To find the average acceleration of the car, we can use the formula for average acceleration, which is defined as the change in velocity divided by the time taken for that change. The formula is: \[ a_{\text{avg}} = \frac{\Delta v}{\Delta t} \] Where: - \( a_{\text{avg}} \) is the average acceleration, - \( \Delta v \) is the change in velocity, - \( \Delta t \) is the change in time. ### Step-by-Step Calculation 1. **Identify the Initial and Final Velocities**: - The car starts from rest, so the initial velocity \( v_i = 0 \, \text{m/s} \). - The final velocity \( v_f = 20 \, \text{m/s} \). 2. **Calculate the Change in Velocity**: - The change in velocity \( \Delta v \) is given by: \[ \Delta v = v_f - v_i = 20 \, \text{m/s} - 0 \, \text{m/s} = 20 \, \text{m/s} \] 3. **Identify the Time Interval**: - The time taken for this change is \( \Delta t = 5 \, \text{s} \). 4. **Substitute Values into the Average Acceleration Formula**: - Now we can substitute the values into the average acceleration formula: \[ a_{\text{avg}} = \frac{\Delta v}{\Delta t} = \frac{20 \, \text{m/s}}{5 \, \text{s}} = 4 \, \text{m/s}^2 \] ### Conclusion The average acceleration of the car during this time is **4 m/s²**. Therefore, the correct option is **B**. ### Explanation of Other Options - **Option A (2 m/s²)**: This value would imply that the car only increased its speed by 10 m/s over 5 seconds, which is incorrect based on the given final speed of 20 m/s. - **Option C (5 m/s²)**: This would suggest that the car reached a speed of 25 m/s in 5 seconds, which is also incorrect since the final speed is 20 m/s. - **Option D (10 m/s²)**: This would imply that the car reached a speed of 50 m/s in 5 seconds, which is far too high given the initial conditions. ### Common Pitfalls - **Misunderstanding the Formula**: Some students might confuse average acceleration with instantaneous acceleration. Remember, average acceleration is calculated over a time interval. - **Forgetting Initial Conditions**: Always ensure to start from the correct initial conditions, especially when the object starts from rest. - **Units**: Ensure that the units are consistent. Here, we used meters per second (m/s) for velocity and seconds (s) for time. ### Revision Summary - Average acceleration is calculated using the formula \( a_{\text{avg}} = \frac{\Delta v}{\Delta t} \). - For this problem, the change in velocity is 20 m/s, and the time interval is 5 seconds. - The average acceleration of the car is 4 m/s², making option B the correct answer. - Always check initial and final conditions carefully to avoid calculation errors.