Ten coins each of mass 10 gm are placed one above the other. The reaction force exerted by 7th coin from the bottom on the 8th coin is (`g= 10 ms^(-2)`)
(a) 0.3 N
(b) 0.2 N
(c) 0.4 N
(d) 0.7 N

Options

A. 0.3 N
B. 0.2 N
C. 0.4 N
D. 0.7 N

Answer

To solve the problem, we need to determine the reaction force exerted by the 7th coin on the 8th coin in a stack of 10 coins, each with a mass of 10 grams.

### Step-by-Step Solution:

1. **Identify the Mass of Each Coin**:
Each coin has a mass of 10 grams. To convert this to kilograms (since SI units are required), we use:
\[ \text{Mass of each coin} = 10 \, \text{grams} = \frac{10}{1000} \, \text{kg} = 0.01 \, \text{kg} \]

2. **Determine the Number of Coins Above the 7th Coin**:
The 7th coin has 3 coins above it (the 8th, 9th, and 10th coins).

3. **Calculate the Total Weight of the Coins Above the 7th Coin**:
The total weight (W) of the 3 coins above the 7th coin can be calculated using the formula:
\[ W = \text{mass} \times g \]
where \( g = 10 \, \text{m/s}^2 \). The total mass of the 3 coins is:
\[ \text{Total mass of 3 coins} = 3 \times 0.01 \, \text{kg} = 0.03 \, \text{kg} \]
Now, substituting into the weight formula:
\[ W = 0.03 \, \text{kg} \times 10 \, \text{m/s}^2 = 0.3 \, \text{N} \]

4. **Determine the Reaction Force**:
According to Newton's third law, the reaction force exerted by the 7th coin on the 8th coin will be equal to the weight of the coins above it. Therefore, the reaction force \( R \) is:
\[ R = W = 0.3 \, \text{N} \]

5. **Conclusion**:
The reaction force exerted by the 7th coin on the 8th coin is \( 0.3 \, \text{N} \).

### Final Answer:
(a) 0.3 N