Chp 2: Kinematics Flashcards
Define distance
actual path travelled by an object
Define displacement
distance from a fixed point in a specified direction. It is a straight line between 2 points.
Define velocity
rate of change of displacement
Define speed
distance travelled per unit time
Define acceleration
rate of change of velocity
Define average speed
total distance travelled over total time taken.
Define average velocity
total change in displacement over total time taken.
the 4 kinematics eqn works only when:
- Travel in straight line
- Constant/uniform acceleration
Steps to solve kinematics problems (a guide)
- Define positive direction
- Write all given data with correct sign
- Identify unknown variables
- Choose kinematics eqn to connect unk variable and given data
- Solve
Explain why acceleration can be greater than 9.81 when the ball is thrown upwards
Initially, the ball is thrown upwards with a certain speed. As air resistance is
not negligible, there will be an additional force acting vertically downwards
besides the gravitational force acting on the ball. Hence the resultant
acceleration that acts downwards will be greater than g.
projectile motion
motion due to a uniform velocity in one direction & a uniformly accelerated motion in a perpendicular direction
acceleration of free fall
acceleration of body towards the surface of earth when the only force acting in it is its weight
Average Acceleration:
Average acceleration refers to the change in velocity over time taken.
Why the path of a mass or a charged particle in a uniform electric field or gravitational
field is the same?
Both particles experience a uniform speed in one direction and uniform force and uniform
acceleration in a perpendicular direction. They move in a parabolic path.
Why in the presence of air resistance, the time taken to reach the maximum height is
shorter than the time taken to return to the original position?
When the ball is on its way up, air resistance acts downwards, opposing the motion of the
object. The resultant force acting downwards is the sum of the air resistance and the
weight of the object. On the downward journey, air resistance acts upward, opposing the motion of the object. The resultant force acting on the object downwards is its weight
minus air resistance. Thus, the acceleration downwards is smaller in magnitude than the
deceleration in the upward journey. Since both journeys cover the same distance, the time
taken for the upward journey is shorter.
Explain what the acceleration of a ball at its maximum height is when an object is thrown
vertically upwards.
At the maximum height, the object is instantaneously at rest, there will not be any drag acting on the object hence the only force acting on the object is its weight. By N2L, ma=mg, thus the acceleration of the ball at its maximum height is 9.81ms-2 downwards.
Explain why a falling object eventually reaches constant velocity in the presence of air
resistance.
When an object falls, there is air resistance acting on the object upwards, since air
resistance increases with speed, as the object accelerates downwards due to gravity, the
air resistance increases. ma=mg-air resistance, as air resistance increases, the acceleration
of the object decreases until the acceleration is zero where the magnitude of the weight of
the object equals to the air resistance, the object is said to be at terminal velocity.
Conditions for a parabolic motion in absence of air resistance.
Horizontally:
Vertically:
Horizontally: Horizontal component of velocity is always constant
Vertically: Vertical acceleration is always constant and downwards, with magnitude equal to 9.81ms-2
If air resistance not negligible how does velocity vary in a projectile motion?
Horizontally:
Vertically:
Horizontally: Horizontal component of velocity decreases to zero
Vertically: Decreases at decreasing rate in the upward journey. Increases at a decreasing
rate in the downward journey, if the building is tall enough, the object will eventually
reach terminal velocity when the weight of the object is equal to the magnitude of air
resistance acting on the object.
Why is the path of a ball thrown at an angle different when there is air resistance and
without air resistance?
With air resistance, some loss in KE is converted to work done against air resistance,
hence not all energy is converted to the GPE, hence maximum height is smaller. In the
horizontal direction, the horizontal velocity is no longer constant due to work done against
air resistance, hence the range is smaller.
