Wednesday, 3 April 2013

What happens when you drop a spring balance with the measured object still on it?

This video (shot at 120 frame-per-second) can be used to show that the spring balance does not measure the weight/gravitational force of the object hanging below rather it is measuring the force exerted by object.


  • The spring extends because the object exerts a force on the spring balance and the spring balance exerts an equal force on the object (Newton's 3rd Law). The force on the object exerted by the spring balance is equal to the gravitational force on the object when the system is in equilibrium (either at rest or at constant velocity). 
  • When the system is accelerated (or free fall in this video), the force exerted on the spring balance by the object is different (zero at free fall) while the gravitational force on the object remains constant

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