Review Physics 131: Lecture 19 Today s Agenda Clicker Question 1: pulled over a frictionless surface by a force of 0 N. What acceleration does block A have? a) Zero m/s b). m/s c) 6.67 m/s d) 60 m/s e) 180 m/s Physics 01: Lecture 17, Pg 1 Physics 01: Lecture 17, Pg Clicker Question 1: pulled over a frictionless surface by a force of 0 N. What acceleration does block A have? Clicker Question : pulled over a frictionless surface by a force of 0 N. What is the tension between blocks B and C, T BC? a) 3.3 N b) 6.67 N c) 13.34 N d) 0 N e) 6.7 N Physics 01: Lecture 17, Pg 3 Physics 01: Lecture 17, Pg 4 Clicker Question : pulled over a frictionless surface by a force of 0 N. What is the tension between blocks B and C, T BC? Problem 11-47 A horizontal spring with k = 100 N/m is compressed by 0.0 m and launches a.5 kg block across a frictionless horizontal surface. After a bit the block encounters a rough surface with K = 0.15. How far will the bo slide before coming to rest? Physics 01: Lecture 17, Pg 5 Physics 01: Lecture 17, Pg 6 Page 1
Problem 11-47 A horizontal spring with k = 100 N/m is compressed by 0.0 m and launches a.5 kg block across a frictionless horizontal surface. After a bit the block encounters a rough surface with K = 0.15. How far will the bo slide before coming to rest? Clicker Question 3: Two blocks of mass m 1 and m (m 1 > m ) slide on a frictionless floor and have the same velocity when they hit a long rough stretch ( > 0), which slows them down to a stop. Which one goes farther? a) m 1 b) m c) they will go the same distance Physics 01: Lecture 17, Pg 7 Physics 01: Lecture 17, Pg 8 Clicker Question 3: Frictional force Car tires are very complicated Technically we only deal with friction of noncompressible objects, for tires it is a different story Friction in general is very complicated F= n Physics 01: Lecture 17, Pg 9 Physics 01: Lecture 17, Pg 10 Physics 01: Lecture 17, Pg 11 Physics 01: Lecture 17, Pg 1 Page
Eample Three blocks are resting on a frictionless surface. Two blocks, one of mass M 1 = 5 kg and the other of M = kg, are held together, compressing a massless spring. When the constraints are removed, block 1 moves to the left with a velocity of m/s. Block moves to the right and collides with block 3, which has a mass of M 3 = 5 kg. These blocks stick together. (a) What is the magnitude of the final velocity of blocks and 3? (b) How much energy was stored in the spring initially? Clicker Question 4: Three blocks are resting on a frictionless surface. Two blocks, one of mass M 1 = 5 kg and the other of M = kg, are held together, compressing a massless spring. When the constraints are removed, block 1 moves to the left with a velocity of m/s. Block moves to the right and collides with block 3, which has a mass of M 3 = 5 kg. These blocks stick together. Which is true concerning the block 1-block -spring system before and after the spring acts (before the collision i with block 3) (A) Mechanical energy is conserved (B) Linear momentum in the -direction is conserved (C) Both (A) and (B) (D) Neither (A) or (B) Conservation of Mechanical Energy, Pg 13 Conservation of Mechanical Energy, Pg 14 Eample Three blocks are resting on a frictionless surface. Two blocks, one of mass M 1 = 5 kg and the other of M = kg, are held together, compressing a massless spring. When the constraints are removed, block 1 moves to the left with a velocity of m/s. Block moves to the right and collides with block 3, which has a mass of M 3 = 5 kg. These blocks stick together. (a) What is the magnitude of the final velocity of blocks and 3? (b) How much energy was stored in the spring initially? Eample Three blocks are resting on a frictionless surface. Two blocks, one of mass M 1 = 5 kg and the other of M = kg, are held together, compressing a massless spring. When the constraints are removed, block 1 moves to the left with a velocity of m/s. Block moves to the right and collides with block 3, which has a mass of M 3 = 5 kg. These blocks stick together. (a) What is the magnitude of the final velocity of blocks and 3? (b) How much energy was stored in the spring initially? Conservation of Mechanical Energy, Pg 15 Conservation of Mechanical Energy, Pg 16 Clicker Question 6: speeds on a frictionless air track. Assume no eternal forces act on the pucks. The total momentum of the system consisting of the two pucks is zero. Clicker Question 7: speeds on a frictionless air track. Assume no eternal forces act on the pucks. The total kinetic energy of the two pucks is zero. Conservation of Mechanical Energy, Pg 17 Conservation of Mechanical Energy, Pg 18 Page 3
