Section 4 - Forces and Energy Flashcards
What is a system?
A single or group of objects
What happenes when a system changes?
Energ is transferred
What is a closed system?
A system with no net change in energy
How can energy be transferred?
Heat
Force
Electricity
Describe the energy transfers that occur when the wind causes a windmall to spin?
The wind does work on the windmill causing it to turn and transferring energy to the kinetic energy store of the windmill
What is the work done equation?
E = F x d Energy = Force x Distance
What does 1 J equal?
1 Nm
What does a force doing work often result in?
A rise in temperature
What is power?
How much work is done per second (rate of energy transfer)
What is the equation for power?
P = E / t Power = Energy / Time
If an electric heater has a power of 600W and it is left on for 3 seconds, how much energy is transferred?
P = E / t E = P x t E = 600 x 3 E = 1800 J
A motor does 4.8 kJ of work in 2 minutes
Find its power output?
4.8 kJ = 4800 J
P = E / t P = 4800 / 120 P = 40 W
A constant force of 20 N pushes an object 20 cm
Calculate the work done
E = F x d E = 20 x 0.2 E = 4 J
An appliance transfers 6000 J of energy in 30 seconds
Calculate its power
P = E / t P = 6000 / 30 P = 200 W
What do interactions between objects cause?
Forces
What are interaction pairs?
A pair of forces that are equal but opposite
What does a free body force diagram show?
An object and all the forces acting on it
What should free body force diagrams include?
Every force acting on the body
What shouldn’t free body force diagrams include?
The forces the object exerts on the rest of the world
What is the resultant force of an object in equilibrium?
Zero
What happens to objects that have no resultant force?
They are either stationary or moving at a steady speed
How do you draw a scale drawing
Set a scale
Draw the forces acting on an object from tip to tail (one at a time)
Then draw a line from the start to the end
Measure the length and the angle
How do you resolve a component force?
Draw it on a scale grid and measure the horizontal and vertical vectors
What must a bearing always be presented as?
A three digit number
What is a moment?
The turning effect of a force
How do you calculate the moment?
M = F x d Moment = Force x Distance
In terms of moment, why is pushing at any other angle bad?
It means a small distance which means a smaller moment
What is the principle of moments/
For an object in equilibrium
The sum of the clockwise moments = The sum of anticlockwise moments
What do levers do?
Transfer the turning effect of a force
How do levers make it easier to do work?
They increase the distance from the pivot at which a force is applied, therefore the smaller the force to give the same moment
What are gears used to do
Transmit the rotational effect of a force from one place to another
Describe the moment of a small gear with a force applied to it?
A small moment
How do gears increase the moment?
If the next gear is larger, the force is applied further from its pivot, so the moment is larger
How does the size of the gear affect its speed?
The larger the gear, the slower it spins
How do you work out how the speeds and moments will change between gears ?
By looking at the gear ratios
The largest gear has 16 teeth and the medium gear has 8
What is the ratio?
16 : 8
4 : 2
2 : 1
If a larger gear and a smaller gear have a ratio of 2 : 1 what does this mean?
For every rotation of the larger gear the smaller gear has rotated twice
A 10N force is applied normal to a door, 85 cm from its hinges
Calculate the moment created
85 cm = 0.85 m
M = F x d M = 10 x 0.85 M = 8.5 Nm
What is the unit for moment?
Nm
What is the ratio of moments of two gears equal to?
The ratio of the gears radii
If a gear has 5N applied to it, what is the force of a second gear that is directly connected it?
5N (the same)
Describe how to convert between J and Nm?
Divide by 1 (do nothing)
What force causes the repulsion of two like electrical charges?
Electrostatic forces of repulsion
A parachuter has a weight of 900 N
At one point during his fall, the air resistance acting on him is 500 N
In what direction is his resultant force at this point?
Down at 400 N (900 - 500 = 400)
If a gear is spinning clockwise, what direction will a second gear directly connected to it spin?
Anti-clockwise (the opposite direction)
How can you improve the efficiency of a machine that uses gears?
Lubrication, reduce friction which will reduce energy loss through heat
