Topic 2, Mechanics Flashcards
Differentiate betw distance and displacement
Define/explain ‘position’
Displacement is a vector quantity (vector from initial to final position)
Distance is dependent on the path taken (length of path travelled)
point in space // vector from origin to the point in space
Differentiate betw speed and velocity
Velocity is a vector quantity
Speed has no direction (only magnitude)
What is instantaneous velocity in terms of instantaneous speed (the relationship between these)
Inst. v = Inst speed + the direction
(e.g. 20m/s east)
Define acceleration
The rate of change of velocity. (vector quantity)
Define velocity
The rate of change of displacement. (vestor quantity)
Units acceleration
Speed units / Time units
e.g. (km/h)/s or (m/s)/s = m/s^2
Does a negative acceleration mean the object is deaccelerating/slowing down?
No, not necessarily!
If no directions are given, neg implies slowing down.
BUT if directions are given it could simply be accelerating in the “opp” (neg) direction.
What happens to an object and its acceleration when the line on a v-t graph crosses the x (the t) axis?
It turns around! If from pos to neg: slows down, truns around (v=0) and speeds up
(on axis y=0 hence v=0 and thus a will = 0
Does a v-t graph tell you about the position of an object?
No! Change in positon yes but not actual position
How do you know if an object is moving left or right from a v-t graph?
Point above t-axis (pos v) = moving RIGHT
Point below t-axis (v neg) = moving to the LEFT
How do you know if an object is slower/faster than another point from a v-t graph?
The closer to the x-axis = the slower it is (lower v), further away=faster. REGARDLESS OF POS/NEG!!
If it goes towards the x axis, it is slowing down
How do you know if the acceleration is to the left or right from a v-t graph?
The sign of the slope (pos/neg)!
Neg slope = neg a = LEFT
Pos slope=pos a=RIGHT
Tell how fast/slow it is going from v-t graph =
= d from t-axis; closer=slower
Find average velocity from x-t graph
delta x / delta t
slope of the secant betw 2 points
(take average betw final and initial point)
Find instantaneous velocity from x-t graph
= slope of the tangent line at the point
(draw tangent line, find 2 points (if grid, where lines meet) and find gradient = inst v)
What do you use a tangent line for?
To calculate ANY RATE!
e.g. v-t graph — inst a = tangent line
How do you draw a tangent line?
Passes through the point
Does not pass through other points (further away) gen.
Goes along the line for a while, same each side
Do objects of different masses dropped from the same height reach the ground at the same time (i.e. does a=g)?
Only if NO AIR RESISTANCE (in vaccuum)
What is an objects weight dependent on (aside from its mass)?
Its LOCATION (as g differs eg moon, everest, surface earth)
mass is not affected
Differentiate betw mass and weight
Weight = m * g
(is dep on location)
Mass only dep matter (not position)
m s^-2 accel due to gravity
N/kg gravitational field strength
How do you find the d at point from v-t graph
What do you get by taking area under the curve (of a certain time interval) of a v-t graph?
The CHANGE of displacement for that time interval = the FINAL “position”, how much distance moved in that time
What is a force?
Pushes/pulls, cause changes in motion
What happens to the motion if the force F and velocity v are
1. In the same direction
2. In opposite direction
3. Perpendicular
- Speeds up
- Slows down
- Turns
Differentiate between contact and field forces
Direct contact on the object vs no contact (eg gravity)
Divide the following forces into contact and field forces.
1. Tension
2. Electrical
3. Gravity
4. Push
5. Normal force
6. Magnetic
7. Frictional
2,3,6 = field, the rest contact
How do you draw a FreeBody Diagram? Give the steps
Draw with scale, SHOW LENGTHS; if same size, show that!
(0.5 draw the object, simplified, assume point … F acting from centre of object (centre of mass))
1. Draw the weight (mg) STRAIGHT DOWN (always, no matter if at angle)
2. Search for points of contact (is it on a surface? = normal force, do you have friction? Is there a string attached? = tension force)
3. STOP!! Don’t do anything else!! ONLY FORCES ON FBD, no motion/velocity/acceleration or anything like that
What do you use a FBD for?
To find the resultant force, Fres
- Define Newton’s 1st law of motion (law of inertia)
- Explain what this means in terms of resultant force
- “An object will remain at rest, or continue to move in a straight line at a constant speed, unless a resultant force acts on it.”
- If Fnet=0, the velocity is constant (or 0) OR if you know the object moves w constant/zero v (no accélération), thé Fres=0.
Define Newton’s 2nd law of motion (+ kinda explain, eq)
“The acceleration of an object is directly proportional to the net (external) force applied, and is indirectly proportional to its mass.”
a=F/m (a prop to F, inversely prop to m, and in the same direction as F)
(Thus also F=ma or F (prop to) ma)
Define Newton’s 3rd law of motion
“Whenever one body exerts a force on another body, the second body exerts exactly the same force on the first body, but in the opposite direction.”
Chris Doner also: action/reaction, 1 interaction 2 forces. Forces equal but opposite; acting on 2 diff bodies, force pairs.
Define work!! (Key definition!!)
Work is the transfer of energy.
Define power (key definition!!)
The rate of transfer//the change in energy. deltaE/deltat
