10.1 - work + energy

what is energy needed to do (to stationary objects)?

• make them move

• change their shape

• warm them up

what is the energy transfer when you lift an object?

you transfer energy from your muscles to the object

what are the energy stores?

• gravitational potential

• kinetic

• thermal

• elastic

• electrostatic

• nuclear

• chemical

• magnetic

what is the energy store for the position of objects in a gravitational field?

gravitational potential store

what is the energy store for moving objects?

kinetic store

what is the energy store for heating / hot objects?

thermal store

what is the energy store for compressed or stressed objects?

elastic store

what are some methods of energy transfer?

• by radiation

• electrical

• mechanical

which energy transfer includes light?

radiation

which energy transfer includes sound?

mechanical

what is a joule (for energy)?

the energy needed to raise a 1 N weight through a vertical height of 1 m

what is a joule (for work done)?

the work done when a force of 1 N moves its point of application by a distance of 1 m in the direction of the force

what is the principle of conservation of energy?

• energy cannot be created or destroyed

• whenever energy is transferred, total energy before = total energy after

what is work done?

• work is done on an object when a force acting on it makes it moves, transferring energy to it

• work done = force x distance moved in the direction of the force

• energy transferred from one energy store to another

what does the amount of work done depend on?

• the force

• the distance moved

how does work done vary with force applied?

the greater the force applied, the more work done

the greater the force applied, the greater / smaller the work done?

the greater the force applied, the greater the work done

how does work done vary with distance moved?

the greater the distance moved, the more work done

the greater the distance moved, the greater / smaller the work done?

the greater the distance moved, the greater the work done

what is the equation for work done?

work done = force x distance moved in the direction of the force

for work done, what is the condition of the distance?

it must be the distance moved in the direction of the force

can we find the work done of an object if it moved in a different direction to the force?

no, only in the same direction

if 1 J of work is done to move an object, how much energy is transferred to the object?

1 J

what happens when energy is transferred from one store to another?

work is done

when is work NOT done?

when the force and distance are perpendicular to each other

what is the work done when putting an object on a shelf?

energy from your muscles → gravitational potential energy of objects on shelf

what is energy store for objects at a height?

gravitational potential energy

what is the energy store for using your muscles?

chemical ?

how do you find the work done when the distance is not acting in the same direction as the force applied?

• if the distance is at an angle, find the horizontal and vertical components using trig

• if the distance is perpendicular to the force, then u can’t do it :P

can you find the work done when the distance is 90 degrees < ? why?

yes, cuz the distance has a horizontal component that you can use to find the work done

what is a force-distance graph?

here

• y = force

• x = distance

• area under graph = work done

what is y on a force-distance graph?

force

what is force on a force-distance graph?

y

what is x on a force-distance graph?

distance

what is distance on a force-distance graph?

x

what is area under the graph on a force-distance graph?

work done

what is work done on a force-distance graph?

area under the graph

what is the force-distance graph for a constant force acting on an object?

here

• linear

• gradient = 0

• non-zero, +ve value for force

• work done can be found by finding area of rectangle

how do you find the work done on a force-time graph where a constant force acts on an object?

here

the graph should give a rectangle, where you can simply find the rectangle’s area for work done

what is the force-distance graph for a variable force acting on an object?

here

• curved line

• non-zero, +ve value for force

• work done for a small amount of distance can be found using strip area = force x change in distance

• total work done can be found by adding area of all strips

how do you find the work done on a force-time graph where a variable force acts on an object?

• work done for a small amount of distance can be found using strip area = force x change in distance

• total work done can be found by adding area of all strips

what is the force-distance graph for stretching a spring?

here

• y = force

• x = extension (distance stretched)

• area under graph = work done = ½ F DELTA L

• straight line through the origin

what is the work done to stretch a spring?

W = ½ F DELTA L

• W = work done

• F = force needed to stretch spring

• DELTA L = amount extended

why is the work done to stretch a spring ½ F DELTA L?

do the graph later idc rn

why is a force-extension graph for stretching a spring a straight line through the origin?

because the force needed is proportional to the extension of the spring (hooke’s law)

what does hooke’s law state?

the force needed is proportional to the extension of the spring

how do we prove hooke’s law?

the force-distance graph for stretching a spring is a straight line through the origin, proving force needed is proportional to extension