You can't directly measure the force of gravity on an object with a single instrument. Instead, you need to measure the object's mass and then calculate the force of gravity using the following equation:

Force of gravity (F) = mass (m) * acceleration due to gravity (g)

Here's how you would do it:

1. Measure the object's mass: Use a scale or balance to determine the object's mass in kilograms (kg).

2. Determine the acceleration due to gravity: The acceleration due to gravity on Earth is approximately 9.81 m/s². This value may vary slightly depending on location.

3. Calculate the force of gravity: Multiply the object's mass by the acceleration due to gravity. The result will be in Newtons (N).

Therefore, the instruments you need are:

* A scale or balance to measure the object's mass.

* A reference value for the acceleration due to gravity, which you can find online or in a physics textbook.

Note: You can also indirectly measure the force of gravity by observing an object's acceleration due to gravity, for example, by dropping it and timing its descent. However, this method is less precise than measuring the mass and calculating the force.