If each washer has a mass of 4.9 g, then the mass of two washers in kilograms would be
.
Given that F = ma, and if 4 washers are attached to the string and car instead of 2, then the applied force on the car will be
. Recall a = 10 m/s2
The total mass of two washers is 2 * 4.9 g = 9.8 g = 0.0098 kg.
The total mass of four washers is 4 * 4.9 g = 19.6 g = 0.0196 kg.
To calculate the applied force on the car when 4 washers are attached:
F = ma
F = (0.0196 kg)(10 m/s^2)
F = 0.196 N
Therefore, the applied force on the car when 4 washers are attached is 0.196 N.