Phys101
Term:122
Online HW-Ch09-Lec04
Q1:
An elastic collision is one in which:
A.
B.
C.
D.
E.
momentum is not conserved but kinetic energy is conserved
total mass is not conserved but momentum is conserved
kinetic energy and momentum are both conserved
momentum is conserved but kinetic energy is not conserved
the total impulse is equal to the change in kinetic energy
Ans:
C
Q2:
Block A, with a mass of 2.0 kg, moves along the x axis with a velocity of 5.0 m/s in
the positive-x direction. It suffers an elastic collision with block B, initially at rest,
and the blocks leave the collision along the x axis. If B is double the mass of A, the
velocity of A after the collision is: (Give your answer in three significant figures
form)
Ans:
�⃗Ai = 5 m/s ı̂
mA = 2 kg; v
0
mB = 2mA = 4 kg ; v
�⃗Bi = 0
mA − mB
2mB
Elastic collision ⇒ vAf =
vAi +
v
mA + mB
mA + mB Bi
⇒ vAf =
2−4
(5) = −1.67 m/s along x
2+4
Q3:
A 3.0 kg and a 2.0 kg carts approach each other on a horizontal air track. They collide
and stick together. After the collision their total kinetic energy is 40 J. The speed (in
m/s) of their center of mass is: (Give your answer in three significant figures form)
𝐀𝐧𝐬:
m1 = 3 kg
m2 = 2 kg
Stick together ⇒ vf is the same for both carts
K f = 40 J ; vcom = ?,
vcom = vf
1
Mvf 2 ; where M = m1 + m2 = 3 + 2 = 5 kg
2
1
so 40 = (5)(vcom )2 ⇒ vcom = 4.00 m/s
2
and K f =
KFUPM-Physics Department
1