AP Physics 1 2.2 Forces and Free-Body Diagrams

Figure for Question 1 — AP Physics 1: Algebra-Based

(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.

(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 $\theta$ .

[ 3 ]

(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.

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.

(i)

What quantities would be measured?

(ii)

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

(iii)

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

(iv)

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

(b)

Describe an experimental procedure that would allow the student to use the fan-block system, with the fan turned on, to find the coefficient of kinetic friction between the block and the track.

[ 3 ]

(i)

On the dot to the right, which represents the fan-block system, draw and label the forces (not components) that are exerted on the system during the experiment described in parts (b)i-iii. Represent each force by a distinct arrow starting on, and pointing away from, the dot.

[Maximum number: 3]

Figure for Question 2 — AP Physics 1: Algebra-Based

(12 points, suggested time 25 minutes)
This problem explores how the relative masses of two blocks affect the acceleration of the blocks. Block A, of mass $m_{A}$ , rests on a horizontal tabletop. There is negligible friction between block A and the tabletop. Block B, of mass $m_{B}$ , hangs from a light string that runs over a pulley and attaches to block A, as shown above. The pulley has negligible mass and spins with negligible friction about its axle. The blocks are released from rest.

Now suppose neither block's mass is much greater than the other, but that they are not necessarily equal. The dots below represent block A and block B, as indicated by the labels. On each dot, draw and label the forces (not components) exerted on that block after release. Represent each force by a distinct arrow starting on, and pointing away from, the dot.

LO 3.A.2.1, SP 1.1;
LO 3.A.3.1, SP 6.4 3 points
Now suppose neither block's mass is much greater than the other, but that they are not necessarily equal. The dots below represent block A and block B, as indicated by the labels. On each dot, draw and label the forces (not components) exerted on that block after release. Represent each force by a distinct arrow starting on, and pointing away from, the dot.

Figure for Answer to Question 2 — AP Physics 1: Algebra-Based

For a correct normal force on block A with acceptable label: $N, F_{\mathrm{N}}$ , "normal force,"
$F_{\text {table }}$ , "table force," or any other label indicating the force is "normal" or comes
from the table

1 point

For correct gravitational forces with acceptable label on both diagrams: $F_{\mathrm{g}}, F_{\text {grav }}, W$ ,
$m g, m_{\mathrm{A}} g$ , "gravity," "grav force," but NOT G or g, and no extraneous forces on
either diagram

1 point

For correct tension forces with acceptable label on both diagrams: "tension," "string
force," $F_{T}, F_{\text {tension }}, F_{\text {string }}, F_{S}, T$ , or some other label indicating that the force
comes from the string or from tension. NOT acceptable: $m_{B} g, F_{m_{B}}$ , "force from
block B" or other indications that the force is "created" by block B1 point

AP Physics 1: Algebra-Based 2.2 Forces and Free Body Diagrams Questions

Exam points

Covered syllabus points

Covered syllabus points

Question 1

Question (a)

Question (i)

Question (ii)

Question (iii)

Question (iv)

Question (b)

Question (i)

Question 2