Mechanics Flashcards
2.1.1 Define displacement, velocity, speed and acceleration
Displacement: The vector quantity from a given fixed point to the position of another point.
Velocity: The ratio of the change of displacement to the time taken. - A vector.
Speed: The ratio of the distance covered to the time taken to cover the distance. - A scalar.
Acceleration: Change of velocity over time. Instantanous = the rate of change with time of the velocity. - A vector.
2.1.2 Explain the difference between instantaneous and average values of speed, velocity and acceleration.
Average speed: Ratio of TOTAL distane travelled to the TOTAL time taken.
Instantaneous: Ratio of distance travelled in an instant.
Average velocity: Ratio of change of displacement to the time taken.
Instantaneous: Velocity in one particular instant.
Average Acceleration: Ratio of change of velocity to the time taken.
Instantaneous: Acceleration in an instant.The rate of change with time of the velocity vector.
2.1.3 Outline the conditions under
which the equations for uniformly
accelerated motion may be applied.
Motion on a straght line and constant acceleration
Equations: v = u + at s = ((u+v)/2)t v^2 = u^2 + 2as s = ut + 1/2at^2 s = vt - 1/2at^2
2.1.4 Identify the acceleration of a body
falling in a vacuum near the Earth’s
surface with the acceleration g of free
fall.
9.81 ms^-2 (ignoring air-resistance)
2.1.5 Solve problems involving the equations of uniformly accelerated motion
GO DO EXAM PROBLEMS OR SOMETHING
2.1.6 Describe the effects of air resistance
on falling objects.
The air resistance is directly proportional to the velocity of the object. Terminal velocity is when the air resistance is equal to the actual acceleration caused by gravity, causing the object to have no net acceleration. The shape and the mass of an object also affects the drag forced.
You are often asked to draw two projectiles where one is affected by air resistance and the other is not. When it is a vertical projectile, the object with air resistance will simply not go up as high. If it is horizontal, it object with air resistance will land at a point closer to the original object. If it is launched at an angle, then you combine both of the previous ones to determine the projectile of the object with air resistance, i.e. it will not go up as high, and will not travel as far.
2.1.7 Draw and analyse distance–time
graphs, displacement–time
graphs, velocity–time graphs and
acceleration–time graphs.
When drawing these graphs, simply plot the data given to you. Time always goes on the x-axis and the other goes on the y-axis.
2.1.8 Calculate and interpret the gradients of displacement–time graphs and velocity–time graphs, and the areas under velocity–time graphs and acceleration–time graphs.
-Slope of displacement - time graphs gives the velocity
- Slope of velocity - time graphs gives the acceleration
- Area under velocity - time graph gives the change in displacement
- Area under acceleration - time graphs gives the change in velocity
NOW DO EXAM QUESTIONS ON THIS
2.1.9 Determine relative velocity in one and
in two dimensions.
All velocity is relative to your frame of refrence. If you’re walking at the speed va = 1 km/h and a car drives past you at vb= 30 km/h, the velocity of the car with respect to you is:
v = vb - va
v = (30 - 1) km/h = 29km/h
The relative velocity is the net velocity. Recall that velocity is the speed in a given direction. Therefore, in one dimension you have to determine in which direction the object is travelling and you must use the ratio between the displacement and time and not the ratio between speed and time. In two dimensions, you have to know the angle between the direction the object is travelling in, and the axis of the direction you are required to calculate the velocity in. You then multiply the speed of the object with the cosine of the angle to determine the velocity.
2.2.1 Calculate the weight of a body using
the expression W = mg.
W = mg You can do that, believe me you can
2.2.2 Identify the forces acting on an
object and draw free-body diagrams
representing the forces acting.
Each force should be labelled by name or given
a commonly accepted symbol. Vectors should
have lengths approximately proportional to their
magnitudes. See sub-topic 1.3. Do some tasks on this.
2.2.3 Determine the resultant force in
different situations.
Net force is the single force whose effect is the same as the combined effect of all individual forces on the body. This is found by vector addition. The resultant of two forces can be maximum the two added (they are both working in the same direction), and minimum the two subtracted from each other (they work in opposite directions)
2.2.4 State Newton’s first law of motion.
When the resultant force on a body is zero, its velocity is constant ie: it will either stand still or move with constant velocity.
2.2.5 Describe examples of Newton’s first
law.
A book lying on table. Anything with constant velocity really.
2.2.6 State the condition for translational
equilibrium.
Equilibrium: the situation when the sum of forces on a body is zero. If an object is at rest, then it is in static equilibrium and, if it is moving with constant velocity, then it is in dynamic equilibrium
2.2.7 Solve problems involving translational
equilibrium
Well go find some problems to solve, my love.The ones with threads and bodys hanging in angles and stuff are cool.
2.2.8 State Newton’s second law of motion.
F=ma. The net force acting on an object (of constant mass) is the product of the objects’ mass and the net acceleration of the object. It can also be written as Delta p/ delta t Figure out why
2.2.9 Solve problems involving Newton’s
second law.
F = ma
How to solve a mechanics problem:
- Draw a free-body fiagram showing all forces acting on each body
- Find net force for each body
- Use Newton’s second law separately on each body, or treat all the bodies as one if convenient.
NB You should check out those two bodies connected with a string problems
2.2.10 Define linear momentum and impulse
Momentum: p = mv HERE’S THE EXPLANATION FOR THE Delta p mentioned above!!!
The product of the mass and velocity of a body, a vector with the same direction a s the velocity.
Impulse: The average force on a body multiplied with the time for which the force was acting. It is the area of a graph of F against t.
2.2.11 Determine the impulse due to a
time-varying force by interpreting a
force–time graph.
Impulse: The average force on a body multiplied with the time for which the force was acting. It is the area of a graph of force against time.
2.2.12 State the law of conservation of linear
momentum.
If the total external force acting on a system is zero then the momentum of the system remains constant.
2.2.13 Solve problems involving momentum
and impulse.
You always know that the total momentum before and after an event is the same if there is no external force acting on the system. By using this you can create an expression to determine certain unknown values.
2.2.14 State Newton’s third law of motion.
If body A exerts a force on body B, then B will exert an equal and opposite force on A.
2.2.15 Discuss examples of Newton’s third
law.
Check out the inclined plane problems (think you might get some of those).
