AP Physics 1 2.2.A Force as an Interaction

2.2.A—Describe a force as an interaction between two objects or systems question 1

(12 points, suggested time 25 minutes) A student in a physics lab has a block with a fan attached to it, as shown in the figure above. The fan has a pivot so that the angle $\theta$ it makes with the horizontal can be adjusted between 0° and 90°. When the fan is pointed horizontally so that $\theta=0^{\circ}$ , the block accelerates from rest along a track, even though there is friction between the block and the track. For the following questions, assume that the student has access to equipment that would usually be found in a school physics laboratory.

Question (a)

(a)

(a) Describe an experimental procedure that the student could use to measure the force F that the air exerts on the fan-block system when the fan is turned on. Assume that the magnitude of this force is the same for all angles $\theta$ .

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Question (i)

(i)

i. What quantities would be measured?

(a)

i, ii, and iii) 2 points

Note: Parts i, ii, and iii are read together

For measuring the mass or weight of the relevant system

1 point

For making a measurement of force (or something from which a force can be calculated)

1 point

with the fan turned on

Note: fan angles of either 0° or 90° need not be measured directly, since these are

given as the minimum and maximum angles possible. If the procedure involves

intermediate angles, then these angles must be measured.

Example 1 (variant - Example 1a in parentheses):

(a)i The downward force exerted by the fan-block system (or "mass", as read on a

balance, of the system)

(a)ii A spring scale (or mechanical balance)

(a)iii Set the fan angle to 90°. With the fan off, hang the system from a spring scale to

find its weight (or place the system on a mechanical balance to find its mass).

Turn the fan on, and record the force from the spring scale (or the mass reading

from the mechanical balance).

Example 2:

(a)i Mass of the fan-block system and low-friction cart. Velocity of the cart as a

function of time.

(a)ii Scale to measure mass. Motion detector to measure velocity as a function of time.

(a)iii Place the system and a low-friction cart on a scale to find the total mass of the

fan-block and cart. Next, on a horizontal surface, place the fan-block on the cart,

set the fan angle to zero, turn on the fan, and release the cart from rest. Use the

motion detector to graph the cart velocity as a function of time.

Question (ii)

(ii)

ii. What equipment would be used for the measurements, and how would that equipment be used? Include a labeled diagram of the experimental setup.

(a)

i, ii, and iii) 2 points

Note: Parts i, ii, and iii are read together

For measuring the mass or weight of the relevant system

1 point

For making a measurement of force (or something from which a force can be calculated)

1 point

with the fan turned on

Note: fan angles of either 0° or 90° need not be measured directly, since these are

given as the minimum and maximum angles possible. If the procedure involves

intermediate angles, then these angles must be measured.

Example 1 (variant - Example 1a in parentheses):

(a)i The downward force exerted by the fan-block system (or "mass", as read on a

balance, of the system)

(a)ii A spring scale (or mechanical balance)

(a)iii Set the fan angle to 90°. With the fan off, hang the system from a spring scale to

find its weight (or place the system on a mechanical balance to find its mass).

Turn the fan on, and record the force from the spring scale (or the mass reading

from the mechanical balance).

Example 2:

(a)i Mass of the fan-block system and low-friction cart. Velocity of the cart as a

function of time.

(a)ii Scale to measure mass. Motion detector to measure velocity as a function of time.

(a)iii Place the system and a low-friction cart on a scale to find the total mass of the

fan-block and cart. Next, on a horizontal surface, place the fan-block on the cart,

set the fan angle to zero, turn on the fan, and release the cart from rest. Use the

motion detector to graph the cart velocity as a function of time.

Question (iii)

(iii)

iii. Describe the overall procedure to be used. Give enough detail so that another student could replicate the experiment.

(a)

i, ii, and iii) 2 points

Note: Parts i, ii, and iii are read together

For measuring the mass or weight of the relevant system

1 point

For making a measurement of force (or something from which a force can be calculated)

1 point

with the fan turned on

Note: fan angles of either 0° or 90° need not be measured directly, since these are

given as the minimum and maximum angles possible. If the procedure involves

intermediate angles, then these angles must be measured.

Example 1 (variant - Example 1a in parentheses):

(a)i The downward force exerted by the fan-block system (or "mass", as read on a

balance, of the system)

(a)ii A spring scale (or mechanical balance)

(a)iii Set the fan angle to 90°. With the fan off, hang the system from a spring scale to

find its weight (or place the system on a mechanical balance to find its mass).

Turn the fan on, and record the force from the spring scale (or the mass reading

from the mechanical balance).

Example 2:

(a)i Mass of the fan-block system and low-friction cart. Velocity of the cart as a

function of time.

(a)ii Scale to measure mass. Motion detector to measure velocity as a function of time.

(a)iii Place the system and a low-friction cart on a scale to find the total mass of the

fan-block and cart. Next, on a horizontal surface, place the fan-block on the cart,

set the fan angle to zero, turn on the fan, and release the cart from rest. Use the

motion detector to graph the cart velocity as a function of time.

Question (iv)

(iv)

iv. Describe how the force can be determined from the measurements described in parts (a)i-iii.

iv) 1 point

For correctly indicating a method to determine the force exerted by air on the fan, using

1 point

the measurements and procedure from parts (a) i, ii, and iii

Example 1 (and 1a), continued from above:

Take the difference in the force readings with the fan off and the fan on. (1a:

Multiply the difference in the mass readings by g ). This difference is the force

exerted by air on the fan.

Example 2, continued from above:

Estimate the average slope of the graph, which is the acceleration. Multiply

acceleration by the total mass to find the force exerted by air on the fan.