constant acceleration irrespective of their masses.This acceleration towards the surface of the Earth, known as the acceleration due to gravity, is donated by g.
Acceleration Due to Gravity
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Its magnitude varies slightly from place to place on the Earth´s surface and is approximately 9.8ms-2
The IB generally allows for an approximation of 10 ms-1 to be used
All of the uniform acceleration equations are applicable to situations of free fall
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The Effects of Air Resistance
Air resistance depends on 2 things Surface area Velocity
Air resistance increases as surface area increasesAir resistance increases as the velocity increases
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Terminal Velocity
As an object falls through the air, it accelerates, due to the force of attraction of the Earth. This force does not change.As the velocity increases, the air resistance, the force opposing the motion, increases, therefore the acceleration decreases.
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If the object falls for long enough, then the air resistance (a force acting upwards) will equal the force of attraction of the Earth (the weight) (a force acting downwards)Now there are no net forces acting on the object (since the two forces balance) so it no longer accelerates, but travels at a constant velocity called its terminal velocity. A sky diver has a terminal velocity of more than 50ms-1 (100 miles per hour)
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Relative Motion
If you are stationary and watching things come towards or away from you, then your stating of velocities is straightforward.If, however you are in motion, either moving towards or away from an object in motion, then your frame of reference is different
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In this case the relative velocity is the velocity of the object relative to your motion.Common examples include cars overtaking Trains going passed platforms