SOLID MECHANICS DYNAMICS TUTORIAL CENTRIPETAL FORCE

Transcription

1 SOLID MECHANICS DYNAMICS TUTORIAL CENTRIPETAL FORCE This work coers elements of the syllabus for the Engineering Council Exam D5 Dynamics of Mechanical Systems C10 Engineering Science. This tutorial examines the relationship between inertia and acceleration. On completion of this tutorial you should be able to Explain and define centripetal and centrifugal acceleration. Explain and define centripetal and centrifugal force. Sole problems inoling centripetal and centrifugal force. Derie formulae for the stress and strain induced in rotating bodies. Sole problems inoling stress and strain in rotating bodies. Analyse problems inoling ehicles skidding and oerturning on bends. It is assumed that the student is already familiar with the following concepts. Newton s laws of Motion. Coulomb s laws of friction. Stiffness of a spring. The laws relating angular displacement, elocity and acceleration. The laws relating angular and linear motion. Basic ector theory. Basic stress and strain relationships. All the aboe may be found in the pre-requisite tutorials.

2 1. CENTRIPETAL AFFECTS 1.1 ACCELERATION AND FORCE Centripetal acceleration occurs with all rotating bodies. Consider a point P rotating about a centre O with constant angular elocity ω (fig. 1). Figure 1 The radius is the length of the line O-P. The tangential elocity of P is = ωr. This elocity is constant in magnitude but is continually changing direction. Let s remind ourseles of the definition of acceleration. change in elocity a = time taken a = t Velocity is a ector quantity and a change in direction alone is sufficient to produce a change. It follows that a point traelling in a circle is continuously changing its direction, elocity and hence has acceleration. Next let s remind ourseles of Newton s second Law of Motion which in its simplest form states Force = Mass x acceleration. It follows that anything with mass traelling in a circle must require a force to produce the acceleration just described. The force required to make a body trael in a circular path is called CENTRIPETAL FORCE and it always pulls towards the centre of rotation. You can easily demonstrate this for yourself by whirling a small mass around on a piece of string. Let s remind ourseles of Newton s Third Law. Eery force has an equal and opposite reaction. The opposite and equal force of centripetal is the CENTRIFUGAL FORCE.

3 The string is in tension and this means it pulls in both directions. The force pulling the ball towards the middle is the centripetal force and the force pulling on your finger is the centrifugal force. The deriation of the formula for centripetal force and acceleration is done by considering the elocity as a ector. Figure Consider the elocity ector before and after point P has reoled a small angle δθ. The magnitude of 1 and are equal so let s denote it simply as. The direction changes oer a small period of time δt by δθ radians. We may deduce the change by using the ector addition rule. Figure The first ector + the change = Final ector. The rule is 1 + δ =. This is illustrated below. Figure 4 δ is almost the length of an arc of radius. If the angle is small, this becomes truer. The length of an arc is radius x angle so it follows that δ = δθ This change takes place in a corresponding small time δt so the rate of change of δ δ θ elocity is = δt δt In the limit as δt dt, δ/δt δt δ dt, δt d d dθ dθ is the acceleration. a = = = ω ω = = rate of change of angle dt dt dt dt d dt

4 Since = ωr then substitute for and a = ωr and this is the centripetal acceleration. Since ω = /R then substitute for ω and a = /R Centripetal acceleration = ωr Centripetal acceleration = /R If we examine the ector diagram, we see that as δθ becomes smaller and smaller, so the direction of δ becomes radial and inwards. The acceleration is in the direction of the change in elocity and so centripetal acceleration is radial and inwards. If point P has a mass M, then the force required to accelerate this mass radial inwards is found from Newton's nd Law. Centripetal force = M ω R or in terms of elocity Centripetal force = M /R Centrifugal force is the reaction force and acts radial outwards. WORKED EXAMPLE No.1 Calculate the centripetal acceleration and force acting on an aeroplane of mass 1500 kg turning on a circle 400 m radius at a elocity of 00 m/s. SOLUTION Centripetal acceleration = /R = 00/400 = 5 m/s. Centripetal force = mass x acceleration = 1500 x 5 = 7.5 kn WORKED EXAMPLE No. Calculate the centripetal force acting on a small mass of 0.5 kg rotating at 1500 re/minute on a radius of 00 mm. SOLUTION ω= πn/60 = x π x 1500/60 = 157 rad/s Cent. acc. = ωr = (157) x 0. = 795 m/s. Cent. force = Mass x acc. = 0.5 x 795 = 697 N

5 WORKED EXAMPLE No. A centrifugal clutch is shown in the diagram. The clutch must transmit a torque of 18 Nm at a speed of 14 re/min. The coefficient of friction µ between the drum and the friction lining is 0.. The radius to the centre of graity of each sliding head is 0.1 m and the inside radius of the drum is 0.5 m. Calculate the required mass of the sliding heads. SOLUTION Figure No.5 Torque = 18 Nm, radius = 0.5 m T = Friction force x radius Friction force = 18/0.5 = 7 N This is diided between two friction pads so each must produce 7/ = 6 N each. From the law of friction, Friction force = µ x normal force Normal force = 6/0. = 10 N The normal force acts in a radial direction and must be equal to the centripetal force. Centripetal force = Mω r Equating forces we hae Mω r = 10 Speed = 14 re/min or 14/60 re/s Radius to centre of graity = 0.1 m Angular elocity ω = π x speed = π x 14/60 = rad/s M x x 0.1 =10 M = 10/( x 0.1) =.58 kg for each sliding head.

