How to Calculate the Force Between Two Touching Objects
Learn to determine the force between two touching objects to be able to complete physics problems. According to Isaac Newton's third law of motion, forces in nature exist in pairs. Specifically, when an object applies a force to another object, an equal force directed in the opposite direction appears. An example is a book sitting on a table. The book exerts a force downward on the table equal to its weight. The table exerts a reaction force, called normal force, back on the book equal to the book's weight.
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Instructions
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Determine the weight of the object applying the initial force in pounds. For example, take a concrete block resting on a wooden incline, with the block weighing 10 lb.
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Convert the weight of the object to a mass in kilograms by dividing by 2.2046, since a kilogram contains 2.2046 pounds. Performing this step leads to 10 pounds divided by 2.2046 lb. per kilogram, or 4.54 kg.
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Measure the angle, in degrees, of the incline using a protractor. Place the flat side of the protractor level with the floor. If the object rests on flat surface the angle is 0. Assume an angle of 20 degrees for this example.
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Multiply the mass times the acceleration due to gravity times the cosine of the angle to arrive at the force the object exerts on the inclined surface. By Newton's third law, this is the same force the inclined surface exerts on the block. The acceleration due to gravity is 9.81 meters per second squared and equals the rate of acceleration of falling objects on Earth. Continuing the exercise, 4.54 kg times 9.81 meters per second squared times the cosine of 20 degrees equals a force of 41.9 newtons. The newton is the unit of force used in science.
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Convert the force to pounds. Observe that when an object rests on an incline, its full weight is not applied to the surface. Multiply by 0.2248 to convert from newtons to pounds. Completing the exercise leads to 41.9 newtons times 0.2248 pounds per newton, or 9.4 lb. Recall the weight of the cement block is 10 lb. which is greater than 9.4 lb.
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Tips & Warnings
Use metric units when performing force calculations to get the correct result.
The newton unit equals a kilogram times meter per second squared.
