P5 Flashcards
Vector quantity
- has magnitude and a direction.
- force, velocity, displacement, acceleration, momentum, etc.
Scalar quantity
- only have magnitude.
- speed, distance, mass, temperature, time, etc.
Vectors
- Usually represented by an arrow: length of the arrow shows the magnitude.
- Direction shows the direction of the quantity.
Contact Forces
When two objects have to be touching for a force to act.
Eg. Friction, air resistance, tension in ropes, normal contact force etc.
Non Contact Forces
The objects don’t need to be touching for the force to act.
Eg. Magnetic force, gravitational force, electrostatic force.
Force
A push or pull on an object that is caused by it interacting with something.
An interaction pair
A pair of forces that are equal and opposite and act on two interacting objects.
Sun and Earth example.
The sun and earth are attracted to each other by the gravitational force.
This is non contact force.
An equal but opposite force of attraction is felt by both the sun and the earth.
Chair example
A chair exerts a force on the ground, whilst the ground pushes back at the chair with the same force (normal contact force).
Equal but opposite forces are felt by both the chair and the ground.
Gravitational Force
Gravity attracts all masses - it’s only noticeable when on of the masses is really big (eg a planet).
Anything near a planet or star is attracted to it very strongly.
2 important effects:
. On the surface of a planet, it makes all things fall towards the ground.
. It gives everything a weight.
Distance
- how far an object has moved.
- scalar quantity, doesn’t involve direction.
Displacement
- vector quantity.
- measures the distance and direction in a straight line from an objects starting point to its finishing point.
- the direction could be to do with a point, eg, towards school, or a bearing (a three digit angle from north, eg 035 degrees)
Spring practical 1
1) measure natural length of spring (when nothing is applied) with a ruler clamped to the stand. Take the reading at eye level and add a marker to the bottom of spring to make the reading more accurate.
2) add a mass to the spring and allow it to come to rest. Record mass and measure new length of spring. The extension is the change in length.
Spring practical 2
3) repeat process until you have enough measurement.
4) plot a force extension graph of your results. It’ll only start to curve if you exceed the limit of proportionality.
Straight line of best fit=linear relationship.
When line bends=non linear
Applying a force…
- May cause object to stretch,compress of bend
- To do so, you need more than one force acting on the object.
- Otherwise the object would simply move in the direction of the applied force instead of changing shape.
Elastic Objects
Objects that can be elastically deformed.
An object has been elastically deformed if it can go back to its original shape and length after the force is removed.
An object has been inelastically deformed if it doesn’t return to its original shape+length after the force is removed
Work is done when
A force stretches/compresses an object and causes energy to be transferred to the EP energy store of object.
If elastically deformed, ALL this energy is transferred to the objects EP energy store.
Extension is directly proportional to force
Spring supported at top with a weight at the bottom STRETCHES.
- Extension of stretched spring/ is directly proportional to the load/force applied.
- Spring constant depends on material stretching - stiffer spring, greater spring constant.
Force équation
Force=spring constant x extension.
Also works for compression, instead of extension, it’s the difference between the natural and compressed lengths.
Scale drawings
- Draw all forces acting on an object, to scale, tip to tale.
- Draw straight line from start of first force to end of last force - this is the resultant force.
- Measure length of resultant force on diagram to find magnitude and angle to find direction of force.
Scale drawings - equilibrium
If all forces acting on an object combine to give a resultant force of 0, the object is in equilibrium.
On scale diagram: means that tip of last force drawn should end where the tail of the first force drawn begins.
Splitting a force into components
Some forces act at awkward angles.
To deal with them, they’re split into 2 components at right angles to each other.
Acting together they have the same effect as the single force.
Resolving a force
- Draw it on a scale grid
- Add the horizontal and vertical components along the grid lines.
- Measure them
Mass
Amount of stuff in object.
Same value anywhere.
Eg object has same mass whether it’s on earth or mars.
Mass isn’t a force.
Measure in kg.
Mass and weight are directly proportional - increasing one increases the other.
Weight
Force acting on an object due to gravity.
Near earth this is caused by gravitational field around earth.
Gravitational field strength varies.
Weight of object is different depending on where it is.
Measured in Newton’s.
Measure using a calibrated spring balance.
Weight equation
Weight=mass x gravitational field strength.
Free body diagrams
Described all forces acting on an isolated object or system.
I.e. every force acting on object or system but none of the forces the object or system exert on the rest of world.
Size of arrow shows relative magnitude of forces, direction shows direction of forces acting on the object.
