Newton's 3rd law: The ball exerts a force on the bat that is equal and opposite to the force exerted by the bat on the ball.
Answer:
a)
1.35 kg
b)
2.67 ms⁻¹
Explanation:
a)
= mass of first body = 2.7 kg
= mass of second body = ?
= initial velocity of the first body before collision = 
= initial velocity of the second body before collision = 0 m/s
= final velocity of the first body after collision =
using conservation of momentum equation

Using conservation of kinetic energy

b)
= mass of first body = 2.7 kg
= mass of second body = 1.35 kg
= initial velocity of the first body before collision = 4 ms⁻¹
= initial velocity of the second body before collision = 0 m/s
Speed of the center of mass of two-body system is given as
ms⁻¹
300/8 = 37.5
37.5 x 12 = 450
New temp. = 450 K
Hope this helps!