Considering the change of units and the definition of weight, the mass of two washers would be 0.0098 kg and if 4 washers are attached to the string, then the applied force on the car will be 0.196 N.

Mass of two washers

If each washer has a mass of 4.9 g, then the mass of two washers will be calculated simply by multiplying the mass of 1 washer by the amount you have of them, in this case 2:

4.9 g×2= 9.8 g

Being 1 kg = 1000 g, you apply the following rule of three to make the change of units: if 1000 g equals 1 kg, 9.8 g equals how many kilograms?

$mass=\frac{9.8 gramsx1 kg}{1000g}$

Solving:

mass= 0.0098 kg

Finally, the mass of two washers would be 0.0098 kg.

Applied force

On the other side, mass is the amount of matter that a body contains and weight is the action exerted by the force of gravity on the body.

The mass of an object will always be the same, no matter where it is located. Instead, the weight of the object will vary according to the force of gravity acting on it.

Weight is calculated as the product of the object's mass and the value of the gravitational acceleration:

W= m×g

In this case, you know:

m= 4.9 g×4= 19.6 g= 0.0196 kg → the mass of 4 washers in kilograms
g= 10 m/s²

Replacing:

W= 0.0196 kg× 10 m/s²

Solving:

W= 0.196 N

Finally, if 4 washers are attached to the string, then the applied force on the car will be 0.196 N.

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