An object is let in free fall from a platform 20m high above Earth's surface. Describe the event in terms of energy and thus determine the speed of the object when it hits ground. Air resistance is negligible and gravitational acceleration is constant.
When the object is at rest on the platform it has no kinetic energy, but only potential energy. The potential energy is mgh where m is the mass of the object, g the gravitational acceleration and h the height of the object before falling. During free fall, the height of the object decreases and so does potential energy, and the speed increases, and with it kinetic energy increases. There is an exchange between potential and kinetic energy. When the object hits the ground there is no potential energy because the height is zero and its energy is only kinetic, 1/2mv2, where v is the speed of the object when it hits the ground. Using the law of energy conservation we deduce that the initial potential energy (mgh) was completely converted in kinetic energy at ground level (1/2mv2) and by equating these two we get v to be sqrt(2gh) so 19.8 m/s.
Related Physics A Level answers
All answers ▸The mercury atoms in a fluorescent tube are excited and then emit photons in the ultraviolet region of the electromagnetic spectrum. Explain (i) how the mercury atoms become excited and (ii) how the excited atoms emit photons.
What is the derivative of distance with respect to time.
A coil is connected to an analogue centre zero ammeter. A magnet is dropped (North pole first) so that it falls vertically and completely through the coil. What would be observe on the ammeter?
A ball is kicked off a cliff at a height of 20m above ground and an angle of 30 degree from the horizontal, it follows projectile motion and lands after a time t. Its velocity at the maximum height it reaches is 20m/s, how long does it take it to land?