Chapter 9: Static Equilibrium Flashcards
What condition or conditions are necessary for static equilibrium?
1) ΣFx = 0
2) ΣFx = 0, ΣFy = 0, Στ = 0
3) Στ = 0
4) ΣFx = 0, ΣFy = 0
ΣFx = 0, ΣFy = 0, Στ = 0
A book weighs 6 N. When held at rest above your head the net force on the book is ________
0 N
A rocket moves through outer space with a constant velocity of 9.8 m/s. What net force acts on it?
The net force is zero.
Muscles that act to extend a limb outward are called _________
extensor
Stress is____
applied force per cross-sectional area
Strain is _____
the ratio of the change in length to the original length
A mass is hung from identical wires made of aluminum, brass, copper, and steel. Which wire will stretch the least?
steel
A copper wire is found to break when subjected to minimum tension of 36 N. If the wire diameter were half as great, we would expect the wire to break when subjected to a minimum tension of _________
9.0 N
A lever is 5.0 m long. The distance from the fulcrum to the weight to be lifted is 1.0 m. If a worker pushes on the opposite end with 400 N, what is the maximum weight that can be lifted?
1600 N
A lever is 5.0 m long. The distance from the fulcrum to the weight to be lifted is 1.0 m. If a 3000 N rock is to be lifted, how much force must be exerted on the lever?
750 N
A boy and a girl are balanced on a massless seesaw. The boy has a mass of 75 kg and the girl’s mass is 50 kg. If the boy sits 2.0 m from the pivot point on one side of the seesaw, where must the girl sit on the other side?
3.0 m
Two children sit on opposite ends of a uniform seesaw which pivots in the center. Child A has mass 60 kg and sits 2.0 m from the center. Child B has mass 40 kg. How far from the center must child B sit for the seesaw to balance?
3 m
(same explanation as Q. 11)
A 10-m uniform beam of weight 100 N is supported by two ropes at the ends. If a 400-N person sits at 2.0 m from the left end of the beam, what is the tension in the left rope?
370 N
A massless scaffold is held up by a wire at each end. The scaffold is 12 m long. A 300-N box sits 4.0 m from the left end. What is the tension in each wire?
left wire = 200 N
right wire = 100 N
A 500-N person stands on a uniform board of weight 100 N and length 8.0 m. The board is supported at each end. If the support force at the right end is three times that at the left end, how far from the right end is the person?
1.6 m
Assuming the lower arm has a mass of 2.8 kg and its CG is 12 cm from the elbow-joint pivot, how much force must the extensor muscle in the upper arm exert on the lower arm to hold a 7.5 kg shot put? (See the figure above.)
1000 N
A bridge piling has an area of 1.250 m2. It supports 1875 N. Find the stress on the column.
1875/1.250 = 1500 N/m^2
A heavy uniform beam is supported in equilibrium by two fulcrums, as shown below. Which fulcrum applies the greater magnitude force to the beam?
The right fulcrum
*The right fulcrum must exert a force equal to both the downward force
of gravity acting at the CM of the beam and the downward force of
the left fulcrum.
States that the sum of
the torques acting on the object, calculated using any reference
point, must be zero
Second condition for equilibrium
A heavy boy and a lightweight girl are balanced on a massless seesaw. If they both move forward so that they are one-half their original distance from the pivot point, what will happen to the seesaw?
Nothing, the seesaw will be balanced. (they exert the same torque)
A 1 kg rock is suspended by a massless string from one end of a 1 m measuring stick. What is the mass of the measuring stick if the system balances at a point of the stick 0.25 m from the end holding the rock?
1 kg
*Since the rock and the center of mass of the stick both have the same
length of lever arm, they must both have the same weight so that the
torques caused by the downward forces of gravity are equal
A mobile hangs in static equilibrium (as shown below). The rods are massless and have lengths as indicated. The mass of the ball at the bottom right is 1 kg. What is the total mass of the mobile?
6 kg
Use torque equilibrium and take the following steps:
(1) Find the big mass on the bottom left (lower rod only) = 3 kg.
(2) Use the entire lower rod assembly (with two masses) to find the mass on top right = 2 kg.
(3) Add up all the masses: 1 kg + 3 kg + 2 kg = 6 kg.
Approximately what magnitude force, 𝐹m, must the extensor muscle in the
upper arm exert on the lower arm to hold a 7.3-kg shot put (shown in the
figure)? Assume the lower arm has a mass of 2.3 kg and its CM is 12.0 cm from the elbow-joint pivot.
Consider the two configurations of identical books (shown below). The CM of each book is indicated by a black dot. Which configuration of books will tip over?
A will tip
A 25-kg piece of equipment can be hung by steel wires of length 1 m, 2 m, or 3 m. If all the wires have the same diameter, which one will stretch the greatest percent?
All three will stretch the same percent.
*have the same value for youngs modulus and are all under the same stress
lA mass of 1000 kg hangs at the end of a steel rod 5.0 m in length. The diameter of the rod is 0.80 cm and Young’s modulus for the rod is 2.10 × 1011 ⁄N m2. What is the elongation of the rods?
How much pressure is needed to compress the volume of an iron block by 0.10%? Express your answer in ⁄N m2, and compare it to atmospheric pressure (1.0 × 10^5 ⁄N m2)
