You go for a ride in an elevator and are surprised at how much lighter you feel

ID: 1495462 • Letter: Y

Question

You go for a ride in an elevator and are surprised at how much lighter you feel when the elevator begins heading down. After the following stop, you place your smart-phone on the ground and run the accelerometer app. The phone indicates that the acceleration has gone down by 2.00 m/s22. The phone is about 250 grams.

1[C]) Consider the system schema that includes the elevator frame and machinery, the Earth, and your phone. Which combination of statements is most accurate? [Enter your answer as a string of the letter options you believe are correct. For instance, if you think options A, D, and E are correct, then enter "ADE"]

A) There is a field force line connecting the phone and the Earth.
B) There is a contact force line connecting the elevator and the phone.
C) There is a field force line connecting the elevator and the Earth.
D) There is a net force line acting on the phone.
E) None of the above

2[Q]) Let j^ be the unit vector along the positive vertical direction. Let mm be the phone's inertia and g=9.8g=9.8 m/s22. Which combination of statements about the scenario is most accurate? [Enter your answer as a string of the letter options you believe are correct. For instance, if you think options A, D, and E are correct, then enter "ADE"]

A) The net force on the phone is 2.45j^ N.
B) F =F floormgj^F=Ffloormgj^
C) The force from the floor on the phone is 0.5j^ N.
D) The phone is temporarily 0.438 lbs lighter when the elevator accelerates down.
E) None of the above

3[W]) Suppose there is another passenger on the elevator with you who weighs 180 lbs. How much lighter do they feel while the elevator accelerates downward? [Enter your answer in units of pounds of force. Note that notation for pounds of force is "lbf"].

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Imagine you are the only person on the whole of planet Earth. Standing on the Earth, at rest, you know that you exert a force down (relative to yourself) on the Earth.

4[C]) Which statements are good explanations for why the force you apply to the Earth fails to accelerate it away from you? [Enter your answer as a string of the letter options you believe are correct. For instance, if you think options A, D, and E are correct, then enter "ADE"]

A) The net force on the Earth is zero.
B) The Earth's push up on you cancels your push down on the Earth.
C) You exert a gravitational force on the Earth that is opposite to the push you exert with your feet on the ground.
D) The Law of Reciprocity ensures that the force you apply to the Earth will be cancelled by a force of equal magnitude and opposite direction.
E) None of the above

A friend of yours struggles to push a heavy crate on a rough (high-friction) carpet. They are getting nowhere, unable to even budge the thing.

5[C]) Which statements are good explanations for your friend's predicament? [Enter your answer as a string of the letter options you believe are correct. For instance, if you think options A, D, and E are correct, then enter "ADE"]

A) The net force on the crate is zero.
B) The net force from friction keeps the crate in place.
C) The impulse exerted on the crate is zero.
D) Your friend can't move the crate due to the Law of Reciprocity. The force you exert leads to an oppposite force on the crate which keeps it from moving.
E) None of the above

You are pulling your niece as she rides in a little go-cart. Her inertia is 29 kg and the cart's is 11 kg. You exert 60.00 N of horizontal force on the taut rope connected to the go-cart. The go-cart rolls close to frictionlessly and started out at rest.

6[C]) Which combination of statements is most accurate (the forces referred to are all along the horizontal direction; all comparisons refer to the magnitudes of the forces involved but the word "magnitude" is not explicit.)? [Enter your answer as a string of the letter options you believe are correct. For instance, if you think options A, D, and E are correct, then enter "ADE"]

A) You exert more force on the rope than it exerts on you since you are accelerating it.
B) The force the rope exerts on the cart is equal to the force exerted on the rope by the cart.
C) The force you exert on the rope is greater than the force exerted by the cart on the rope since the cart accelerates.
D) The force exerted by the cart on your niece is greater than the force exerted by the rope on the cart.
E) None of the above.

7[C]) After about 1.7 seconds, the rope snaps. Which combination of statements about the scenario is most accurate (assume friction is negligible)? [Enter your answer as a string of the letter options you believe are correct. For instance, if you think options A, D, and E are correct, then enter "ADE"]

A) The net force on the cart after the rope snaps is non-zero since the cart keeps moving.
B) The cart stops accelerating after the rope snaps.
C) The cart's momentum changes after the rope snaps.
D) The cart continues to exert a non-zero force on your niece in order to keep her moving along with the cart.
E) None of the above

8[W]) You were about 1.8 meters ahead of the cart when the rope snapped. How much time do you have to get out of the way of the cart?

9[W]) You are working out the details in the design of a decorative lighting fixture for a big, office highrise elevator system.

A cord attached to the elevator ceiling connects to a flat, brass square-shaped dish which weighs 13 lbs. Crystal-studded bulb-holders and the lights themselves hang from each of the four corners of the dish. They each weigh 3 lbs.

The cords connecting the lights to the square dish are to be made of the same material and load-bearing strength of the cord connecting the dish to the ceiling. You must select a type of cord that is strong enough to hold the lighting fixture, even in the event of an emergency stop.

The elevator's emergency brakes will accelerate a descending elevator upwards by a third of a gg.

What is the minimum amount of force that must be supported by the cord material you need to choose? [NOTE: You may ignore the weight of the cords themselves in thinking about this scenario.]

10[W]) What is the vertical component of the force exerted by one of the corner cords on the dish during an emergency stop?

Explanation / Answer

Part 1)

In this case the acceleration of the elevator is subtracted from the acceleration of gravity, so the person cula sioente lighter and cell mark lower gravity acelracion, notice that this system is not inertial

so the successfully warped expressions must involve land and some of the bodies

A correct expressions

Part 2

Part 3)

Wo = m g

Wo = 180 lb (4.448 N/ 1 lb) =800.64 N

m = 800.64/9.8

m = 81.70 Kg

W = 81.70 2

W = 163.40 N

This is the apparent weight of the person

Part 4)

The strength of the body to the ground is applied to the land

the strength of the earth to the body is applied to the body

Verdad C

Part 5)

The friction force counteracts the force applied

True A B

Part 6

True A B

Part 7)

After breaking the rope the force exerted is zero

Verda E

Part 8

Usamos

y = vo t + ½ a t2

a = 0

y =Vo t

t =y / Vo

T = 1.8/

Part 9

W = 13 +3 *4 = 25 lb 1 N/ 4.448 lb

W = 111.2 N

W = mg

m = W/g

m = 111.2/9.8 = 11.35 Kg

F = m at

a = g + g/3 = 4/3 g = 4.3 * 9,8

a =13 m/s2

Part 10

The vertical force

F = m g

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You go for a ride in an elevator and are surprised at how much lighter you feel

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