AP Physics 1 Exam Free Response Example.

Two identical blocks, A and B , are on a horizontal surface, as shown above. There is negligible friction between the surface and the blocks. As shown in Figure 1, block A is initially moving toward block B with speed vA=v0 and block B is initially at rest. Figure 2 shows the blocks in contact as they collide a short time later. In both figures, the location of the center of mass of the two-block system is indicated by the X that is labeled CM.

(a)

i. On each of the dots, which represent the blocks while they are in contact during the collision, draw a single arrow to indicate the direction of the net force, if any, exerted on each block. The net force must be represented by a distinct arrow starting on, and pointing away from, the appropriate dot. If the net force is zero for either block, write “zero” under the dot representing that block.

ii. During the time that the blocks are in contact, describe whether the center of mass of the two-block system is speeding up, slowing down, or staying the same.

____ Speeding up ____ Slowing down ____ Staying the same

Briefly describe, in terms of a basic law of physics, whether the center of mass of the two-block system is speeding up, slowing down, or staying the same.

(b) The surface is tilted to an angle of 37 degrees from the horizontal, as shown above in Figure 3. The blocks are each given a push so that at the instant shown, they are moving toward each other.

i. Each block has a mass of 0.20kg. Calculate the magnitude of the net force exerted on the two-block system at the instant shown above.

ii. At the instant shown in Figure 3, the blocks are moving toward each other with the same speed of 0.35m/s. The blocks collide 0.50 seconds later. What is the speed of the two-block system’s center of mass just before the blocks collide?

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[zelo_s]

Quick question, why should the mass in the very last part of the FRQ be 0.2 instead of 2*0.2 = 0.4? Is the CM considering the 2 Masses?

[stevebosh]

The cos and sin thing blew my mind... Very simple but I was never taught it that way

[clintn.kildepstein]

It’s the day of my physics test and my teacher doesn’t teach jack. Thanks sir!

[ananyaanair]

Awesome video! Mr. Finn, do you think you'll be posting any more AP P1 content before June 2?

[gavintetrault]

The final velocity should be 3m/s and not 3 m/s^2 right?

[karendenny]

2:46 If there was friction, that is considered an external force right? And if so would the Vcm be decreasing ?

[timothy_liu]

In part a, how are the two objects able to have a net force in opposite directions? Won't the contact force and normal force of each block just cancel out causing the two blocks to just remain stationary when they collide?

[MarckBuffalo]

6:24 is 10m/s a substitute for 9.8m/s? And if not where did it come from

[spinze]

For the last part, why is v0 = 0, and not 0.35? In the problem it says they are initially moving with a speed of 0.35 m/s.

[clou]

Mister, I got 2.9m/s as my final answer. All I did differently was use the original values as best as I could instead of round in-between steps. Is this fine?

[asifchowdhury]

For the last part, why would you use 1.2 instead of 2.4?

[SC]

For part c, why is the center of mass have v equal to zero?

[samyang]

in the last part, why is fnet 1.2 and not 2.4?

[moose-]

Bit nit-picky, but velocity's units should have been m/s, not m/s^2

[ananyavangoor]

For the free body diagrams drawn on the inclined plane, wouldn't the force normal have to be drawn from the base of the block and not the center?

[goondocx]

Thank you so much for the explanation!

[Bcam]

on part bii. I thought the fnet was 2.4? Why is 1.2 used?

[samyang]

in the last part, why didnt we use the speed given by 0.35?

[mom]

For part 1 of B, why is the net force 2.4N. Shouldn't it have been 1.2N because the system as a whole is experiencing 1.2N of force?

[ahmadabualaish]

Im confused on why u didnt use the 2.4 in place of Fnet