Final answer:
The magnitude of the force exerted on the block with mass 9 kg by the block with mass 4 kg is 49 N.
Step-by-step explanation:
The magnitude of the force exerted on the block with mass 9 kg by the block with mass 4 kg can be calculated using Newton's second law of motion, which states that force equals mass times acceleration (F = ma).
First, let's find the acceleration of the system. Since the horizontal surface is frictionless, the only force acting on the system is the gravitational force pulling the blocks downward. The force of gravity can be calculated using the formula F = mg, where m is the mass of the object and g is the acceleration due to gravity (9.8 m/s²).
For the 9 kg block, the force is F₁ = 9 kg × 9.8 m/s² = 88.2 N.
For the 4 kg block, the force is F₂ = 4 kg × 9.8 m/s² = 39.2 N.
Therefore, the magnitude of the force exerted on the 9 kg block by the 4 kg block is the difference between the forces, F = F₁ - F₂ = 88.2 N - 39.2 N = 49 N.