SAT II Physics : Linear Motion

Study concepts, example questions & explanations for SAT II Physics

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Example Questions

Example Question #1 : Kinematics Equations

Q6

If the block of mass \(\displaystyle m\) slides up a frictionless incline plane and is pulled by mass \(\displaystyle M\) which is falling then what is the acceleration of the block on the ramp if \(\displaystyle m= 20kg\) and \(\displaystyle M=40kg\) and the angle of incline is 30 degrees?

Possible Answers:

\(\displaystyle a=2g\)

\(\displaystyle a=g\sqrt{2}\)

\(\displaystyle a=\frac{1}{2}g\)

\(\displaystyle a=\frac{\sqrt{3}}{2}g\)

\(\displaystyle a=\frac{5}{6}g\)

Correct answer:

\(\displaystyle a=\frac{1}{2}g\)

Explanation:

Begin by making a free body diagram for each block:

Q62

Use the diagram to write an equation for net force on each block:

\(\displaystyle F_{net} =F_{t}-mg \sin (30)\)

\(\displaystyle F_{net} = Mg -F_{t}\)

Since \(\displaystyle F=ma\) for the block on the ramp and \(\displaystyle F=Ma\) for falling block we can substitute into these equations:

\(\displaystyle ma =F_{t}-mg \sin (30)\)

\(\displaystyle Ma = Mg -F_{t}\)

Then add the equations to get:

\(\displaystyle Ma+ma = Mg-mg\sin (30)\)

Next rearrange to isolate \(\displaystyle a\):

\(\displaystyle a = \frac{M-msin(30)}{M+m}\)

Substitute the given values from the question and solve:

\(\displaystyle a = \frac{40-20sin(30)}{40+20}\)

\(\displaystyle a=\frac{1}{2}g\)

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