3. MOTION Flashcards
Define Instantaneous speed
The speed of an object over a very short interval of time.
Define Acceleration
The rate of change of velocity.
Define Stopping distance
The total distance travelled from when the driver first sees a reason to stop, to when the vehicle stops.
Define Thinking distance
The distance travelled between the moment when you first see a reason to stop, to the moment when you use the brake.
Define Braking distance
The distance travelled from the time the brake is applied until the vehicle stops.
Define Free fall
When an object is accelerating under gravity, with no other force acting on it it is said to be in free fall.
What are all the equations i need for this topic?
(9)
v = ∆x/∆t
average speed(ms^-1) = distance travelled(m) / time taken(s)
v = ∆s/∆t
average velocity(ms^-1) = change in displacement(m) / time taken(s)
a = ∆v/∆t
acceleration(ms^-2) = change in velocity(ms^-1) / time taken(s)
suvat equations:
v = u + at
s = vt - 1/2 at^2
s = ut + 1/2 at^2
s = 1/2(u+v)t
v^2 = u^2 + 2as
thinking distance = speed x reaction time
Write down everything you know about distance-time graphs
(4)
Distance-time graphs are used to represent the motion of objects
Distance is on the y-axes
Time is on the x-axes
Speed is the gradient of the graph
A stationary object is represented by a horizontal straight line
An object moving at a constant speed is represented by a straight sloping line
Instantaneous speed is the speed of an object over a very short interval of time
To find the instantaneous speed on a distance-time graph, draw a tangent on the graph at that point, then find the gradient of the tangent.
(the greater the gradient, the greater the instantaneous speed)
Write down everything you know about displacement-time graphs
(2)
They are used to represent the motion of objects
Displacement is on the y-axes
Time is on the x-axes
Velocity is the gradient of the graph
Write down everything you know about velocity-time graphs
(2)
Velocity is on the y-axes
Time is on the x-axes
Acceleration is the gradient of the graph
Displacement is the area under the graph
A straight line of zero gradient = constant velocity/ zero acceleration
A straight line of constant positive gradient = constant acceleration
A straight line of constant negative gradient = constant deceleration
A curve with changing gradient = changing acceleration
Write down everything you know about the equations of motion
(3)
The ‘suvat’ equations are used for calculating quantities involving motion in a straight line at a constant acceleration including motion of bodies falling in a uniform gravitational field without air resistance
s = displacement or distance travelled (m)
u = initial velocity (ms^-1)
v = final velocity (ms^-1)
a = acceleration (ms^-2)
t = time taken for the change in velocity (s)
v = u + at
s - vt - 1/2at^2
s = ut + 1/2at^2
s = 1/2(u+v)t
v^2 = u^2 +2as
Write down everything you know about stopping distance (car)
(5)
Stopping distance = thinking distance + braking distance
Stopping distance = the total distance travelled from when the driver first sees a reason to stop, to when the vehicle stops
Thinking distance = the distance travelled between the moment when you first see a reason to stop, to the moment when you use the brake
thinking distance = speed x reaction time (for a vehicle moving at a constant speed)
Braking distance = the distance travelled from the time the brake is applied until the vehicle stops
Factors that influence these distances:
- speed of the vehicle
- condition of brakes
- condition of tyres
- condition of road
- weather conditions
- alertness of driver
Write down everything you know about Free fall and g
(3)
Objects with a mass exert a gravitational force on each other
An object is said to be in free fall when it is accelerating under gravity with no other force acting on it.
The value of g depends on a number of factors including:
- altitude
- latitude
- the geology of the area
Write down everything you know about ways of determining g
(4)
The basic idea is to drop a heavy ball over a know distance and time its decent
The problem is it happens very quickly
Electromagnet and trapdoor:
An electromagnet holds a small steel ball above a trapdoor
-> when the current is switched off, a timer is triggered, the electromagnet demagnetises, and the ball falls.
-> When it hits the trapdoor, the electrical contact is broken and the timer stops.
The value for g is calculated form the height of the ball and the time taken
INACCURACY: Tiny delays in the release of the steel ball because of the finite time taken for the magnet to demagnetise. Presence of air resistance
ACCURACY MAY BE IMPROVED BY: using a heavier ball and a much longer drop.
Light gates:
Two light beams, one above the other, with detectors connected to a timer.
-> When the ball falls through the first beam, it interrupts the light and the timer starts.
-> When the ball falls through the second beam a known distance further down, the timer stops.
Taking pictures:
A small metal ball is dropped form rest next to a metal rule, and its fall is recorded on video
-> The position of the ball at regular intervals is then determined by examining the recording.
Write down everything you know about Projectile motion
(6)
How far a projectile travels depends on
- height above ground/sea level
- initial velocity of projectile
(assuming no air resistance)
The vertical and horizontal motions of the projectile are independent from each other.
For a projectile assuming no air resistance:
- vertical velocity changes due to acceleration of free fall
-> vertical displacement and time of flight can be calculated using equations of motion
- horizontal velocity remains constant
-> horizontal acceleration = gcos90 = 0
To work out speed of a projectile:
v = √(vx^2 + vy^2)
To work out angle of projectile from speed:
ø = tan^-1(vy/vx)
horizontal velocity = vcosø
vertical velocity = vsinø
