physcs test 1 conceptual

Elevator motion (signs of velocity ⃗v and acceleration ⃗a) : Up, speeding up

⃗v>0, ⃗a>0

Elevator motion (signs of velocity ⃗v and acceleration ⃗a) : Up, constant speed

⃗v>0, ⃗a= 0

Elevator motion (signs of velocity ⃗v and acceleration ⃗a) : Up, slowing down

⃗v>0, ⃗a<0.

Elevator motion (signs of velocity ⃗v and acceleration ⃗a): Down, slowing down

⃗v<0, ⃗a>0

A box is pulled across a rough floor at constant velocity by a horizontal rope. The net

force on the box is

zero. Constant velocity implies zero net force (Newton’s 1st)

Newton’s third law tells us

action and reaction forces act on different objects but are equal in magnitudeNewton’s 3rd law.

A net force F produces acceleration a on mass m. If the same net force acts on mass

3m, the acceleration is

Answer: a/3.

F = ma ⇒ a = F/m. For mass 3m, a′ = F/(3m) = a/3.

A car rounds a flat curve at constant speed. Which must be true?

The net force points toward the center of the curve. Centripetal

force is required for circular motion.

A ball is thrown straight upward. At the very top of its path,

velocity = 0, acceleration = − g. At the top instant vertical

velocity is zero but gravity still acts downward.

An elevator moves downward but slows as it approaches the ground floor. The tension

in the supporting cable is

greater than mg. Elevator moving downward but slowing im-

plies upward acceleration, so tension ¿ weight.

True or False: If an object moves with constant speed around a circle, it is not acceler-

ating

False. Constant speed around a circle still has centripetal acceler-

ation (direction changes).

Two ice skaters push off each other. Which statement is correct?

Each skater exerts the same force on the other. (Newton’s 3rd law

again.)