P2.2 The Kinetic Energy of Objects Flashcards
What is meant by ‘work done’ in physics?
When a forces moves an object through a distance, energy is transferred and work is done
Can you explain ‘work done’ in more detail? (4)
- If something moves, something else is providing some sort of ‘effort’ to move it
- The thing putting in the effort needs a supply of energy (e.g. food, fuel)
- The thing does ‘work’ by moving the object - and transfers the energy it receives as fuel into other forms
- The energy transferred can be useful or wasted, but it is still work done
What is ‘work done’ measured in?
It is equal to energy transferred and is measured in Joules
What is the formula to work out ‘work done’?
W = F x d W = work done (J) F = force (N) d = distance (m)
What is gravitational potential energy?
The energy an object has due to it’s vertical position in a gravitational field
What happens to the GPE when an object is lifted up from the ground?
Work is done against the form of gravity and the object gains GPE
What is formula to work out GPE?
GPE (J) = mass (kg) x gravitational field strength (N/kg) x height (m)
Anything that is moving has kinetic energy, what is the formula to find kinetic energy?
KE = 1/2 x mass (kg) x speed² (m/s)
What does kinetic energy depend on?
The mass and speed of an object
What happens when the brakes are pressed in a car?
When a car brakes its kinetic energy is transferred into heat energy by friction between the brake pads and the wheels, and work is done
Work done by brakes = energy transferred, what does this mean?
1/2 x mass x speed² = braking force x braking distance (or F x d)
What is an objects GPE converted to when it falls?
Kinetic energy
What is the formula to show the transfer of energy when an object is falling? (GPE = KE)
Mass x gravitational field strength x height = 1/2 x mass x max speed²
What other forms of energy are transferred from a falling object?
Sound and heat - heat is transferred due to the friction between the object and the particles in the air
What is power?
The rate of doing work - how much per second
What is meant powerful machine?
One which transfers a lot of energy in a short period of time
What is the formula for power?
Power (watts) = work done or energy transferred (J) ÷ time taken (seconds)
What is one watt equal to?
1 Joule of energy transferred
How can you work out the power output of a person?
You may have to use other equations to find the energy transferred, but it is possible
What is momentum?
A property of moving objects
What is the difference between momentum and speed?
Momentum has a size and direction (unlike speed)
What is the equation for momentum?
Momentum (kg m/s [direction]) = mass (kg) x velocity (m/s)
What is conservation of momentum?
The total momentum before an event (like a collision or explosion) is the same as after the event
What different types of collisions are there?
Objects bumping into each other and sticking or objects bouncing off of each other
What happens when a force acts on an object?
There is a change in momentum
How do traditional brakes work?
The brakes transfer kinetic energy (of the car) into heat and sound energy
How do regenerative brakes work?
In some electric and hybrid cars, instead of coverting the KE into heat energy the brakes put the vehicle’s motor in reverse - which slows the wheels
What is a benefit of regenerative brakes?
The motor converts the kinetic energy into electrical energy that is stored as chemical energy in the motor - thus storing the energy of braking rather than wasting it
What happens when a car crashes?
It slows down very quickly - this means that a lot of KE is converted into other forms of energy in a short amount of time which could be fatal for the people inside
Where are crumple zones on a car found?
At the front and back of the car
How do crumple zones work?
They crumple on impact - crumple zones increase the impact time, decreasing the force produced by the change in momentum
What are side impact bars?
strong metal tubes fitted into car door panels
How do side impact bars help to reduce the force on the passengers?
They direct the KE of the crash towards other areas of the car away from the passengers and towards areas like crumple zones
How do seatbelts help reduce the force on the passengers?
Not only do they stop passengers from hitting hard surfaces in the car or being thrown out the windscreen - but they also stretch slightly increasing the time taken for the wearer to stop, which reduces the force acting on the chest
How do airbags help to reduce the force on the passengers?
They slow you down more gradually and prevent you from hitting hard surfaces in the car
