CH3 Flashcards
Define speed, the equation used to calculate speed, and the respective SI units.
Speed is defined as the rate of change of distance. The equation used is:
speed = distance / time
The SI units are ms-1.
v = Delta x/ Delta t
Define displacement.
The displacement of an object is the distance it has travelled in a given direction, so it is a vector with both magnitude and direction.
Define avg velocity, the equation used to calculate
velocity, and the respective SI units.
The velocity of an object is defined as the rate of change of displacement, or speed in a given direction, making velocity a vector.
The equation used is velocity = change in displacement/time and the SI units are
ms-1.
v = Delta s/ Delta t
Define acceleration, the equation used to calculate acceleration, and the respective SI units.
Acceleration is defined as the rate of change of velocity, making it a vector.
The equation used is acceleration = change in velocity / time and the SI units are ms-2.
a = Delta v/ Delta t
where Dv is the change in velocity and Delta t the time taken for that change
How can acceleration be determined?
Either by calculation of with a velocity time graph (v-t graph)
How can acceleration be determined from a velocity time graph?
SInce a = Delta v/Delta t, it can be determined by the gradient of the v-t graph
What does a straight, horizontal line represent on a displacement-time graph?
A stationary object.
What does a line with a constant, non-zero gradient represent on a displacement-time graph?
An object moving with constant velocity.
What does a curved line represent on a displacement-time graph?
Acceleration (if gradient is increasing) or
deceleration (if gradient is decreasing).
What does a straight, horizontal line represent on a velocity-time graph?
An object moving with constant velocity or zero acceleration
What does a line with a constant,
non-zero gradient represent on a velocity-time graph?
An object that is accelerating (positive gradient) - constant acceleration
or decelerating (negative gradient) - constant deceleration
What does a curved line represent on a velocity-time graph?
A curve with changing gradient: acceleration is changing
What does the area under a velocity-time graph represent?
Displacement.
How do you calculate displacement in a non linear velocity time graph?
Count the squares that are completely or nearly completely under the line (pg 30 book)
What does the area under an acceleration-time graph represent?
Velocity
Describe how g of can be determined using light gates.
- Set up the light gates one above the other, with detectors connected to a timer
- when the ball falls through the first beam, it interrupts the light and the times starts
- When the ball falls through the second beam known distance further down, the timer stops
Describe a way of determing g apart from light beams and trapdoor
Camera
Describe how light gates can also be used to investigate conservation of momentum.
● Place two carts on a linear air track (to reduce friction) with repelling
magnets so that they do not stick together.
● Attach card to the top of each cart so that they break the beams of the
light gates when they pass.
● Keep one cart stationary and push the other towards it, measuring its
velocity before the collision.
● Then measure the velocity of both carts after the collision and
calculate the momentum before and after.
Define ‘g’.
The acceleration of free fall, ‘g’, is the acceleration of an object in response to the gravitational attraction between the Earth and the object. g = 9.81m s-2.
Describe the experiment in which one can determine ‘g’ using an electromagnet.
- An electromagnet holds a steel ball suspended over a trapdoor.
- When the current is switched off a timer is triggered
- the electromagnet demagnetises and the ball falls
- The value of g is calculated from the height of the fall and the time taken.
● As the ball was initially resting, u = 0.
● The distance and time are known, so we can use a SUVAT equation:
s = ut + ½ at2 t
● Calculate ‘a’ which, in this case, is ‘g’.
A ball is projected of a castle at 6m s-1. How does its horizontal velocity change from its launch until it hits the ground?
The horizontal velocity remains the same as there is no acceleration in the horizontal direction.
In projectile motion, what is the vertical acceleration?
The vertical acceleration is equal to gravitational field strength (g) downwards.
What is meant by instantaneous speed and how can it be found?
It is the speed of an object over a very short interval of time.
It is found by drawing the tangent in a distance time graph at that specific time, then determining the gradient of the tangent
What are the 4 SUVAT equations? and what does SUVAT stand for?
S = Displacement or distance travelled
u = initial velocity
v = final velocity
a = acceleration
t = time taken for the change in velocity
v = u + at
s = ut + 1/2 a(t)2
s = 1/2 (u+v)t
(v)2 = (u)2 + 2as
Define thinking distance
Distance travelled between the moment when you first see a reason to stop to the moment when you use the brake
Define Breaking distance
Distance travelled from the time the brake is applied until the vehicle stops
Formula for thinking distance
Thinking distance = speed * reaction time
What are the two components of projectile motion and what is their characteristic?
Projectile Motion has both a vertical and a horizontal component.
These components are independent form each other. So the key to analyzing two-dimensional projectile motion is to break it into two motions and analyze them separately.
In projectile motion what is the acceleration in the horizontal direction? Motivate your answer
There’s no acceleration in the horizontal direction since gravity does not pull projectiles sideways, only downward. Air resistance would cause a horizontal acceleration, slowing the horizontal motion, but since we’re going to only consider cases where air resistance is negligible we can assume that the horizontal velocity is constant for a projectile.
How is horizontal velocity for a projectile. Motivate it
The horizontal component of a projectile’s velocity remains constant due to the absence of any horizontal forces acting on it. This is a consequence of Newton’s first law of motion, also known as the Law of Inertia, which states that an object in motion will stay in motion at a constant velocity unless acted upon by an external force
What is the acceleration equal to in the vertical component of projectile motion?
Constant acceleration due to gravity (g)
Time of Flight and horizontal distance travelled calculations: A ball is fired horizontally from a cliff at 44.1m above the sea. The initial horizontal velocity is 304 ms-1. Calculate 1. Time of flight and 2. horizontal distance travelled
- Use equation s = ut + 1/2 a(t)2 to calculate the time
- The initial vertical velocity u = 0 (u = 304 * cos90degrees = 0)
- substitute: s=1/2a(t)2
(t)2=2s/a= 2*44.1/9.81= 8.991(s)2
t=3s
2. - there is no acceleration in the horizontal direction.
- so horizontal distance = Horizontal velocity * time
- horizontal distance = 304*3= 912 m
See book pg 42 to calculate: Velocity of object when it hits the ground, Range
