Self Check: Statics
First: Try with no notes or neighbors.
If you have to: Use your notes and/or textbook.
As a last resort:Ask your neighbors for help.
The purpose of this is for YOU to figure out WHAT YOU DON’T KNOW!
‘Cause you will need to know how to do these on the test!
SC 1: Forces in equilibrium.
Forces 1, 2, and 3 are in equilibrium. Force 1 has a magnitude of 35 N and is acting at an angle of 57° above the positive x-axis. Force 2 is 25 N along the negative y-axis.
- Define equilibrium (WRT forces).
- Sketch the problem (assumed that you will do this)
- Label the diagram (again, assumed!)
- Determine the sum of forces 1 and 2.
- Find force 3.
- If the angle of force 1 is decreased while force 2 remains unchanged, how will force 3 initially react? Be specific, describing the effect on angle and magnitude of both x and y components.
- If the magnitude of force 2 increases, how will force 3 initially react? Again, be specific.
Key:
- Forces in balance.
Forces that sum to zero. - Labels above.
- See box:
- See box:
- The angle will decrease
[move toward the x-axis and eventually move above the x-axis].
The magnitude in the x direction will increase.
The magnitude in the y direction will decrease
[move toward the x-axis and eventually move above the x-axis]. - The angle will decrease.
[move toward the x-axis and eventually move above the x-axis].
The magnitude in the x direction will remain unchanged.
The magnitude in the y direction will decrease.
[move toward the x-axis and eventually move above the x-axis].
First: Try with no notes or neighbors.
If you have to: Use your notes and/or textbook.
As a last resort:Ask your neighbors for help.
The purpose of this is for YOU to figure out WHAT YOU DON’T KNOW!
‘Cause you will need to know how to do these on the test!
SC2: Torques
A 65 kg student exerts a force of 55 N on the end of a 25 kg door 74 cm wide. What is the magnitude of the torque if the force is exerted (a) perpendicular to the door, and (b) at a 30º angle to the face of the door? Is the door in equilibrium? Explain!
Now the student’s older sibling (who didn’t take physics!) pushes against the door to resist the motion. She doesn’t understand torques, and therefore pushes against the door in its center not at its end. At least she pushes perpendiculary to the door! With what force must she push to balance the force applied by her younger sibling in both problems above?Is the door in equilibrium? Explain!
KEY:
- The door is not in equilibrium as there is no counter (or “anti”?) torque.
One torque cannot be in equilibrium!
First: Try with no notes or neighbors.
If you have to: Use your notes and/or textbook.
As a last resort:Ask your neighbors for help.
The purpose of this is for YOU to figure out WHAT YOU DON’T KNOW!
‘Cause you will need to know how to do these on the test!
SC3: Forces and Torques in equilibrium: Using both simultaneously!
[P(9):20] A shop sign weighing 245 N is supported by a uniform 155-N beam as shown in Fig. 9–54. Determine the:
- mass of the beam [15.8],
- tension in the guy wire [708],
- horizontal and vertical forces exerted by the hinge on the beam [580,-6],
- force exerted by the hinge on the beam [580, 0.593° below pos. x], and
- force exerted on the wall by the hinge [same as d].
(For help, you may want to look at Example 9-6)
KEY:
The beam is in equilibrium. Use the conditions of equilibrium to calculate the tension in the wire and the forces at the hinge. Calculate torques about the hinge, and take counterclockwise torques to be positive.