The average resisting force that must act on `5kg` mass to reduce its speed from 65 to `15ms^-1` in `2s` is

Options

A. (a) `12.5N`
B. (b) `125N`
C. (c) `1250N`
D. (d) None of these

Explanation

To solve the problem of finding the average resisting force that must act on a 5 kg mass to reduce its speed from 65 m/s to 15 m/s in 2 seconds, we can follow these steps:

### Step 1: Identify the given values
- Initial velocity (u) = 65 m/s
- Final velocity (v) = 15 m/s
- Mass (m) = 5 kg
- Time (t) = 2 s

### Step 2: Calculate the acceleration
We can find the acceleration (a) using the formula:
\[ a = \frac{v - u}{t} \]
Substituting the values:
\[ a = \frac{15 \, \text{m/s} - 65 \, \text{m/s}}{2 \, \text{s}} = \frac{-50 \, \text{m/s}}{2 \, \text{s}} = -25 \, \text{m/s}^2 \]

### Step 3: Calculate the resisting force
The average resisting force (F) can be calculated using Newton's second law:
\[ F = m \cdot a \]
Substituting the values:
\[ F = 5 \, \text{kg} \cdot (-25 \, \text{m/s}^2) = -125 \, \text{N} \]
Since we are interested in the magnitude of the resisting force, we take the absolute value:
\[ F = 125 \, \text{N} \]

### Conclusion
The average resisting force that must act on the 5 kg mass is **125 N**.

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