10.2 Kinetic energy and potential energy Flashcards
What is kinetic energy
The energy of an object due to its motion
The faster an object moves, the more kinetic energy it has
Kinetic energy equation
Ek=1/2mv^2
What is potential energy
The energy of an object due to its position
If an object is raised through a vertical height Δh at steady speed, the force needed to raise it is equal and opposite to its weight mg. Therefore…
…the work done to raise the object=mgΔh=force*distance moved
The work done on the object increases its gravitational potential energy
ΔEp=mgΔh
If an object is released above the ground and air resistance is negligible, the object gains speed as it falls, potential energy therefore decreases and its kinetic increases
After falling through a vertical height, its kinetic energy is equal to its loss of potential energy
1/2mv^2=mgΔh
When does the equation for kinetic energy NOT hold
Speeds approaching the speed of light
Einstein’s theory of special relativity tells us that the mass of an object increases with speed and that the energy E of an object can be worked out from the equation E=mc^2 where c is the speed of light in free space and m is the mass of the object
When does the equation involving kinetic and potential energy hold
ONLY if the change of height is much smaller than the Earth’s radius
The force of gravity on an object decreases with increased distance from Earth
Equation relating to pendulum bob being released where height above equilibrium position =h0
1/2 mv^2=mg(h0-h)
If a fairground vehicle was initially at rest at the top of the track and its speed is v at the bottom of the track, then at the bottom of the track:
Its kinetic energy=1/2mv^2
Its loss of potential energy=mgh, where h is the vertical distance between the top and the bottom of the track
The work done to overcome friction and air resistance=mgh-1/2mv^2
