2 Kinematics Flashcards
Not revised here: Pg 15 - 17
Define distance. Is it a scalar or a vector quantity?
Distance represents the total length of the path an object travels. It is a scalar quantity.
Define displacement. Is it a scalar or a vector quantity?
Displacement is the distance moved in a specified direction from a reference point. It is a vector quantity.
Define speed. Is it a scalar or a vector quantity? How can average speed be found?
The instantaneous speed of an object is the rate of change of distance travelled with respect to time.
(v = dx/dt in ms^-1) It is a scalar quantity. Average speed equals the total distance travelled by an object/total time taken.
Define velocity. Is it a scalar or a vector quantity? How can average velocity be found?
The instantaneous velocity of an object is the rate of change of displacement with respect to time.
(v = ds/dt in ms^-1) It is a vector quantity. Average velocity equals the change in displacement/total time taken.
Define acceleration. Is it a scalar or a vector quantity? How can average acceleration be found?
The instantaneous acceleration of an object is the rate of change of velocity with respect to time.
(a = dv/dt in ms^-2) It is a vector quantity. Average acceleration equals the change in velocity/total time taken.
Which two factors affect the sign of acceleration? When is acceleration positive? When is acceleration negative?
The sign of acceleration depends on whether the object is:
1) speeding up or slowing down? (accelerating or decelerating)
2) moving in the positive or negative direction? (displacement)
Acceleration is positive when the object is accelerating in the direction of positive displacement or decelerating in the direction of negative displacement. Acceleration is negative when the object is accelerating in the direction of negative displacement or decelerating in the direction of positive displacement.
Using velocity and acceleration, how do we decide if an object is slowing up or speeding up?
When velocity and acceleration are in the same direction, the object is speeding up. When velocity and acceleration are in opposite directions, the object is slowing down.
Sketch the (i) displacement-time, (ii) velocity-time and (iii) acceleration time graphs of an object travelling at increasing speed and constant acceleration. Give an example of when this occurs.
Releasing a ball at rest at a height
Graph on pg 6 of notes
Sketch the (i) displacement-time, (ii) velocity-time and (iii) acceleration time graphs of an object travelling at decreasing speed and constant acceleration. Give an example of when this occurs.
Throwing a ball upwards
What is the significance of the gradients of the (i) displacement-time (s-t), (ii) velocity-time (v-t) and (iii) acceleration-time (a-t) graphs of an object?
(i) Instantaneous velocity
(ii) Instantaneous acceleration
(iii) No significance
What is the significance of the area under the graph of the (i) displacement-time (s-t), (ii) velocity-time (v-t) and (iii) acceleration-time (a-t) graphs of an object?
(i) No significance
(ii) Change in displacement
(iii) Change in velocity
What criteria must be fulfilled before the equations of motion can be used?
One dimensional motion
Constant acceleration (same magnitude and direction)
State the four equations of motions
v = u+at
s = 1/2(u+v)t
s = ut+1/2at^2
v^2 = u^2+2as
Note: When (iv) is multiplied by m/2, it gives the equation that the change in K.E numerically equals to the work done by a constant F.
What is free fall?
When an object is falling in the absence of air resistance, it is undergoing free fall.
What does an object experience when it is being thrown upwards or downwards or released from rest in the absence of air resistance?
The object experience a constant acceleration directed downwards (towards the centre of the earth) with a magnitude of g = 9.81ms^-1 when it is being thrown upwards or downwards or released from rest. This is known as the acceleration due to free fall and it is assumed to be a constant value near the Earth’s surface.
What happens to an object when it moves through a fluid (presence of air resistance)?
When an object moves through a fluid (liquid/gas), it experiences a drag force/air resistance which acts opposite in direction to velocity.
Under non-turbulent conditions, how is the drag force related to velocity?
The drag force is proportional to the instantaneous velocity. (FD = kv, where k is a constant)
At higher velocities, how is the drag force related to velocity?
When turbulence sets in, the drag force is proportional to the square of the velocity. (Fd = k’v^2, where k’ is a constant)
Describe the motion of an object released from rest in a uniform gravitational field where there is air resistance.
At the point of release, velocity is 0 and the only force acting on the object at this instant is the gravitational force. The object falls with an acceleration of g. As the body gains velocity, air resistance which is dependent on velocity starts to increase. The resultant force on the body = mg-Fd = ma. The body is still accelerating but acceleration is now less than g (a = g-Fd/m). Eventually, drag force increases to a point where is it equal in magnitude to the weight of the body and resultant force equals 0, acceleration of the body equals 0 and the body attains a constant velocity which is called the terminal velocity.
Why is air resistance negligible for heavier objects?
a = g-Fd/m where g almost equals a.
amount of air resistance an object experience depends on its speed, its cross-sectional area, its shape and the density of the air.
Sketch the graphs of a-t, v-t and s-t for an object released from rest in a uniform gravitation filed where there is air resistance.
Pg 13 of notes
Why is the time of flight upwards for a bouncing ball less than its time of light downwards?
On its way up, the drag force acts as an extra retarding force to the object’s motion. Hence, velocity reduces to zero faster as the drag force is acting in the same direction as the object’s weight. Resultant force = Fd+mg = ma(up), a(up) = g+Fd/m, a(up) more than g. On the object’s way down after reaching maximum displacement, the drag force acts is in the opposite direction to the weight of the ball. Velocity increases at a slower rate. Resultant force = ma(down), a(down) = g-Fd/m, a(up) less than g. Hence, a(up) is greater than a(down). During the flight upwards, a shorter time is needed to cover the same distance. Thus, the time of flight upwards for a bouncing ball less than its time of light downwards.
What is projectile motion?
It is a curved path in a vertical plane that involves motion in the x and y-directions simultaneously and the vertical and horizontal motions of a projectile are independent of each other.
What can you say about the time taken for an object to travel in the x and y-directions?
They are the same.