Equilibrium and Elasticity Ch 12 all Equilibrium An

Equilibrium and Elasticity Ch 12 (all)

Equilibrium An object is in equilibrium when: - The vector sum of all the external forces that act the body must be zero. Fnet = 0 - The vector sum of all the external torques that act on the body, measured about any possible point, must all be zero. Tnet = 0 Objects in Equilibrium: 1) A book resting on a table 2) A hockey puck sliding with constant velocity across a frictionless surface 3) The rotating blades of a ceiling fan 4) The wheel of a bicycle that is traveling along a straight path at constant speed An object is in static equilibrium if it follows the rules of equilibrium, and the linear momentum of the body must be zero. Objects in Static Equilibrium: 1) A book resting on a table

Center of Gravity The gravitational force Fg on a body effectively acts at a single point, called he center of gravity (cog) of the body. If g is the same for all elements of a body, then the body’s cog is coincident with the body’s center of mass (com).

Elasticity All real “rigid” bodies are to some extent elastic, which means that we can change their dimensions slightly by pulling, pushing, twisting, or compressing them. A body is placed under eithe stress or strain, which are proportional to each other because stress=module x strain.

Tension and Compression For simple tension or compression, the stress on the object is defined as F/A, where F is the magnitude of the force applied perpendicularly to an area A on the object. The strain, or unit deformation, is then the dimensionless quantity ∆L/L, the fractional change in a length of the specimen. The modulus for tensile and compressive stresses is called the Young’s modulus and is represented in engineering practice by the symbol E. F/A = E ∆L/L

Shearing In the case of shearing, the stress is also a force per unit area, but the force vector lies in the plane of the area rather than perpendicular to it. The corresponding modulus, which is given the symbol G in engineering practice, is called the shear modulus. F/A = G ∆x/L

Hydraulic Stress The stress is the fluid pressure p on the object, and pressure is a force per unit area. The strain is ∆V/V where V is the orginial volume of the specimen and ∆V is the absolute value of the change in the volume. The corresponding modulus, with symbol B, is called the bulk modulus of the material. p = B ∆V/V

20 Questions

Which of the following is not a requirement for an object to be in equilibrium? A) linear momentum is constant B) energy is conserved C) angular momentum is constant § B) energy is conserved

Explain what each symbol in the formula p = B ∆V/V stands for and when this formula might be used Used to determine the hydraulic stress experienced by an object p = pressure B = the bulk modulus of the object ∆V = the change in volume from the force V = the original volume

Because forces are applied to the end points of a rod in opposite direction, the stress experienced is _______. Tension

A steel rod has a radius R of 9. 5 mm and a length of 81 cm. A 62 k. N force F stretches it along its length. What is the stress experienced by the rod? stress = F/A stress = F/πR 2 stress=(6. 2 x 104)/π(9. 5 x 10 -3) stress = 2. 2 x 108

To find the tension of T 1, where would the best pivot point be? C A A T 1 B

Which disks are in equilibrium? F F 2 F A B only F F 3 F 2 F 2 F F B C

Which way will the seesaw tip? 5 kg A Towards B 3 kg 1 m 2 m B

Since the constant of proportionality is called a modulus of elasticity, stress equals… modulus x strain

The bending of a wave as it enters a new medium is called______: Refraction

Find the tension in T. The box has a mass of M and the bar has a mass of 3 M. 2 d tnet = Fr M 1 r 1 + M 2 r 2 = T y r 3 M 2 d + 3 M 3 d = T(sinØ) 6 d 11/6 M = T(sinØ) T = 11 M/6(sinØ) Ø 6 d Ty = T (sinØ)

A 50 kg man is on one side of a seesaw and is 2 meters away from the pivot point, a 40 kg man wants to balance the force, how far from the pivot point should he stand? t 1 = t 2 x 1 m 1 g = x 2 m 2 g (2)(50)(9. 8) = (x 2)(40)(9. 8) X 2 = 2. 5

Find the tension in T 2. T 1 Pivot Point (blue beam is mass less) tnet = RF tnet =-(r 1 xmg)+(Fxr 2) r 1 x mg = F x r 2 3 x 5 x 9. 8 = F x 7 F = 21 N T 2 5 kg 3 m 4 m

2 kg Is the center of mass near A, B, or C? 3 kg A 5 kg C B The center of gravity is closest to C.

Which of the following are in equilibrium? A) A car accelerating B) A hockey puck at constant speed C) A book resting on a desk Both B and C. A is not because it is not at constant speed.

If a 10 kg mass is placed on a rod 5 meters away its pivot point, when is the torque experienced? t t = = x Fg x m g (5) (10) (9. 8) 490 Nm

When force is applied to a bridge, it results in: A) Tension B) Compression C) Both

The stress with is force per unit area and the force vector lies in the plane is the case of _____. Shearing

Which equation is the modulus for tension and compression? A) F/A = G ∆x/L B) p = B ∆V/V C) F/A = E ∆L/L

Which equation is the modulus for shearing? A) F/A = G ∆x/L B) p = B ∆V/V C) F/A = E ∆L/L

Which equation is the modulus for hydraulic stress? A) F/A = G ∆x/L B) p = B ∆V/V C) F/A = E ∆L/L

10 Questions

A book resting on a table is in _______ equilibrium. Static

The single point in which gravity is effectively acting on for any object is called the _____. Center of Gravity

When an object is being pulled apart by two forces in opposite direction parallel to its plane, the object is under ______. Tension

An object being acted on by two opposite forces perpendicular to its plane is said to be under a _____ stress. Shearing

When a object being acting upon by a surrounding fluid is said be to under a ________. Hydraulic Compression

If g is the same for all elements of a body, then the body’s center of gravity is coincident with the body’s ______. Center of Mass

All real “rigid” bodies are to some extent _____. Elastic

For simple tension or compression, the stress on the object is defined as ____. F/A (Force / Area)

The translational motion of a body is governed by Newton’s _______ law in its linear momentum form. Second (Fnet = d. P/dt)

Name three elastic moduli used to describe the elastic behavior of an object. • Tension and Compression • Shearing • Hydraulic Stress