To solve the problem, we need to determine the force exerted by the floor of the elevator on the body when the elevator is accelerating upward. Let's break this down step-by-step. ### Step 1: Understand the Forces Acting on the Body When the elevator is at rest or moving at a constant speed, the only forces acting on the body are its weight (downward) and the normal force exerted by the floor (upward). The weight of the body is given as 80 N, which is the force due to gravity acting on it. When the elevator accelerates upward, the normal force (let's call it \( F_N \)) must not only balance the weight of the body but also provide the additional force required for the upward acceleration. ### Step 2: Apply Newton's Second Law According to Newton's second law, the net force acting on an object is equal to the mass of the object multiplied by its acceleration (\( F_{net} = m \cdot a \)). 1. **Calculate the mass of the body**: The weight of the body is given by the formula: \[ W = m \cdot g \] where \( W \) is the weight (80 N), \( m \) is the mass, and \( g \) is the acceleration due to gravity (10 m/s²). Rearranging this gives: \[ m = \frac{W}{g} = \frac{80 \, \text{N}}{10 \, \text{m/s}^2} = 8 \, \text{kg} \] 2. **Calculate the net force when the elevator accelerates upward**: When the elevator accelerates upward with an acceleration \( a = 5 \, \text{m/s}^2 \), the net force acting on the body can be expressed as: \[ F_{net} = m \cdot a = 8 \, \text{kg} \cdot 5 \, \text{m/s}^2 = 40 \, \text{N} \] ### Step 3: Determine the Normal Force The normal force must counteract both the weight of the body and provide the net force for the upward acceleration. Therefore, we can set up the equation: \[ F_N - W = F_{net} \] Substituting the known values: \[ F_N - 80 \, \text{N} = 40 \, \text{N} \] Now, solving for \( F_N \): \[ F_N = 80 \, \text{N} + 40 \, \text{N} = 120 \, \text{N} \] ### Conclusion The force exerted by the floor on the body as the elevator moves upward with an acceleration of 5 m/s² is **120 N**. Therefore, the correct option is **C. 120 N**. ### Explanation of Other Options - **A. 40 N**: This option is incorrect because it does not account for the weight of the body. The normal force must be greater than the weight when the elevator accelerates upward. - **B. 80 N**: This option represents the weight of the body when the elevator is at rest or moving at constant speed. It does not consider the additional force required for upward acceleration. - **D. 160 N**: This option is incorrect as it suggests an excessive normal force that would imply a downward acceleration, which is not the case here. ### Revision Summary - The normal force in an accelerating elevator must counteract both the weight of the body and provide the necessary force for acceleration. - Use \( F_N = W + F_{net} \) to find the normal force when the elevator accelerates. - Remember that weight is calculated using \( W = m \cdot g \). - Always check the direction of forces when dealing with acceleration in elevators.