Clicker Question 8: speeds on a frictionless air track. Assume no eternal forces act on the pucks. The mechanical energy of the system will be conserved during the collision. Clicker Question 9: speeds on a frictionless air track. Assume no eternal forces act on the pucks. After the two pucks collide the total momentum will not necessarily be the same as before the collision. Conservation of Mechanical Energy, Pg 19 Conservation of Mechanical Energy, Pg 0 Clicker Question 10: speeds on a frictionless air track. Assume no eternal forces act on the pucks. With no eternal horizontal forces acting on the system consisting of the two pucks, the net impulse delivered to each puck must be zero over the entire collision. Clicker Question 11: Mike applied 10 N of force over 3 m in 10 seconds. Joe applied the same force over the same distance in 1 minute. Who did more work? J F t p av f p i a) Mike b) Joe c) both did the same work Conservation of Mechanical Energy, Pg 1 Conservation of Mechanical Energy, Pg Clicker Question 1: In a baseball game, the catcher stops a 90-mph pitch. What can you say about the work done by the catcher on the ball? a) catcher has done positive work b) catcher has done negative work c) catcher has done zero work Clicker Question 13: A spring is compressed by a ball an amount = 0.5 m. The system is released and the ball is shot out down a frictionless table. At the end of the table the ball encounters a hump that brings it up to a height of 0.6 m. If the mass of the ball is 10 kg and the k for the spring is 600 N/m what is the final speed of the ball? a).33 m/s b) 4.56 m/s c) 3.45 m/s d) 1.79 m/s e) 5 m/s vf? Conservation of Mechanical Energy, Pg 3 Conservation of Mechanical Energy, Pg 4 Page 4
E f = E i K f + U f =K i + U i ½k f + ½mv f + mgy f = ½k i + ½mv i + mgy i ½k(0) + ½mv f + mg(.6) = ½k(.5) + ½m(0) + mg(0) ½mv f + = mg(.6) ½k(.5) ½mv f = ½k(.5) -mg(.6) mv f = k(.5) - mg(.6) v f = (k/m)(.5) - g(.6) v v f =3. (m/s) f = 1.79 m/s Clicker Question 16: A rifle bullet of mass m = 3.00 10 - kg traveling at v b = 40.0 m/s collides with and embeds itself in a pendulum of mass M =.88 kg, initially at rest and suspended vertically by massless strings of length L =.00 m. As the bullet collides with the block, what is conserved for the block/bullet system? (The bullet/block collision is so fast the pendulum does not have time to swing up until after the collision) (a) Mechanical Energy (b) Linear Momentum (c) Both (a) and (b) (d) Neither (a) or (b) Conservation of Mechanical Energy, Pg 5 Conservation of Mechanical Energy, Pg 6 Clicker Question 17: A rifle bullet of mass m = 3.00 10 - kg traveling at v b = 40.0 m/s collides with and embeds itself in a pendulum of mass M =.88 kg, initially at rest and suspended vertically by massless strings of length L =.00 m. After the bullet is imbedded in the block and the pendulum begins to swing, what is conserved for the block/bullet system? (a) Mechanical Energy (b) Linear Momentum (c) Both (a) and (b) (d) Neither (a) or (b) : a -D Collision George is driving south along Spadina at 18.0 m/s in a flatbed truck heavily loaded with bluestone, while John is driving a VW at 4.0 m/s east along Bloor. Because John is teting on his cell phone, he does not see the red light and collides with the front of the truck. During the collision, the vehicles lock together. If George's truck has a total mass of 830 kg and John's has a total mass of 10 kg, find the velocity of the interlocked vehicles immediately after the collision. Truck Conservation of Mechanical Energy, Pg 7 8 Initial m T = 830 kg Final Truck v T = 18 m/s m tot = 9540 kg Tru uck m B = 10 kg y v f v B = 4 m/s v f =?? v fy 9 30 Page 5
Clicker Question 14: After the accident, which is true about the angle of the velocity of the two cars relative to the eastward direction? a) <45 m tot = 9540 kg b) > 45 v f =?? c) = 45 Tr ruck Clicker Question 15: What is the correct equation for the -direction momentum? a) m B v B = (m T + m B )v f b) m T v T = (m T + m B ) v f c) m B v B + m T v T = (m T + m B ) v f d) m B v B = (m T + m B )v f Sin 31 3 -direction object 1 object object 1 object mv i + mv i = mv f + mv f m Bv B = m B v f + m T v f m B v B = (m B + m T ) v f y-direction object 1 object object 1 object mv i + mv i = mv f + mv f m Tv T = m B v yf + m T v yf m T v T = (m B + m T ) v yf v f mbvb m m B T 10kg4m/s 9540 kg 5.37m/s v vf mt vt 830kg18m/s m m B T 9540 kg 15.7m/s 33 34 Final -D Collision I John George m tot = 9540 kg v f = 5.37 m/s f v f =?? v fy =15.7 m/s 5.37m / s 15.7m / s 16.6m s v f / 15.7 arctan 71 5.37 35 Page 6