Torque AP Physics 1 FRQ – Exam Style Questions etc.
Torque AP Physics 1 FRQ
Unit 5: Torque and Rotational Dynamics
Weightage : 10-15%
Exam Style Practice Questions, Torque AP Physics 1 FRQ
Question
A box of uniform density weighing 100 newtons moves in a straight line with constant speed along a horizontal surface. The coefficient of sliding friction is 0.4 and a rope exerts a force F in the direction of motion as shown above.
a. On the diagram below, draw and identify all the forces on the box.
b. Calculate the force F exerted by the rope that keeps the box moving with constant speed.
c. A horizontal force F’, applied at a height 5/3 meters above the surface as shown in the diagram above, is just sufficient to cause the box to begin to tip forward about an axis through point P. The box is 1 meter wide and 2 meters high. Calculate the force F’.
▶️Answer/Explanation
Ans:
a) FBD. Fn pointing up, Fg pointing down, fk applied to base of box pointing left
b) Constant speed → a=0.
Fnet = 0 F – fk = 0 F – uFn = 0 F – (0.4)(100)=0 F = 40 N
c) The force F’ occurs at the limit point of tipping which is when the torque trying to tip it (caused by F’) is equal to the torque trying to stop it from tipping (from the weight) using the tipping pivot point of the bottom right corner of the box.
(F’)(5/3 m) = Fg (0.5m)
(F’) (5/3) = (100)(0.5)
F’ = 30 N
Question
The diagram below shows a beam of length 20.0 m and mass 40.0 kg resting on two supports placed at 5.0 m from each end.
A person of mass 50.0 kg stands on the beam between the supports. The reaction forces at the supports are shown.
(a) State the value of N1 + N2
(b) The person now moves toward the X end of the beam to the position where the beam just begins to tip and reaction force N1 becomes zero as the beam starts to leave the left support. Determine the distance of the girl from the end X when the beam is about to tip.
▶️Answer/Explanation
Ans:
(a) Simple application of Fnet(y) = 0 N1 + N2 – mbg – mpg = 0
N1 + N2 = (40)(9.8) + (50)(9.8) = 882 N
(b) 
Apply rotational equilibrium
(mbg) • r1 = (mpg) • r2
(40) (5m) = (50) (r)
r = 4m from hinge
→ 1 m from point X