# What is the correct kinetic energy in the following case?

We dropped a golf ball from 50 cm above the ground into a sand pit. It took 0.046 seconds for the free fall. We calculated s=d/t and substituted that into KE=1/2mv ^2 , we got 9.2 joules

But when I worked out velocity with v=√2as (instead of speed) I got the kinetic energy as 2.2 joules. Which is right ?

Asked on by leila324

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justaguide | College Teacher | (Level 2) Distinguished Educator

Posted on

When a ball is falling under gravity, there is a force acting on it due to the gravitational pull of the Earth. This force accelerates the ball as it falls down.

In calculating the kinetic energy of the ball when it hits the sand pit, you require the velocity(or speed) of the ball when it has dropped down 50 cm. This cannot be calculated using the formula s = d/t as that formula is valid only for a body moving at a constant speed, not if the body is under acceleration.

If the kinetic energy has to be calculated we need the mass of the ball in addition to its velocity. That also has not been provided.

You can calculate the final velocity of the ball as (2*0.5/0.46) m/s

This velocity can get be substituted into the expression for kinetic energy to calculate it.

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