Practice Test: Mr Rogers AP Physics C  Friction 


Online Quiz  Friction 8B66D878 

AP Physics Friction Practice Test 

1  )  A block of mass = 2M is stacked atop a block of mass = M. The sliding COF between the blocks = 0.8 and between the lower block and floor is 0.1. A force = F is applied to the top block parallel to the ground. Find the acceleration of the lower block. a = 1.3g  
2  )  Find the acceleration of a box of mangos sliding down a slope of 40 degrees. Assume the dynamic COF = .2 and the static COF = .3. Would doubling the number of mangos in the box have any effect on the acceleration? No, a = g ( sinβ  μ_{d }cosβ ). Note that mass is not in the equation.  
3  ) 


4  )  Find the static COF if the box of mangos starts to slide if the angle of the slope = 30 degrees. static COF = tan 30^{º }or 0.577  
5  )  The tires on Martha's 4WD SUV have a static COF = 0.7 and a dynamic COF = 0.5 with the road surface. What is her maximum acceleration and minimum stopping distance given a starting velocity of 40 m/s? a_{max} = 0.7g (Note that the maximum acceleration comes from the max friction force, which is the max static friction. With antilock brakes the wheels do not lock up and slide. They roll slower and slower as the vehicle stops, at which time they stop rolling.), Min stopping distance = 114 m  
6  ) 
If Martha's SUV is only 2WD, what is the effect on her maximum
acceleration (speeding up) and minimum stopping distance (slowing down)? Assume the COF's the same
for all wheels
and that the car's mass is evenly distributed (half over the front tires and
half over the back tires). Her maximum acceleration
(speeding up) is cut in half and her minimum stopping distance doubled.
Note that in reality there is a separate friction force acting on each
wheel. If the weight of the vehicle is evenly distributed, each driven wheel
will have a normal force of 1/4 mg making its static friction force = 1/4
μ_{s}mg.
Using kinematics analysis with a_{max} based on the maximum static friction:
Obviously, cutting the stopping acceleration in half doubles the stopping distance.


7  ) 
Juan pushes downward on a box of mass = M with a force = 2Mg
that makes a 45 degree angle with the horizon. By contrast Susan pulls
upward on a similar box with a similar force and angle. Both boxes sit on a
surface with a sliding COF = 0.2 and a static COF = 0.3. Find the
acceleration of both boxes.


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