Fundamentals 2

Question 1

A car speeds up while moving east. The two horizontal forces acting on the car are the force of the engine and the force of friction. What is the direction of the car’s acceleration?

Answer 1

east. (When an object speeds up, the acceleration is in the direction of motion.)

Question 2

A ball is thrown up and to the right with speed v at an angle of 30o above the horizontal.

What is the horizontal component of the ball’s velocity as it reaches the ground?

Answer 2

v cos (30)

Question 3

A ball is thrown up and to the right with speed v at an angle of 30o above the horizontal.

What is the vertical component of the ball’s acceleration immediately after it’s released?

Answer 3

g, i.e. 10 m/s/s.

Question 4

Write the definition of torque. (Not Newton’s second law for rotation, the equation that defines torque.)

Answer 4

force times lever arm.

Question 5

Define power.

Answer 5

energy per time. (Equivalently, work per time, or energy used in one second.)

Question 6

An object moves downward and slows down. Which is correctly set equal to ma? Choose one.

(A A) (up forces) – (down forces)

(B B) (down forces) – (up forces)

Answer 6

A. (When an object slows, acceleration is opposite the direction of motion, so acceleration and unbalanced force are upward; I suggest never trust students with a negative sign, so always start N2L with the bigger force minus the smaller force.)

Question 7

Write the formula for gravitational potential energy.

Answer 7

mgh. (That’s near Earth’s surface. Physics C students, or P1 students as of 2025 or 2026, may choose to write negative GmM/d, which is valid everywhere as long as you take the zero of PE to be a very long distance away from a planet.)

Question 8

Which type of coefficient of friction can take on any value up to a maximum that depends on the surfaces in contact?

Answer 8

static.

Question 9

Object A moves left with momentum 2.0 N·s; object B moves right with momentum 3.0 N·s. What is the total momentum of the two-object system?

Answer 9

1.0 Ns. (Momentums in opposite directions subtract, i.e. momentum is a vector.)

Question 10

Give a specific example of a situation in which a moving object’s acceleration is NOT constant.

Answer 10

object on a curved track; anything in simple harmonic motion; an object falling where air resistance is important; etc.