Fundamentals 8 Flashcards
A person of mass 75 kg stands on a platform scale in an elevator which is moving down and speeding up. Is the reading in the scale greater than, less than, or equal to 750 N?
less than. (But not “because the elevator is moving downward.” No! The elevator has downward acceleration because it’s moving down and speeding up. Therefore acceleration is in the direction of unbalanced force, which is down; and the weight is greater than the upward force of the scale.)
An object is 3 m above a table, and 6 m above the floor. The object is moved horizontally from a position directly above the table to a position above the floor without the table in the way. Has the potential energy of the object-earth system increased, decreased, or remained the same?
remained the same. (Gravitational potential energy only changes if the distance from Earth’s center changes. You can’t change your reference point for potential energy within the same problem!)
A normal force is the force of ____ on ____.
force of a surface on an object.
A string pulls at angle of 30 degrees above the horizontal with tension 100 N. What is the horizontal component of the tension in the string?
100 N cos 30 degrees. (Which equals 87 N, but that’s not important.)
Write the formula for rotational kinetic energy, defining each term briefly.
(1/2)Iw^2. Here I is the rotational inertia, and w [omega] is the angular speed.
A satellite moves in an elliptical orbit around Earth, from position A closest to Earth to position B farthest from Earth. Does the angular momentum of the satellite about the satellite’s center increase, decrease, or remain the same when moving from A to B?
remain the same. (The force of the earth acts directly at the satellite’s center, and thus has no lever arm, and applies no torque on the satellite. No net torque means no change in angular momentum.)
A satellite moves in an elliptical orbit around Earth, from position A closest to Earth to position B farthest from Earth. Does the kinetic energy of the satellite increase, decrease, or remain the same when moving from A to B?
decrease. (The gravitational potential energy increases, because the satellite gets farther from Earth’s center; so by mechanical energy conservation, kinetic energy must decrease.)
I throw a ball upward with initial speed 10 m/s. Is the highest position reached greater than, less than, or equal to 10 m above where I threw it?
less than. (The relevant equation here is d = v^2/2a because the object comes to rest… that’s d = (10)^2/2(10) = 5 m. Point is, just because acceleration is 10 m/s/s and velocity is 10 m/s doesn’t mean that displacement is also 10 m!)
An object moves in a circular path and speeds up. Describe the direction of the object’s acceleration.
a component is toward the circle’s center, and another component is in the direction of motion. (So, if I were to draw the acceleration vector, it would point not straight toward the circle’s center, but is angled above the center in the direction the object is moving. Don’t know how to get a picture in the comments, though! :-) )
What are the units of mass?
kg. (Or grams, though kilograms are standard.)
