# Physics funnies part A

Some physics cartoons previously tweeted by physic fairy-godmom are included here for your physics enjoyment.   # Practice MC problem 1-24 Do this this one by process of elimination.

I hope you noticed it says ,”Select TWO answers.”

Is it A)?  No, we don’t know the height above the surface from which the rock was dropped or time till it the water so we can’t determine the speed at entry.

B)? No, because we don’t know its speed or acceleration.

Confident? Stop here and answer C) and D). Want to check? Continue …

C)? Yes, at this point it tells us the rock is moving at a constant velocity, so we know its acceleration is 0.

D)? Yes, we can calculate ΔE = mgh2 -mgh1 with h2-h1=vt so                           h2-0=(4m/s)(3s) and we are given m=0.2kg and g=10m/s². We don’t actually have to do the math though.

So we were right– it’s C) and D). # Practice MC problem 1-21 Turning up the volume on a radio increases the amplitude of the wave.  The frequency, period, wavelength remain the same.

Increasing the amplitude means that the displacement of the diaphragm will be larger, so that could be answer A) or C)…what happens to the speed of the motion of the diaphragm?

If the maximum displacement has increased but the frequency of the wave remains the same, then the diaphragm is traveling through a larger distance in the same amount of time–which means the speed has increased– # Practice MC problem 1-13 and 1-14 OK, the plot is of v vs. t and has 4 sections: AB, BC, CD, DE.

13. How do you get acceleration from a plot of v vs t? You calculate the slope.  They make it even easier by asking for AVERAGE acceleration, because you don’t have to worry about the shape of the curve; just calculate a=v/t by counting “rise over run” i.e. y/x on the plot to get average acceleration for each section. SO

average acceleration for AB is 8/0.6 = 13.33

average acceleration for BC is 0/0.4 = 0

average acceleration for CD is -8/0.4 = -20

average acceleration for DE is -4/0.6 = -6.67

Read the question again and notice that they want us to compare the MAGNITUDE of the acceleration, which means absolute value, so for example the magnitude of the average acceleration for CD is 20 which is larger than the magnitude of the average acceleration for AB which is 13.33.

Now rank them in order from highest to lowest magnitude and get CD>AB>DE>BC which is answer D).

14.  How do you calculate distance traveled? Use x=vt+½at² to calculate the distance traveled in each section.  Use time and average velocity from graph, and average acceleration calculated in 13 above so

average velocity for AB is 4m/s and time is 0.6s so x = 4(0.6)+0.5(13)(0.6²) = 6.5m

average velocity for BC is 8m/s for 0.4s so x = 8(0.4)+0.5(0)(0.4²) = 3.2m

average velocity for CD is  4m/s for 0.4s so x = 4(0.4)+0.5(-20)(0.4²) = 0m

average velocity for DE is -2m/s for 0.6s so x = -2(0.6)+0.5(-6.7)(0.6²) = -2.4m

Now choose the one with largest magnitude which is AB which is answer A). # Practice MC problem 1-17 Start with the equations for Fg and Fe from the equation sheet: and note that both Fg and Fe are inversely proportional to r², that is , they have the same dependence on r.

The first situation has a fixed r and an associated Fg1 and Fe1.

When only r changes, we get  a new Fg2 and Fe2 and now Fg2=n*Fg1.

Since Fg and Fe have the same dependence on r,  now Fe2=n*Fe1. # Practice MC problem 1-5 Does the net force on the block equal the vector sum of the individual sources?

1. Translation: ΣF=ma and ΣF= Fweight+Fspring, but does ma=Fweight+Fspring?

2. Identify how to calculate the forces using Fspring=-kΔx , restoring force of spring, and Fweight=mg

3. Realize that the net force ΣF is ma=mg-kΔx.  To see if the value of F is equal to the sum of Fweight and Fspring we need data to fill in all variables on both sides of the equation including a (from the left side, ma) and Δx from the right side- g is known and k is given).  So we need measurements of acceleration and position only.  We didn’t use velocity at all.

# Practice MC problem 1-4 This is a strange one that probably looks nothing like any problem you solved this year.  But look more closely…the graph is a plot of v vs. t, which is very familiar.

The squiggly lines show v v.s t for each individual sphere- but what would be the v vs. t for the center of mass of the whole system?  Most likely it would be a straight line through each of the intersecting points on the plot – a straight line from (0,0) through (0.1,0.25), (0.2,0.5),…,(0.5,1.25). So from this data we can generate a plot of v vs. t for the system.

Can we calculate angular speed from this?

Yes, using v=ωR (not on equation sheet, so memorize please), we can solve for the angular speed ω.

Can we calculate linear momentum?

Yes, using p=mv (which is on the equation sheet FYI), we can calculate the linear momentum p, so we can calculate the change in linear momentum, Δp=mv2-mv1, with v2 from plot at t=0.5s and v1 from plot at t=0s.