IB Physics Topic 2 Mechanics
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Define displacement, velocity, speed and acceleration.
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DISPLACEMENT: change in position of an object from origin, in a particular direction, vector VELOCITY: rate of change of displacement with respect to time, vector, velocity=change of displacement/time SPEED: rate of change of distance with respect to time, scalar, speed=distance/time ACCELERATION: rate of change of velocity with respect to time, vector, acceleration=change of velocity/time
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Explain the difference between instantaneous and average values of speed, velocity and acceleration.
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AVERAGE VALUE: change over a period of time INSTANTANEOUS VALUE: value at a given instant in time
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Outline the conditions under which the equations for uniformly accelerated motion may be applied.
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Needs to have constant acceleration, neglecting retarding forces such as air resistance and frictions. IN LINEAR MOTION
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Identify the acceleration of a body falling in a vacuum near the Earth's surface with the acceleration g of free fall.
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In the absence of air resistance, all falling objects have the SAME acceleration of free fall, INDEPENDENT of their mass.
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Solve problems involving the equations of uniformly accelerated motion.
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v=u+at a=(v-u)/t average velocity=s/t (other equations given in formula book)
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Define air resistance.
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Force of air that pushes against a falling object
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Describe the effects of air resistance on falling objects.
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Velocity does not keep rising, eventually reaches a maximum or TERMINAL VELOCITY because acceleration=zero
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Define terminal velocity.
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The maximum velocity reached by a falling object due to air resistance, when acceleration=zero
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Determine relative velocity in one and in two dimensions.
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•If two things are moving in same straight line but at different speeds, relative velocity is addition or subtraction.
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Define a force.
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•A force gives rise to velocity change (acceleration). •A force can cause deformation. •The direction of the acceleration is the same as the direction of the force which produced it
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Calculate the weight of a body.
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W = mg.
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Identify the forces acting on an object and draw free-body diagrams representing the forces acting.
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•Only one object is chosen •All forces on that object are shown
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Determine the resultant force in different situations.
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FLAT HORIZONTAL SURFACE: normal force is equal and opposite to the gravitational force INCLINED PLANE: Draw free-body diagram, the weight and normal reaction force are not balanced, and a resultant force acing in direction of the slope exits. Use trigonometry to work out the relationship between the different forces.
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Measuring forces in springs.
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•The simplest experimental method for measuring the size of a force is to use the extension of a spring. •Up to the elastic limit, the extension, x of a spring is proportional to the tension force, F. F∝x •Constant of proportionality k is called the spring constant ∴F=kx
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State Newton's first law of motion.
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An object continues in uniform motion in a straight line or at rest unless a resultant external force acts on it.
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State the condition for translational and equilibrium.
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TRANSLATIONAL: •The resultant force on an object is zero ∑F=0 •Forces are vector quantities, a zero resultant force means no force IN ANY DIRECTION
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State Newton's second law of motion.
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•The resultant force is proportional to the rate of change of momentum. F=∆p/∆t •The resultant force is proportional to the acceleration, this is an experimenta law F=ma
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Define linear momentum and impulse.
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LINEAR MOMENTUM: the mass of an object multiplied by its velocity, p=mv IMPULSE: change in momentum, force multiplied by duration of force Impulse=Δp=FΔt
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Determine the impulse due to a time-varying force by interpreting a force-time graph.
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Area under graph
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State the law of conservation of linear momentum.
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Total linear momentum of a system of interacting particles remains constant provided there is no resultant external force.
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State Newton's third law of motion.
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When two bodies A and B interact, the force that A exerts on B is equal and opposite to the force that B exerts on A.
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Important notes of Newton's third law.
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Fab=-Fba •the two forces in the pair act on DIFFERENT OBJECTS •the forces are the same type
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Outline what is meant by work.
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W=Fs •whenever work is done, energy is transferred from one place to another or transformed from one form to another W=Fscosθ for when force applied at angle to resulting displacement.
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What is the work done in compressing or extending a spring?
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½kx²
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Determine the work done by a non-constant force by interpreting a force-displacement graph.
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Area under curve is work done.
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Outline what is meant by kinetic energy.
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KE=½mv²=p²/2m
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Outline what is meant by change in gravitational potential energy.
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Energy object has due to its position in a gravitational field. ΔEp=mgΔh
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State the principle of conservation of energy.
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•Energy cannot be created nor destroyed. it simply changes from one form of energy to another. •Total energy of a closed system is constant •There is no change in total energy in the universe
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Describe the amount of energy transferred.
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Equal to the work done.
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What is elastic potential energy.
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Elastic potential energy=½kx²
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Distinguish between elastic and inelastic collisions.
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ELASTIC: No mechanical energy is lost, KE is conserved. INELASTIC: Kinetic energy is not conserved, energy is lost. If they stick together, COMPLETELY INELASTIC. MOMENTUM IS ALWAYS CONSERVED IN ALL COLLISIONS
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Define power.
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•Rate at which work is done. •Rate at which energy is converted. Power=Energy transferred/time=WD/time
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Define and apply the concept of efficiency.
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EFFICIENCY: is the ratio of useful energy to the total energy.
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Draw a vector diagram to illustrate that the acceleration of a particle moving with constant speed in a circle is directed towards the centre of the circle.
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•Direction is changing all the time •Velocity of object is constantly changing •object in uniform circular motion MUST be accelerating even if speed is constant •Has CENTRIPETAL ACCELERATION caused by the CENTRIPETAL FORCE which acts towards the centre of the circle.
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Apply the expression for centripetal acceleration.
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a=v²/r a=4π²r/T²
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Identify the force producing circular motion in various situations.
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•Caused by centripetal force •the direction of centripetal acceleration is the same as the direction of centripetal force which is always changing Centripetal force=ma(centripetal) =mv²/r towards the centre of the circle
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For circular motion, what is the net horizontal force along the direction of motion and normal to the direction of motion.
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ALONG: zero NORMAL: non zero
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Does an object in circular motion do work?
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The centripetal force does NOT do any work as: WD=force x distance IN THE DIRECTION OF THE FORCE