6 SELF ASSESSMENT EXERCISE No.1 1. A centrifugal clutch is similar to that shown in figure 5. The clutch must transmit a torque of 5 Nm. The coefficient of friction µ between the drum and the friction lining is 0.4. Each sliding head has a mass of 0.4 kg acting at a radius of 0.15 m. The inside radius of the drum is 0.18 m. Calculate the minimum speed required. (Answer 5.78 rad/s or 51.7 re/min). A rotating arm has a sliding mass of kg that normally rests in the position shown with the spring uncompressed. The mass is flung outwards as it reoles. Calculate the stiffness of the spring such that the mass compresses it by 0 mm when the arm reoles at 500 re/min. (Answer 8.4 N/mm) Figure 6

7 STRESS and STRAIN in ROTATING BODIES.1 STRESS Wheels experience stresses in them because the centripetal reaction (centrifugal force) tends to stretch the material along a radius. This can cause the wheel to disintegrate if it runs to fast. The effect might be catastrophic in a grinding wheel or flywheel. Een if the wheel does not disintegrate, the stress will cause it to expand. The blades on a turbine or compressor rotor will stretch slightly under this strain and might touch the casing. Consider a bar of uniform cross section A rotating about its centre as shown. Figure 7 The radius to the tip is R. Consider a small length δr at radius r. The mass of this element is δm = ρaδr. The centrifugal force produced by this mass is δf. δf = δm ωr = ρaδrωr In the limit as δr dr this becomes df = ρadrωr The centrifugal force acting on this section due to the mass between r and R is then found by integration. R [ R r ] F = ρaω r dr = ρaω r The tensile stress induced at this section (at radius r) is the force per unit area. F [ R r ] σ = = ρaω A A [ R r ] σ = ρω

8 . STRAIN Consider the same element (figure 7). Suppose the element δr increases in length by δx. In the limit as δr dr, the strain in this short length is then ε = dx dr ρω = σ ρω = [ R r ] E E [ R r ] ε = E The extension of the short length is ρω dx = σ = [ R r ] dr E For the length from r to R, the extension is found by integrating again. ρω x = E ρω x = E ρω x = E R [ R r ] r R R ρω dr = E R R r R r r R r + r r R r WORKED EXAMPLE No. 4 A bar 0.5 m long with a uniform section is reoled about its centre. The density of the material is 7 80 kg/m. The tensile stress in the material must not exceed 600 MPa. Calculate the speed of rotation that produces this stress. Go on to calculate the extension of the bar. The elastic modulus is 00 GPa. SOLUTION [ R r ] σ = 600x10 = ρω The maximum stress will be at the middle with r = 0 and R = 0.5 m σ = 600 x 10 = 780ω ω =.45 x 10 ω = 1566 Conerting to re/min N = ω x 60/π = re/min 6 [ ] rad/s The extension of one half of the bar from r = 0 to R = 0.5 m is ρω R r 780 x 1566 x 0.5 x = R r 0 0 E + 9 = x 00 x 10 + x = or 0.5mm

9 SELF ASSESSMENT EXERCISE No. 1a. The blade of a turbine rotor has a uniform cross section. The root of the blade is attached to a hub of radius r. The tip of the blade has a radius R. Show that the stress at the root of the blade due to centrifugal force is gien by σ = ρω [ R r ] 1b. The hub radius is 00 mm and the tip radius is 0 mm. The density of the blade material is 8 00 kg/m. Calculate the stress at the root when the rotor turns at re/minute. (157.8 MPa) 1c. Derie the formula for the extension of the blade tip x gien below. ρω x = E R R r r + where E is the elastic modulus. 1d. Calculate the extension of the tip gien that E = 06 GPa. ( mm)

10 . APPLICATION TO VEHICLES.1 HORIZONTAL SURFACE Figure 8 When a ehicle traels around a bend, it is subject to centrifugal force. This force always acts in a radial direction. If the bend is in a horizontal plane then the force always acts horizontally through the centre of graity. The weight of the ehicle is a force that always acts ertically down through the centre of graity. The diagram shows these two forces. The centrifugal force tends to make the ehicle slide outwards. This is opposed by friction on the wheels. If the wheels were about to slide, the friction force would be µw where µ is the coefficient of friction between the wheel and the road. Figure 9 The tendency would be for the ehicle to oerturn. If we consider the turning moments inoled, we may sole the elocity which makes it oerturn. If the ehicle mass is M its weight is Mg. The radius of the bend is R. Figure 10 Consider the turning moments about point O The moment of force due to the weight is W x d or M g d The moment due to the centrifugal force is C.F. x h Remember the formula for centrifugal force is C.F. = (M/R) The moment due to the centrifugal force is C.F. x h = (M/R) x h When the ehicle is about to oerturn the moments are equal and opposite. Equating the moments about point O we get Rearrange to make the subject. = (g d R/h)½ M = (M/R) x h = Mg d

