Fundamentals 7

Question 1

Under what conditions are the forces on an object balanced?

Answer 1

when acceleration is zero; that is, when an object is in equilibrium, moving in a straight line at constant speed.

Question 2

Write two expressions for angular impulse.

Answer 2

angular impulse is change in angular momentum; angular impulse is torque times time.

Question 3

The equation for the weight of an object on earth’s surface is GMm/r^2. How does the variable G in this equation change on the moon’s surface, which is about 1/3 the diameter and 1/100 the mass of earth?

Answer 3

it does not. (Big G is the universal gravitational constant, which is, well, constant throughout the universe.)

Question 4

Define “mechanical energy”.

Answer 4

kinetic energy plus potential energy. (This includes all forms of kinetic energy, rotational and translational.)

Question 5

An object on a vertical spring oscillates in simple harmonic motion. It’s highest position is 30 cm above the ground; its lowest position is 20 cm above the ground. At what position above the ground is the object’s speed largest?

Answer 5

25 cm. (At the equilibrium position of the spring, the potential energy of the spring-object-earth system is zero. Thus, all the energy of the system there is kinetic energy. The equilibrium position in simple harmonic motion is halfway between the two extreme positions.)

Question 6

How do you determine the location of an object from a velocity-time graph?

Answer 6

you can not. (You can determine displacement from the area under a velocity-time graph, but you cannot determine position.)

Question 7

Write the equation for translational kinetic energy.

Answer 7

(1/2)mv^2.

Question 8

An airplane has been cruising at a speed of 200 m/s for 10 s. What is the airplane’s acceleration?

Answer 8

zero. (Acceleration is the change in speed every second; this airplane doesn’t change speed at all.)

Question 9

A diagram shows a ball of mass m moving at speed v toward point P. The ball is a distance x from point P. What is the angular momentum of the ball about point P?

Answer 9

zero. (The angular momentum of a point object is mvr, where r is the distance of closest approach between the line of motion and the rotational axis. Here the line of motion goes directly through point P, so r = 0.)

Question 10

How do you find instantaneous angular velocity from an angular position vs. time graph?

Answer 10

take the slope of a tangent line.