Résultant force
Most cases, there are at least 2 forced acting on an object along any direction.
If you have many forces acting at single point, you can replace with a single force - this is resultant force.
If forces are parallel, the overall effect is found by adding those going in same direction and subtracting any going in opposite direction.
Résultant force - work done
To make something move, force must be applied.
The thing applying the force needs fuel or food.
Force does work to move object and energy is transferred from one store to another.
‘Work done’ and ‘energy transferred’ are the same.
Work done equation
Work done=force x distance
When a force moves an object through a distance, ENERGY IS TRANSFERRED and WORK IS DONE on an object.
1 joule of work is done when a force of 1 newton causes an object to move 1 metre.
Elastic equation
1/2 x spring constant x extension squared
Momentum
= mass x velocity.
- greater mass/greater velocity=more momentum
- vector quantity
- in a closed system, total momentum before event = total momentum after:
. This is called conservation of momentum.
Reaction time practical
- Sit with arm on table.
- Other person drops ruler without warning.
- Catch ruler as quick as possible.
- Measurement on ruler = how far it dropped in time it took you to react.
- Longer distance, longer reaction time.
- Repeat experiment, calculate mean.
Thinking distance
How far car travels during drivers reaction time.
Affected by:
. Your speed.
. Your reaction time.
Braking distance
Distance taken to stop under the braking force. Affected by: . Your speed. . Weather/road surface. . Tyre condition. . How good your brakes are.
Stopping distance
= thinking distance + braking distance.
This is the emergency stop.
Maximum force is applied by brakes in order to stop car in shortest possible distance.
Speed limits affect speed and stopping distance.
Friction - car brakes
.Braking causes friction.
.The work done between brakes and wheels transfers energy from KE stores of wheels to TE stores of brakes.
.Brakes increase temp.
.Faster vehicles = more energy in KE, more work needs to be done to stop it.
.So greater braking force is needed.
Inertia
. The tendency for an object to continue in the same state of motion - steady speed.
. An object’s inertial speed measures how difficult it is to change velocity of object. Can be found using:
Mass = force/ acceleration.
Newton’s third law
When 2 objects interact, the forces they exert on each other are equal and opposite.
E.g. push a trolley, it pushes back just as hard.
Newton’s first law
- If the resultant force on a stationary object is 0, the object will remain stationary. If the resultant force on a moving object is 0, it’ll carry on moving at the same velocity.
- A non 0 resultant force produces acceleration in direction of force.
- This acceleration can be in changing direction.
Newton’s second law
Resultant force = mass x acceleration.
- larger resultant force = more acceleration: force and acceleration are directly proportional.
- acceleration is also inversely proportional to mass of object. Larger mass accelerates less.
Friction
- always acts in opposite direction to movement.
- to travel at steady speed, driving force needs to balance frictional forces.
- you get friction between 2 surfaces in contact/when an object passes through a fluid.
- reduce it by using a lubricant.
Drag
- the resistance you get in a fluid (gas or liquid)
Eg air resistance. - most important factor in reducing it is keeping object shape streamlined.
. This lets the fluid flow easily across object, reducing drag. - frictional forces from fluids increase speed, always!!!!
Terminal velocity
- at first, gravity is much more than frictional force slowing so it accelerates.
- as speed increases friction builds up.
- this reduced acceleration until frictional forces = accelerating force.
- it’s reached terminal velocity.
- less streamlined = lower t.v.
- large surface area = lower t.v.
Distance time graphs
- gradient = speed
- flat section = stationary
- straight uphill = steady speed
- curves = acceleration/deceleration
- steepening curve = speeding up
- levelling off curve = slowing down
- find gradient of tangent to find speed at certain point.
Velocity time graphs
- gradient = acceleration
- uphill = acceleration
- downhill = deceleration
- curve means changing acceleration
- area under any section of graph = distance travelled in that time interval.
- is section under graph is irregular then find area by counting squares under line and multiple no. by value of 1 square
Acceleration
= change in velocity/time
- deceleration is just negative acceleration
- constant acceleration sometimes called uniform acceleration.
- acceleration due to gravity is uniform for all in free fall. = 9.8 roughly
- equation for uniform acceleration:
Final velocity2 - initial velocity2 = 2 x acceleration x distance
Typical speeds - for estimations
- person walking =1.5m/s
- person running = 3m/s
- person cycling = 6m/s
- car = 25m/s
- train = 30m/s
- plane = 250m/s