11 If the ehicle was about to slide sideways without oerturning then the centrifugal force would be equal and opposite to the friction force. In this case µw = M/R µmg = M/R = (µgr) ½ Comparing the two equations it is apparent that it skids if µ<d/h and oerturn if µ>d/h WORKED EXAMPLE No.5 A wheeled ehicle traels around a circular track of radius 50 m. The wheels are m apart (sideways) and the centre of graity is 0.8 m aboe the ground. The coefficient of friction is 0.4. Determine whether it oerturns or slides sideways and determine the elocity at which it occurs SOLUTION Oerturning = (g d R/h)½ = (9.81 x 1 x 50/0.8)½ = 4.76 m/s Sliding sideways = (µgr)½ = (0.4 x 9.81 x 50)½ = 14 m/s It follows that it will slide sideways when the elocity reaches 14 m/s.

12 . BANKED SURFACE The road surface is flat and inclined at θ degrees to the horizontal but the bend is still in the horizontal plane. The analysis of the problem is helped if we consider the ehicle simply as block on an inclined plane as shown. Figure 11 The C.F. still acts horizontally and the weight ertically. The essential distances required for moments about point O are the ertical and horizontal distances from O and these are h' and d' as shown. When the ehicle is just on the point of oerturning the moments about the corner are equal and opposite as before. h' = (h d tanθ) cosθ d' = (h tanθ + d) cosθ Figure 1 Equating moments about the corner we hae By applying trigonometry to the problem you should be able to show that M h' /R = Mg d' h' = (h d tanθ) cosθ /R = g (h tanθ + d) cosθ d + tanθ Make the subject. = gr h d 1 tanθ h This is the elocity at which the ehicle oerturns. If the ehicle slides without oerturning, then the total force parallel to the road surface must be equal to the friction force. Remember the friction force is µn where N is the total force acting normal to the road surface. Resole all forces parallel and perpendicular to the road. The parallel forces are F cosθ and W sinθ as shown. The normal forces are F sinθ and W cosθ as shown. The friction force opposing sliding is µn Figure 1

13 The forces acting parallel to the surface must be equal and opposite when sliding is about to occur. Balancing all three forces we hae µn + Wsin θ = F cosθ The total normal force N is the sum of the two normal forces. N = Wcosθ + F sinθ Substituting µ Wcosθ + Fsinθ + Wsinθ = Fcosθ { } µwcosθ + µfsinθ + Wsinθ = Fcosθ µmgcosθ + µm sinθ + Mgsinθ = M cosθ R R µgcosθ + µ sinθ + gsinθ = cosθ R R µ + µ tanθ + tanθ = Rg Rg Rg Rg { µtanθ 1} { 1 µtanθ} = Rg = tanθ + µ { µ + tanθ} { 1 µtanθ} = tanθ µ This gies the elocity at which the ehicle slides.

14 WORKED EXAMPLE No.6 A motor ehicle traels around a banked circular track of radius 80 m. The track is banked at 15o to the horizontal. The coefficient of friction between the wheels and the road is 0.5. The wheel base is.4 m wide and the centre of graity is 0.5 m from the surface measured normal to it. Determine the speed at which it oerturns or skids. SOLUTION 1. OVERTURNING d 1. + tanθ + tan( 15) = gr h = 9.81X d 1. 1 tanθ 1 tan( 15) h 0.5 Hence = 76.6 m/s. SKIDDING { µ + tanθ} { 1 µ tanθ} = Rg = 80x9.81x { tan15} { tan15} Hence = 6. m/s The ehicle will skid before it oerturns.

15 SELF ASSESSMENT EXERCISE No. 1. A motor ehicle traels around a banked circular track of radius 110 m. The track is banked at 8o to the horizontal. The coefficient of friction between the wheels and the road is 0.4. The wheel base is m wide and the centre of graity is 0.6 m from the surface measured normal to it. Determine the speed at which it oerturns or skids. ( 66. m and 4.9 m ). A ehicle has a wheel base of.1 m and the centre of graity is 1.1 m from the bottom. Calculate the radius of the smallest bend it can negotiate at 10 km/h. The coefficient of friction between the wheels and the track is 0.. (For skidding R = 77.5 m and for oerturning R = m hence the answer is 77.5 m). Repeat question gien that the bend is banked at 10o. (For skidding R = 5. m and for oerturning R = 8. m hence the answer is 5. m)