Fundamentals 11 Flashcards
A meterstick of mass m hangs horizontally. How much torque is provided by the earth about the meterstick’s center of mass?
zero. (The force of the earth acts at the meterstick’s center of mass, so has no lever arm about that position.)
How do you determine speed from a position-time graph?
slope of the tangent
A block attached to a string oscillates as a pendulum in simple harmonic motion. Describe the direction of the unbalanced force on the block throughout its motion.
There will be a component of unbalanced force acting along the curved path, directed toward the equilibrium position. This is the “restoring force” for simple harmonic motion, which is zero at the equilibrium position and gets larger as the distance from equilibrium gets larger.
There will also be a component of unbalanced force toward the center of the circular path. This is the “centripetal force” for an object in circular motion. This force, equal to mv^2/r, will be zero on the edges where the block has no speed for an instant; and will be maximum at the equilibrium position where speed is greatest.
A block slides along the floor to the right, slowing down due to friction. What is the direction of the block’s acceleration?
left. (When an object slows down, its acceleration is opposite the direction of motion.)
A block slides along the floor to the right, slowing down due to friction. What is the Newton’s third law force pair to the force of friction on the block?
the force of the block on the floor. (Friction is the force of the floor on the block, so that pairs with the force of the block on the floor.)
A person stands on a scale in an elevator. Give all ways the elevator could be moving if the scale reading is greater than the person’s weight.
moving up and speeding up; or, moving down and slowing down. (Unbalanced force is up, which means acceleration is up. When an object speeds up, its acceleration is in the direction of motion; when an object slows down, its acceleration is opposite the direction of motion.)
A 1 kg block falls from a table to the floor 1 m below; air resistance is negligible. Consider the block-earth system. How much work was done by external forces on the block during its fall?
zero. (The only force on the block is its weight, that is, the force of the earth on the block. This is internal to the block-earth system. No external forces act on the system at all, let alone do work on the system.)
The unbalanced force on an object is in the direction of ________.
acceleration.
Under what conditions is the angular momentum of a system conserved?
When no net external torque acts on the system.
Planet A and Planet B orbit one another. Planet A is four times the mass of planet B. What is the ratio of the net force on planet A to the net force on planet B?
1:1. (Newton’s Third Law.)
