Section 4 - Mechanics Flashcards
Define scalar
Something with only a magnitude
Define vector
Something with a magnitude and a given direction
Give examples of scalars
Mass, temperature, time, length, distance, speed, energy
Give examples of vectors
Displacement, velocity, force, acceleration, momentum
What are the 2 methods of combining vectors?
Scale drawings, Pythagoras and trigonometry
Describe the method of adding vectors together with scale drawings
Draw a scale diagram with a ruler and protractor, then measure the missing side with a ruler and find the missing angle with a protractor
Describe the method of adding vectors together with Pythagoras and trigonometry
Sketch the vectors end to end, draw a line from the tail of the first vector to the tip of the second vector to give the resultant. Can use Pythagoras to get the magnitude, and trig to get the angle
What does the horizontal component equal when an item is travelling at an angle theta to the horizontal?
V cos theta
What does the vertical component equal when an item is travelling at an angle theta to the horizontal?
V sin theta
What do free body force diagrams show?
All forces on a single body
What should a free body force diagram show?
All forces that act on a body, but not the force that it exerts on the rest of the world
What is a moment?
A turning effect of a force
What happens when moments aren’t balanced?
The object will turn
Define a moment
force times the perpendicular distance from the point to the line of action of the force
State the principle of moments
For a body to be in equilibrium, the sum of the clockwise moments about any point equals the sum of anti clockwise moments about the same point
What forces act in a lever?
An effort force acts against a load force by means of a rigid object rotating around a pivot
What is a couple?
A pair of coplanar forces
What are coplanar forces?
Forces of equal magnitude but in opposite directions
What does a couple produce?
No resultant linear force, but does produce a turning effect
Define the moment of a couple?
The size of one of the forces times the perpendicular distance between the lines of action of the forces
Define mass
The amount of matter in it
What does a greater mass lead to in terms of motion?
The greater resistance to a change in velocity
What happens to the mass when the gravitational field changes?
It doesn’t change
What is weight?
A force experienced by a mass due to a gravitational field
The happens to the weight when the gravitational field changes?
It varies
What is the centre of mass?
A point where the mass is thought to be most concentrated
What are the 2 methods of finding the centre of mass?
Symmetry or experiment
How can you find the centre of mass by symmetry?
It is at its centre, and halfway through the thickness
How can you find the centre of mass by experiment?
- Hang the object freely from a point and draw a vertical line downwards from the point of suspension
- Hang the object from a different point and repeat the process
- The COM is where the lines cross
What does the stability of an object depend on?
COM and base area
When will an object topple over?
When the line of action of its weight falls outside its base area
What conditions make an object more stable?
Wider base and low centre of mass
Define speed
How fast something is moving
Define displacement
How far an object’s travelled from its starting point in a given direction
Define velocity
The rate of change of an object’s displacement
Define acceleration
The rate of change of an object’s velocity
What does acceleration look like on a displacement-time graph?
A curved line
What does faster acceleration look like on a displacement-time graph?
A steeper gradient
What does slower acceleration look like on a displacement-time graph?
A shallower gradient
What does deceleration look like on a distance-time graph?
A decreasing gradient
What does the gradient of a displacement-time graph tell you?
Velocity
How can you find the velocity on a curved displacement graph?
Draw a tangent and find the gradient of a tangent
What does the gradient of a velocity-time graph tell you?
Acceleration
What does the area under a velocity-time graph tell you?
Displacement
What does the area under an acceleration-time graph tell you?
Change in velocity
What are the advantages of data loggers?
- More accurate
- Much higher sampling rate than humans
- Can see data in real time
What is free fall?
When there is gravity acting on an object and nothing else
What affects the vertical component motion?
Gravity
Is the horizontal component motion affected by gravity?
No, it just moves at a constant speed
How would you solve a problem if a projectile is launched at an angle?
- Resolve initial velocity into horizontal and vertical components
- Use vertical component to work how long it is in the air/how high it goes
- Use horizontal component to work out how far it goes horizontally
Describe how you would use the horizontal component to work out time when a projectile is launched at an angle
- Halfway through the ball’s flight the v=0
- use v = u + at, and multiply by 2
State Newton’s 1st law
The velocity of an object will now change unless acted upon by a resultant force
State Newton’s 2nd law
Force equals mass times acceleration
Who proved that all objects fall at the same rate?
Galileo
State Newton’s 2rd law
Each force has an equal and opposite reaction force
What is friction?
A force that opposes motion
What are the 2 main types of friction?
Dry friction between solid surfaces
Fluid friction
What is a fluid?
Something that can flow - air or a gas
In fluid friction, what does the force depend on?
The viscosity of the fluid
What is fluid friction also known as?
Drag
What does the magnitude of fluid friction depend on?
The speed and the shape
What are the main 3 facts about fluid friction?
- Act in the opposite direction to the motion
- Can never speed things up/start things moving
- Convert KE into heat and sound
Which direction does lift act in?
Perpendicular to fluid flow
What occurs at terminal velocity in a car?
Driving force equals friction force
Describe the 3 stages of driving from rest up to terminal velocity
- Car accelerates from rest, with resultant and driving force forward
- As speed increases, friction increases which reduces resultant force on car and reduces acceleration
- Car reaches speed where frictional force equals driving force -> No resultant force
Describe the velocity time graph for a parachute
The skydiver accelerates up to its terminal velocity, and then the parachute opens and there is a sharp decrease in velocity and then there is a terminal velocity after the parachute has opened
When is momentum conserved?
When no external forces act on the system
What is an elastic collision?
When momentum and KE are conserved
What is an inelastic collision?
Some of KE is converted into other forms of energy, but momentum is conserved
What is an impulse?
Change in momentum
How can you calculate impulse from a force-time graph?
The area under it
What does reducing the impact time of a collision do to the force?
Increases the force
State some vehicle safety features
Crumple zones - The front and back of the car can crumple, making the car take longer to stop
Seat belts - Stretch slightly, increasing time taken for wearer to stop and spread force around body
Air bags - Slow down passengers more gradually, and preventing them from hitting hard surfaces in the car
When is work done?
When energy is transferred
Why do you normally need a force to move something?
As you have to overcome another force
What is work?
The energy that’s been changed from one form to another
Is the force always in the same direction as the movement?
No
What does the area under a force-displacement graph represent?
Work done
What is power?
Energy transferred per second
State the principal of energy
Energy cannot be created or destroyed. It can be transferred from one form to another but the total amount of energy in a closed system will not change
What is kinetic energy?
The energy of anything moving
What is gravitational potential energy?
The energy you gain of you lift something up
