Section 4 - Forces and Energy Flashcards
What is a system?
A system is a portion of the universe that has been chosen for studying the changes that take place within it in response to varying conditions. A system may be complex, such as a planet, or relatively simple, as the liquid within a glass
What happens when a system changes?
Energy is transferred - it can be transferred into or away from the system, between different objects in the system or between different types of energy stores
Some energy is dissipated and stored in less useful ways
What is effeciency?
the proportion of the total energy supplied that ends up in useful energy stores
What is a closed system?
A system thats defined so that the net change in energy is zero
How does energy transferred by heating change depending on the system?
- When the system is the pan of water
E.g. A pan of water on a gas camping stove
When the system is the pan of water, energy is transferred into the system by heating to the thermal energy stores of the pan and water, which increases their temperature.
How does energy transferred by heating change depending on the system?
- When the system is the pan of water and the camping stove
E.g. A pan of water on a gas camping stove
When the system is the camping stove and pan, energy is transferred from the chemical energy store of the camping stove to the thermal energy store of the pan and the water, increasing their temp.
What are the energy transfers during lifting a box?
1) A box is lifted off the floor. The box is the system
2) As the box is lifted, work is done against gravity
3) This causes energy to be tranferred to the box’s kinetic and GPE store
How does a toothbrush (the system) transfer energy?
It transfers energy electrically from the chemical energy store of its battery to the kinetic energy store of its bristles
- some energy is transferred out of the system to the surroundings due to sound and heat
How does a hairdryer (the system) transfer energy?
It transfers energy into the system electrically from the main supply to the KE store of the fan inside it.
It then transfers energy electrically to the thermal energy store of the heating element and some energy is transfered away by sound
What is work?
the measure of energy transfer when a force (F) moves an object through a distance (d)
energy transferred (mechanically)
when is work done?
when a force moves an object through a distance, work is done on the object and energy is transferred (mechanically)
What is work mesured in?
J
-> One joule of work is when a force of one newton causes an object to move a distance of one metre
1J = 1Nm (newton metre)
What is the equation for work done?
Work done (J) = Force (N) x Distance (m)
Via friction mechanically, what energy store is the energy transfered to?
Thermal
What is power?
The rate of energy transfer
How much work done per second
What is the unit for power?
W - Watt
1w = 1J/s
What is the formula for power?
Power (w) = Work done (J) / Time taken (s)
P = E / t
What is a normal contact force?
An object at rest on a surface exerts a force on the surface. The reaction force acts at right angles to the surface.
So:
ʌ - normal contact force
I
I
BLOCK
———— Surface
What can interacting magnetic fields cause?
Attraction or repulsion between magnetic objects - the electrostatic force causing attraction and repulsion between electrical charges is due to interactions between their electrical fields
In free body force diagrams where should the arrows be?
Coming out of the object, only representing the forces acting on it
If all the forces on an object are balanced, what is the object in?
Resultant force = 0
Equilibrium
-> If in a diagram, if the forces line up like vectors to put the object in the same place, its resultant
What is the moment?
the turning effect of a force
Me :)
What is the equation for the moment of a force?
Moment of a force (Nm) = force (N) x Distance (m)
(the work done equation)
