Mechanics Flashcards
Give examples of scalar and vector quantities
Scalar: mass, temperature, time, length
Vectors: force, displacement, acceleration, momentum
How to calculate average speed?
Total distance / total time
What is relative speed?
The comparison of speed between two objects
If your car goes left at 30mph and a car goes left at 40mph, what is the speed relative to your car?
10mph left
If your car goes right at 30mph and a car goes left at 40mph, what is the speed relative to your car?
70mph left
If a plane flies right at 120mph and wind goes left at 10mph, what is the resultant velocity?
110mph right
How to work out the velocity if the forces are on a right angle
Draw it on a graph, create a triangle then use pythag to find the hypotenuse (as you connect vectors end to end)
For the direction, do trig
What is displacement?
The distance from the start to the finish, in a straight line
What is a projectile?
Something that is projected or dropped and only has gravity acting on it (ignoring air resistance)
What is a trajectory?
The path a projectile takes through the air
It is always a parabola (a rainbow shape)
How do you calculate a projectiles trajectory?
Calculate the horizontal bit (which is a constant velocity as gravity doesn’t act on it)
Then the vertical bit (which will increase as gravity accelerates it)
Then add them together
What angle should you push on an object to get maximum moment?
Right angle, as any other angle means a smaller moment because the perpendicular distance between the line of action and pivot is smaller
If an object is suspended in air, where would the centre of mass be?
Directly below the suspension
If it swings it will continue swinging until it reaches below the suspension again
Why does the centre of mass rest below the suspension of a suspended object?
Because there is no moment, as the pivot is the suspension and is in line with the line of action
How do you find the centre of mass for any flat shape via suspension?
Hang it once and see where it stops moving
Then hang it in a different position and wait for it to stop moving
The place where the two lines both go through is the centre of mass
How to find the centre of mass for simple 2d shapes
Via lines of symmetry
What happens if the anti-clockwise moment equals the clockwise moment?
The object won’t turn as it is balanced
What happens if the anti-clockwise moment is not equal to the clockwise moment?
There is a resultant moment and the object turns
What makes a stable object? (moment)
A wide base and low centre of mass
When will an object tip over due to moments?
If it’s centre of mass moves beyond the edge of its base
What is a centripetal force?
The force that keeps something moving In a circle
What are the three forces that can cause a centripetal force?
Friction, tension and gravity
What is the centripetal force of a car going round a bend?
Friction, as the car is drawn towards the middle of the bend due to the friction of the tires
What three things can change the size of a centripetal force?
Mass - the heavier an object is the greater the force to keep it in a circle
Speed - the faster an object is the greater the force needed to keep it
Radius - the smaller the circle, the more turns the object has to do so a greater force is needed
What are geosynchronous satellites?
Satellites that are put in high orbit and rotate with the earth, therefore staying in the same place above the earth
These are normally communications as they’re always in the same place so tv and phones can find it
What are low-polar orbit satellites?
Satellites that are closer to the earth and spin round both poles as the earth rotates the other way beneath them
They go much faster as gravity is stronger and can take pictures of the earth as they’re low
Used for weather and spying
How does ‘orbit’ work?
Orbit is the balance of the forward motion of the object (velocity) and the force (gravity) pulling it into the circle
This subsequently makes it go round in circles
Why do planets further away from the sun move slower?
Gravity makes things move faster as the pull is stronger so there’s a stronger force of attraction
If you move away from the sun the gravity is weaker so you move slower
Why does a comet speed up when it orbits close to the sun?
Because as it gets closer, it speeds up due to gravity
Do transformers work on DC?
No, only AC as they use electromagnetic induction
What are the three types of transformers?
Step up
Step down
Isolating
What do step up transformers look like?
They have more turns on the secondary coil than the primary coil
What do step down transformers look like?
They have more turns on the primary coil than the secondary
How do transformers work?
The primary coil produced a magnetic field which stays within the iron core
This means nearly all of it passes through the secondary coil (hardly any is lost)
There is AC in the primary coil the field in the iron core is constantly changing direction
This rapidly changing field is then felt by the secondary coil
This induced an alternating voltage in the secondary coil
The number of turns on the two coils determine whether the voltage is greater or less than the voltage in the primary
What if you supplied DC to the primary coil?
You’d get nothing out of the secondary, as although there’s still a field it is not alternating
What is the iron core in transformers used for?
Transferring the changing magnetic field
No current flows through it
Are transformers efficient?
They’re nearly 100% efficient
