Translational Kinetic Energy AP Physics 1 FRQ – Exam Style Questions etc.
Translational Kinetic Energy AP Physics 1 FRQ
Unit: 3. Work , Energy and Power
Weightage : 10-15%
Exam Style Practice Questions, Translational Kinetic Energy AP Physics 1 FRQ
Question
A 0.5 kilogram object rotates freely in a vertical circle at the end of a string of length 2 meters as shown above. As the object passes through point P at the top of the circular path, the tension in the string is 20 newtons. Assume g = 10 meters per second squared.
(a) On the following diagram of the object, draw and clearly label all significant forces on the object when it is at the point P.
(b) Calculate the speed of the object at point P.
(c) Calculate the increase in kinetic energy of the object as it moves from point P to point Q.
(d) Calculate the tension in the string as the object passes through point Q.
▶️Answer/Explanation
Ans:
(a) 
(b) Apply Fnet(C) = mv2/ r … towards center as + direction
(Ft + mg) = mv2/r (20+0.5(10))=(0.5)v2/ 2 v = 10 m/s
(c) As the object moves from P to Q, it loses U and gains K. The gain in K is equal to the loss in U.
∆U = mg∆h = (0.5)(10)(4) = 20 J
(d) First determine the speed at the bottom using energy.
Ktop + Kgain = Kbottom ½ mvtop2 + 20 J = ½ mvbot2 vbot = 13.42 m/s
At the bottom, Ft acts up (towards center) and mg acts down (away from center)
Apply Fnet(C) = mv2/ r … towards center as + direction
(Ft – mg) = mv2/r (Ft – 0.5(10)) = (0.5)(13.42)2/ 2 Ft = 50 N
Question
A 2-kilogram block initially hangs at rest at the end of two 1-meter strings of negligible mass as shown on the left diagram above. A 0.003-kilogram bullet, moving horizontally with a speed of 1000 meters per second, strikes the block and becomes embedded in it. After the collision, the bullet/ block combination swings upward, but does not rotate.
a. Calculate the speed v of the bullet/ block combination just after the collision.
b. Calculate the ratio of the initial kinetic energy of the bullet to the kinetic energy of the bullet/ block combination immediately after the collision.
c. Calculate the maximum vertical height above the initial rest position reached by the bullet/block combination.
▶️Answer/Explanation
Ans:
a) Apply momentum conservation perfect inelastic. pbefore = pafter m1v1i = (m+M)vf vf = 1.5 m/s
b) KEi / KEf ½ m v1i2/ ½ (m+M)vf2 = 667
c) Apply conservation of energy of combined masses K = U ½ (m+M)v2 = (m+M)gh h = 0.11 m