MAE 242 Dynamics – Section I Dr. Kostas Sierros

Design project 1 …because of the make – up quiz…

Make-up quiz

• Make – up quiz will take place next

Tuesday (18 th September)

• People who solved the Quiz 1 problem will get bonus points • People that did not do well on Quiz 1, will start from scratch • Next Monday (17 th September) I can offer a help session.

What time (after 5:00 PM) do you prefer??

Cylindrical coordinates (13.6)

This approach to solving problems has some external similarity to the normal & tangential method just studied. However, the path may be more complex or the problem may have other attributes that make it desirable to use cylindrical coordinates Equilibrium equations or “Equations of Motion” in cylindrical coordinates (using r, q , and z coordinates) may be expressed in scalar form as:  F r = ma r  F q = ma q  F z = ma z

Cylindrical coordinates (13.6) continued…

If the particle is constrained to move only in the r – q plane (i.e., the z coordinate is constant), then only the first two equations are used (as shown below). The coordinate system in such a case becomes a polar coordinate system. In this case, the path is only a function of q.

 F r = ma r  F q = ma q Note that a fixed coordinate system is used, not a “body-centered” system as used in the n – t approach

Tangential and normal forces

If a force

r = f (

q

)

P

causes the particle to move along a path defined by , the normal force

N

exerted by the path on the particle is always perpendicular to the path’s tangent. The frictional force

F

always acts along the tangent in the opposite direction of motion. The directions of

N

and by using the angle y

F

can be specified relative to the radial coordinate

Determination of angle ψ

The angle y , defined as the angle between the extended radial line and the tangent to the curve, can be required to solve some problems. It can be determined from the following relationship; tan y = r d q dr = dr r d q If y is positive, it is measured counterclockwise from the radial line to the tangent. If it is negative, it is measured clockwise

Problem 1

Problem 2

Problem 3

Kinetics of a particle: Work & Energy Chapter 14 Chapter objectives

• Develop the principle of work and energy and apply it in order to solve problems that involve force, velocity and displacement • Problems that involve power and efficiency will be studied • Concept of conservative force will be introduced and application of theorem of conservation of energy, in order to solve kinetic problems, will be described

Lecture 8

•

Kinetics of a particle: Work and Energy

(Chapter 14) 14.1-14.3

Material covered

•

Kinetics of a particle: Work & Energy

The work of a force Principle of Work and Energy Principle of Work and Energy for a system of particles

…Next lecture…

Power and efficiency, conservative forces and potential energy, conservation of energy …and

MAKE – UP QUIZ

Today’s Objectives

Students should be able to: 1.

Calculate the work of a force 2.

Apply the principle of work and energy to a particle or system of particles

Applications I

A roller coaster makes use of gravitational forces to assist the cars in reaching high speeds in the “valleys” of the track How can we design the track (e.g., the height, h, and the radius of curvature, r to control the forces experienced by the passengers?

Applications II

Crash barrels are often used along roadways for crash protection. The barrels absorb the car’s kinetic energy by deforming If we know the typical velocity of an oncoming car and the amount of energy that can be absorbed by each barrel, how can we design a crash cushion?

Work and Energy

Another equation for working kinetics problems involving particles can be derived by integrating the equation of motion (

F

= m

a

) with respect to displacement By substituting a t = v (dv/ds) into F t yield an equation known as the = ma t , the result is integrated to principle of work and energy This principle is useful for solving problems that involve force , velocity , and displacement . It can also be used to explore the concept of power To use this principle, we must first understand how to calculate the work of a force

Work of a force (14.1)

A force does work on a particle when the particle undergoes a displacement along the line of action of the force Work is defined as the product of force and displacement components acting in the same direction . So, if the angle between the force and displacement vector is q , the increment of work dU done by the force is; dU = F ds cos q By using the definition of the dot product and integrating, the total work can be written as; U 1-2 =

r 1



r 2 F

• d

r

Work of a force (14.1) continued…

If

F

is a function of position (a common case) this becomes U 1-2 = s 1  s 2 F cos q ds If both F and q are constant (F = F c ), this equation further simplifies to U 1-2 = F c cos q ( s 2 - s 1 ) Work is positive if the force and the movement are in the same direction . If they are opposing , then the work is negative . If the force and the displacement directions are perpendicular, the work is zero

Work of a weight

The work done by the gravitational force acting on a particle (or weight of an object ) can be calculated by using; U 1-2 =  y 2 y 1 - W dy = - W (y 2 - y 1 ) = - W D y The work of a weight is the product of the magnitude of the particle’s weight and its vertical displacement. If D y is upward , the work is negative since the weight force always acts downward

