TOPIC 4- Forces And Energy Flashcards
Efficiency of an energy transfer
The proportion of total energy supplied that ends up in useful energy stores.
(Useful energy/total energy)
A closed system
A system with a net change in energy of zero.
How is energy transferred by heating?
Energy is transferred to the thermal energy stores of the object, increasing its temperature.
What happens when a force moves an object through a distance?
Work is done on the object and so energy is transferred mechanically from one store to another.
Work done (J) formula
Force (N) * distance moved in direction of force (M)
One joule of work
Force of one newton causes an object to move a distance of one meter.
1J =1Nm
Why does work done often lead to a rise in temperature?
Energy is dissipated to the thermal energy stores of the moving object and its surroundings.
Efficiency formula
Useful energy transferred by device / total energy supplied to device.
Power definition (Unit?j
Rate of energy transfer (how much work is done per second).
Measured in watts
Power formula
Work done (J) / time taken (s)
P=E/T
Force
A vector (push/pull) that is caused by it interacting with something.
Contact forces
Objects that need to be touching for a force to act.
Non-contact forces
Forces that can act between objects that aren’t touching.
Usually caused by interacting fields.
What happens to interacting magnetic fields?
Cause attraction or repulsion between magnetic objects and electrostatic force between charges is caused by the fields interacting.
Newton’s third law
Whenever 2 objects interact, they both exert an equal but opposite force called an interaction pair.
Free body force diagram
An isolated body and all forces acting on it.
Includes every force acting on the body but none it exerts. Sizes of arrows show relative magnitudes of forces and their directions.
Resultant force
The overall force on a point/object.
Resultant force of objects in equilibrium
Zero
How to use scale drawings to find resultant forces
- ) draw a scale drawing of the forces acting (w a sensible scale) w arrows.
- )draw net force from point of each end of forces and measure the length to scale to find the force in N.
- ) use a protractor to measure the direction as a bearing.
How do you split a force into components?
Split the angle into two components perpendicular to each other, acting together will have the same effect as the single force.
Moment
The turning effect of a force.
Moment of a force formula
Force (N) * distance (m)
Nm = N*m
How do you achieve the maximum moment?
Push at right angles to the spanner, meaning a larger distance. (Normal to direction of the force)
Sum of clockwise moments =
The sum of anti-clockwise moments
Levers
Transfer the turning effect of a force so that it is easier to do work (increases distance from pivot at which force is applied)
How does lever length affect force needed to give a moment?
Longer the lever, smaller the force needed to give the same moment.
Gears
Circular cogs w ‘teeth’ around edge which interlock to cause others to turn upon turning (in opposite direction).
What are gears used for?
To transmit the rotational effect of a force from one place to another.
How does gear size affect the moment?
Speed of spin?
Larger gears give larger moments as the distance from the pivot is further.
Larger gear = slower spin
