0.4 Mechanics Flashcards
What is a scalar?
A quantity that only has amount (magnitude).
What is a vector?
A quantity with both direction and magnitude.
Name 6 scalars.
1) Time
2) Mass
3) Temperature
4) Length
5) Speed
6) Energy
Name 6 vectors.
1) Weight
2) Acceleration
3) Force
4) Displacement
5) Velocity
6) Momentum
What are two different ways to combine vectors?
- Drawing scale diagrams
- Using pythagoras and triganometry
When answering questions about vectors, if the vector given uses directions of a compass, then you should what?
Then your answer must be given as a bearing.
How do you show forces acting on an object?
Drawing a free body diagram.
What does it mean for an object to be in equilibrium?
When all forces acting on the object are balanced and cancel each other out.
- This also includes rotational forces (moments)
- The resultant force will therefore be 0 aswell as acceleration.
What is the official definition of a moment?
The force multiplied by the perpendicular distance from the pivot to the line of action of the force.
What is the equation for a moment?
M = F x d
Moment = force x perpendicular distance from pivot to the line of action of force
What is the Principle of moment?
If a system is in equilibrium,
the sum of the clockwise moments = the sum of the anticlockwise moments
What is a couple?
A pair of forces that are equal in size and act parallel to each other but in opposite direction.
What is the centre of mass?
The single point where all the weight is considered to act through.
What does uniform mean?
The density of the material is the same throughout, resulting in the centre of mass always being in the middle.
What is the definition of displacement?
Distance and direction from the starting point in a straight line.
What is the definition of speed?
How fast something is going regardless of direction.
What is the definition of velocity?
How fast something is going with direction.
What is the definition of acceleration?
The rate of change of velocity.
If a object is not accelerating, what formulae can we use?
Speed = Distance / Time
However if an object is accelerating, what equations can we use?
SUVAT equations
What does the gradient of a displacement time graph represent?
Velocity
What does a curved line represent in a displacement time graph?
Changing velocity / acceleration
What does the gradient of a velocity time graph represent?
Acceleration
What does the area under the line represent in a velocity time graph?
Displacement
What are the two methods to work out the area under the graph?
1) Divide up into triangles and rectangles (for a straight line)
2) Count the squares (use for curved lines)
What does a curved line on a velocity time graph show?
A change of acceleration
What does the area under the line of an acceleration time graph represent?
Change in velocity
What does Newtons 1st Law state?
If there are no resultant forces acting on an object it will either:
1) Stay at rest
2) Travel at a constant speed in a straight line
What does Newtons 2nd Law state?
The acceleration of an object is proportional to the resultant force acting on it: F = m x a
- Where there is an acceleration ther must be a force
- Where there is a force there is an accereleration
What does Newtons 3rd Law state?
If object A exerts a force on object B, then object B exercts an equal but opposite force on object A
What are the rules for Newtons 3rd Law?
1) Two objects involved
2) Same type of force
3) Same size but in opposite directions
What does freefall mean?
When an object is in freeall the only force acting on that object is its weight.
What is air resistance proportional to?
Surface area and speed
Why when there is no acceleration do all objects accelerate at the same rate?
Because acceleration is independent of mass.
How did Galileo record freefall ‘accurately’?
He rolled balls with different masses down a slope.
- Acceleration is still caused by gravity but rolled slower, making it easier to time and for air resistance to be negligible.
- He timed how long it took the balls to roll down different lengths of the slope and found the distance travelled was always the same.
What is a projectile?
An object where the only force acting on it is it’s weight, therefore it is in freefall.
Is there a horizontal force acting on a projectile?
No, as we are assuming that air resistance is negligible.
If there is no horizontal force acting on a projectile, then what happens to the horizontal velocity?
It will stay constant.
How do you calculate the horizontal component (projectiles)?
Horizontal component = horizontal velocity x time in air
Why will the projectile accelerate in the vertical direction?
Because the vertical force of weight is acting on it, and acceleration is typically due to g=9.81m/s^2
If the projectile is fired horizontally then what is the vertical speed be?
Speed will be 0
For every second a projectile is in flight how much velocity will be gained?
9.81 ms^-1
Is the vertical and horizontal components of motion related?
No they are completely independent.
What difference will air resistance will air resistance make to a projectiles path if it is no negligible?
- The horizontal velocity will slowly decrease to 0 as a horizontal force is now acting.
- It will therefore not travel as far horizontally.
- The projectile will gain vertical speed more slowly.
What is the simple definition on for friction?
- It is a force that opposes motion
- It turns kinetic energy into thermal energy
- It can never speed things up or start them moving.
What are the two types of friction?
1) contact friction
2) fluid friction
What is contact friction?
Friction between two solid surfaces.
What is fluid friciton?
Better known as drag, which is caused by a gas or a liquid.
What does drag increase with?
- Larger surface area (of the projectile)
- Larger speed (proportional to speed squared)
- Larger density (viscosity) of fluid
- Nothing to do with mass
What is lift?
Lift is an upwards force on an object moving through a fluid. It happens when the shape of an object causes the fluid flowing over it to change direction.
What direction does lift act?
Perpendicular to the direction of fluid flow (relative to the object)
What direction does drag act?
Drag acts parallel to the direction of the fluid flow.
What is terminal velocity?
The maximum speed of an object that occurs when the resistive and driving forces acting on the object are equal to each other.
What is linear momentum?
The product of an objects mass and velocity.
It is a vector quantitiy.
What is the equation of momentum?
Momenmtum = mass x velocity
What does the principle of (conservation of) linear momnetum state?
The momentum before collision = total momentum after collision, provided no external forces act.
What does inelastic collision mean?
- Momentum conserved
- Kinetic energy NOT conserved
- Most collisions in real life are inelastic
- Energy is lost as heat and sound
What does elastic collisions mean?
- Momentum conserved
- Kinetic energy conserved
What happens in an elastic collision between two objects of the same mass?