Work of a spring force

When stretched, a linear elastic spring develops a force of magnitude F s k is the spring stiffness = ks, where and s is the displacement from the unstretched position The work of the spring force moving from position s 1 s 2 is; s 2 s 2 U 1-2 = s 1  F s ds =  s 1 k s ds = 0.5k(s 2 ) 2 - 0.5k(s 1 ) 2 to position If a particle is attached to the spring, the force F s particle is opposite exerted on the to that exerted on the spring. Thus, the work done on the particle by the spring force will be negative or U 1-2 = – [ 0.5k (s 2 ) 2 – 0.5k (s 1 ) 2 ]

http://www.mech.uwa.edu.au/DANotes/springs/intro/springAnimation.gif

Spring forces

It is important to note the following about spring forces: 1.

The equations just shown are for linear springs only! Recall that a linear spring develops a force according to F = ks (essentially the equation of a line) 2.

The work of a spring is not just spring force times distance at some point, i.e., (ks i )(s i ).

students often fall into!

Beware , this is a trap that 3.

Always double check the sign of the spring work after calculating it. It is positive work if the force put on the object by the spring and the movement are in the same direction

Principle of work and energy (14.2 & 14.3)

By integrating the equation of motion, the principle of work and energy  F t = ma t = mv(dv/ds), can be written as  U 1-2 = 0.5m(v 2 ) 2 – 0.5m(v 1 ) 2 or T 1 +  U 1-2 = T 2  U 1-2 is the work done by all the forces acting on the particle as it moves from point 1 to point 2. Work can be either a positive or negative scalar T 1 and T 2 are the kinetic energies of the particle at the initial and final position, respectively. Thus, T 1 = 0.5 m (v 1 ) 2 m (v 2 ) 2 . The kinetic energy is always a and T positive scalar 2 = 0.5 (velocity is squared!) So, the particle’s initial kinetic energy plus the work done by all the forces acting on the particle as it moves from its initial to final position is equal to the particle’s final kinetic energy

Principle of work and energy (continued…)

Note that the principle of work and energy (T 1 +  U 1-2 not a vector equation ! Each term results in a scalar value = T 2 ) is Both kinetic energy and work have the same units, that of energy! In the SI system, the unit for energy is called a where 1 J = 1 N·m. In the FPS system, units are ft·lb joule (J), The principle of work and energy cannot be used, in general, to determine forces directed normal to the path, since these forces do no work The principle of work and energy can also be applied to a system of particles by summing the kinetic energies of all particles in the system and the work due to all forces acting on the system

Example

Given:

A 0.5 kg ball of negligible size is fired up a vertical track of radius 1.5 m using a spring plunger with k = 500 N/m. The plunger keeps the spring compressed 0.08 m when s = 0

Find: Plan:

The distance s the plunger must be pulled back and released so the ball will begin to leave the track when q = 135° 1) Draw the FBD of the ball at q = 135°.

2) Apply the equation of motion in the n-direction to determine the speed of the ball when it leaves the track.

3) Apply the principle of work and energy to determine s

Example (continued)

Solution:

1) Draw the FBD of the ball at q = 135° t

N

2) The weight (

W

) acts downward through the center of the ball. The normal force exerted n 45° by the track is perpendicular to the surface. The friction force between the ball and the

W

track has no component in the n-direction Apply the equation of motion in the n-direction.

ball leaves the track at q = 135°, set N = 0 Since the => +  F n = ma n = m (v 2 / r ) => W cos45° = m (v 2 / r ) => (0.5)(9.81) cos 45° = (0.5/1.5)v 2 => v = 3.2257 m/s

Example (continued)

3) Apply the principle of work and energy ( q = 0) and position 2 ( q between position 1 = 135°). Note that the normal force (

N

) does no work since it is always perpendicular to the displacement direction. (Students: Draw a FBD to confirm the work forces) 0.5m (v 1 ) 2 T 1 +  U 1-2 – W D y – (0.5k(s 2 ) 2 = T 2 – 0.5k (s 1 ) 2 ) = 0.5m (v 2 ) 2 and v 1 = 0, v 2 = 3.2257 m/s s 1 = s + 0.08 m, s 2 = 0.08 m D y = 1.5 + 1.5 sin 45° = 2.5607 m => 0 – (0.5)(9.81)(2.5607) – [0.5(500)(0.08) 2 – 0.5(500)(5 + 0.08) 2 ] = 0.5(0.5)(3.2257) 2 => s = 0.179 m = 179 mm

Homework

Hibbeler

13.2

13.18

13.22

13.25

13.43

13.50

To be handed in Tuesday 18 th September