Question

circular motion and gravity tf review1. which of the following statements are true of an object moving in a circle at a constant speed?include all that apply.a. the object experiences a force which has a component directed parallel to the directionof motion.b. inertia causes objects to move in a circle.c. there can be a force pushing outwards on the object as long as the net force is inwards.d. because the speed is constant, the acceleration is zero.e. the acceleration and the net force vector are directed perpendicular to each other.f. if the net force acting upon the object is suddenly reduced to zero, then the objectwould suddenly depart from its circular path and travel tangent to the circle.g. the acceleration of the object is directed tangent to the circle.2. a physics teacher ties an eraser to the end of a string and thenwhirls it in a counter-clockwise circle. if the teacher lets go of thestring, then the eraser hits a student (or several students) in theclassroom. if the string is let go when the eraser is at point x onthe diagram at the right, then which 2 student(s) in a row will theeraser hit? write the initials in this space:3. which of the following statements are true about gravitational force? identify all that apply.a. the gravitational force only acts between very, very massive objects.b. the gravitational force between an object and the earth is inversely related to the distancebetween the objects and the earths center.c. the gravitational force acting upon an object is the same as the weight of the object.d. the gravitational force between two objects is independent of the mass of the smaller ofthe two objects.e. if object a gravitationally attracts object b with a force of x newtons, then object b will alsogravitationally attract object a with the same force of x newtons.f. the doubling of the separation distance (measured from the center) between two objectswill halve (½) the gravitational force between the objects.g. if an object is placed two earth-radii above the surface of the earth, then the force ofgravitational attraction between the object and the earth will be one-fourth the magnitudeas on earths surface.h. orbiting astronauts do not experience a force of gravity; this explains why they feelweightless.

Explanation:

Step1: Analyze circular motion statements

For uniform circular motion (constant speed):

• A: Parallel force would change speed, false.
• B: Inertia resists change, not circular motion, false.
• C: Outward "force" is inertia, not real, false.
• D: Speed constant but direction changes, acceleration non-zero, false.
• E: Net force/acceleration point inward, perpendicular to tangential velocity, true.
• F: No net force = straight tangent path, true.
• G: Acceleration is centripetal (inward), not tangent, false.

Step2: Analyze eraser tangent path

At point X, tangent direction is right, hitting AF then SH.

Step3: Analyze gravitational force statements

Use Newton's law of gravitation $F_g = G\frac{m_1m_2}{r^2}$:

• A: Gravity acts on all masses, false.
• B: Force inversely proportional to $r^2$, not $r$, false.
• C: Gravitational force on object = its weight, true.
• D: Force depends on both masses, false.
• E: Newton's 3rd law: equal/opposite force, true.
• F: $r$ doubles, force becomes $\frac{1}{4}$, not $\frac{1}{2}$, false.
• G: Distance from Earth center is $3R$, force is $\frac{1}{9}$ of surface, false.
• H: Astronauts experience gravity (it provides centripetal force), false.

Answer:

1. E, F
2. AF, SH
3. C, E