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Dynamics Chris Parkes October 2013 Dynamics Work/ Kinetic Energy Potential Energy Conservative forces Conservation laws Momentum Centre-of-mass Impulse Part II – “We are to admit no more causes of natural things than such as are both true and sufficient to explain their appearances.” READ the Textbook!

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Work & Energy Work = Force F times Distance s, units of Joules[J] More Precisely, W=F.x –F,x Vectors so W=F x cos –Units (kg m s -2 )m = Nm = J (units of energy) –Note 1: Work can be negative e.g. Friction Force opposite direction to movement x –Note 2: Can be multiple forces, uses resultant force ΣF –Note 3: work is done on a specific body by a specific force (or forces) The rate of doing work is the Power [Js-1 Watts] Work is the change in energy that results from applying a force F s x F So, for constant Force

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Example A particle is given a displacement in metres along a straight line. During the displacement, a constant force Find (a) the work done by the force and (b) the magnitude of the component of the force in the direction of the displacement. in newtons acts on the particle.

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r F θ F cos θ

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Work-Energy Theorem The work done by the resultant force (or the total work done) on a particle is equal to the change in the Kinetic Energy of the particle. Meaning of K.E. K.E. of particle is equal to the total work done to accelerate from rest to present speed suggests Work Done by Varying Force W=F.x becomes

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Energy, Work Energy can be converted into work –Electrical, chemical, or letting a weight fall (gravitational) Hydro-electric power station mgh of water I n terms of the internal energy or potential energy Potential Energy - energy associated with the position or configuration of objects within a system Note: Negative sign Potential Energy, U

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Reference plane mg U g = 0 h - h U g = mgh U g = - mgh Gravitational Potential Energy particle stays close to the Earth’s surface and so the gravitational force remains constant. No such thing as a definitive amount of PE Choice of zero level is arbitrary

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This stored energy has the potential to do work Potential Energy We are dealing with changes in energy 0 h choose an arbitrary 0, and look at p.e. This was gravitational p.e., another example : Stored energy in a Spring Do work on a spring to compress it or expand it Hooke’s law BUT, Force depends on extension x Work done by a variable force

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Consider small distance dx over which force is constant F(x) dx Work W=F x dx So, total work is sum 0 X Graph of F vs x, integral is area under graph work done = area F X dx

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Elastic Potential Energy Unstretched position X -X For spring,F(x)=-kx: F x X Stretched spring stores P.E. ½kX 2

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Potential Energy Function Reference plane k x mg FsFs

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Conservative Forces –A system conserving K.E. + P.E. (“mechanical energy”) But if a system changes energy in some other way (“dissipative forces”) –e.g. Friction changes energy to heat, reducing mechanical energy –the amount of work done will depend on the path taken against the frictional force –Or fluid resistance –Or chemical energy of an explosion, adding mechanical energy Conservative & Dissipative Forces Conservation of Energy K.E., P.E., Internal Energy

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Conservative forces frictionless surface

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Example A 2kg collar slides without friction along a vertical rod as shown. If the spring is unstretched when the collar is in the dashed position A, determine the speed at which the collar is moving when y = 1m, if it is released from rest at A.

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Properties of conservative forces Work done on moving round a closed path is zero Work done by friction force is greater for this path The work done by a conservative force is independent of the path, and depends only on the starting and finishing points The work done by them is reversible A B

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Forces and Energy e.g. spring Partial Derivative – derivative wrt one variable, others held constant Gradient operator, said as grad(f)

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Minimum on a potential energy curve is a position of stable equilibrium - no Force Glider on a linear air track Negligible friction

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Maximum on a potential energy curve is a position of unstable equilibrium U

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Linear Momentum Conservation Define momentum p=mv Newton’s 2 nd law actually So, with no external forces, momentum is conserved. e.g. two body collision on frictionless surface in 1D before after m1m1 m2m2 m1m1 m2m2 v0v0 0 ms -1 v1v1 v2v2 For 2D remember momentum is a VECTOR, must apply conservation, separately for x and y velocity components Initial momentum: m 1 v 0 = m 1 v 1 + m 2 v 2 : final momentum Also true for net forces on groups of particles If then

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Energy Conservation Need to consider all possible forms of energy in a system e.g: –Kinetic energy (1/2 mv 2 ) –Potential energy (gravitational mgh, electrostatic) –Electromagnetic energy –Work done on the system –Heat (1 st law of thermodynamics) Friction Heat Energy can neither be created nor destroyed Energy can be converted from one form to another Energy measured in Joules [J]

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Collision revisited We identify two types of collisions –Elastic: momentum and kinetic energy conserved –Inelastic: momentum is conserved, kinetic energy is not Kinetic energy is transformed into other forms of energy Initial K.E.: ½m 1 v 0 2 = ½ m 1 v ½ m 2 v 2 2 : final K.E. m1m1 v1v1 m2m2 v2v2 See lecture example for cases of elastic solution 1.m 1 >m 2 2.m 1

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Impulse Change in momentum from a force acting for a short amount of time (dt) NB: Just Newton 2 nd law rewritten Where, p 1 initial momentum p 2 final momentum Approximating derivative Impulse is measured in Ns. change in momentum is measured in kg m/s. since a Newton is a kg m/s 2 these are equivalent Q) Estimate the impulse For Andy Murray’s serve [135 mph]?

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Centre-of-mass Average location for the total mass Position vector of centre-of-mass Mass weighted average position Centre of gravity – see textbook

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dmdm r x y z Rigid Bodies – Integral form dm is mass of small element of body r is position vector of each small element.

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Momentum and centre-of-mass Differentiating position to velocity: Hence momentum equivalent to total mass × centre-of-mass velocity Forces and centre-of-mass Differentiating velocity to acceleration: Centre-of-mass moves as acted on by the sum of the Forces acting

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Internal Forces Internal forces between elements of the body and external forces –Internal forces are in action-reaction pairs and cancel in the sum –Hence only need to consider external forces on body In terms of momentum of centre-of-mass

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Example A body moving to the right collides elastically with a 2kg body moving in the same direction at 3m/s. The collision is head-on. Determine the final velocities of each body, using the centre of mass frame. 4kg 6ms -1 2kg 3ms -1 C of M

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4kg 6 ms -1 2kg 3 ms -1 C of M Lab Frame before collision Centre of Mass Frame before collision 5 ms -1 4kg 1 ms -1 2kg 2 ms -1 C of M Centre of Mass Frame after collision 4kg 1 ms -1 2kg 2 ms -1 C of M

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