Mechanical Equilibrium
What can you correctly say about a pair of vectors that add together to equal zero?
You can correctly say the vectors are equal in magnitude and opposite in direction.
Consider a book that weighs 15N at rest on a flat table. How many newtons of support force does the table provide? What is the net force on the book in this case?
Because the net force is 0 and we are told that the book is pushing down on the table with a force of 15 newtons, we must conclude that the table is pushing up on the book with a force of 15 newtons. The book is at rest on the table, those of us that believe in Newton’s 1st law of motion know that the net force on the book must be zero.
A hockey puck slides across the ice at a constant speed. Is it in equilibrium? Why or why not?
Yes. If the puck moves in a straight line with unchanging speed, the forces of friction are negligible. Then the net force is practically zero, and the puck can be treated as in dynamic equilibrium.
Can force be expressed in units of pounds and also in units of newtons?
Yes.
A monkey hangs stationary at the end of a vertical vine. What two forces act on the monkey? Which, if either, is greater?
The upward force is the tension in the vine. The downward force is caused by the gravity. Both are equal when the monkey hangs in equilibrium.
When you stand at rest on a bathroom scale, how does your weight compare with the support force from the scale?
The scale shows the support force. There is a downward pull by gravitational forces and upward by the floor.
A block is being pushed with a force of 10N to make it slide at a constant velocity. What is the friction force created when the block is moving?
10N. The block is in equilibrium because it's moving at a CONSTANT velocity.
What is the net force on an object that is pulled with forces of 80 newtons to the right and 80 newtons to the left?
The net force is 0 newtons
Can an object be in mechanical equilibrium when only a single nonzero force acts on it? Explain.
No. Because in a mechanical equilibrium the net force must be 0. When only a single nonzero force acts on an object, the net force can't be zero.
A force of gravity pulls downward on a book on a table. What force prevents the book from accelerating downward?
The support force acting on the table keeps the book from accelerating downwards.
In order to slide a heavy cabinet across the floor at a constant speed, you exert a horizontal force of 600N. Is the force of friction between the cabinet and the floor greater than, less than, or equal to 600N? Defend your answer.
The friction force is equal to 600N for constant speed. Only then will sum of the force equals 0.
Mass is a vector quantity. (True/ False)
False. Mass only has magnitude and not direction.
Two people each pull with a force of 300 N on a rope in a tug-of-war. What is the net force on the rope?
The net force on the rope is zero.
Suppose that you jounce up and down while weighing your-self on a bathroom scale. Which varies: the upward support force or the force of gravity on you?
The support force from the scale is changing while you jounce up and down the scale.
Is it easier to move a 10N rock or a 100N boulder? Please explain.
It's easier to move the lighter object because the support force is weaker on the lighter object.
A single vector can be replaced by two vectors in the x- and y-directions that are at right angles to each other. The original vector is the ____________ vector of these two x and y vectors.
Resultant Vectors
Two people each pull with a force of 300 N on a rope in a tug-of-war. How much force is exerted on each person by the rope?
The force exerted by the rope on each person is 300 N which is in the opposite direction.
Why is your weight reading from the scale is not as good if you're moving?
Only when you are in equilibrium will the support force on you correctly show your weight. And it is equal to the force of gravity acting on you.
When a ball is tossed straight up, it momentarily comes to a stop at the top of its path. Is it in equilibrium during this brief moment? Why or why not?
At the top of its path and everywhere else along its path, the force of gravity acts to change the ball’s motion. Even though, for a moment, stops at the top, the net force on the ball is not zero and it therefore is not in equilibrium.
Write the equilibrium rule for forces in symbolic notation on the whiteboard.