Module 3: C3 - Motion Flashcards
Equation for Speed
Speed (ms^-1) = Distance (m) / Time (s)
Speed, Velocity, Instantaneous Speed Definitions
Speed:
Is the distance travelled per unit of time
Velocity:
Is the displacement travelled per unit of time
Instantaneous Speed:
Is the speed at a given instant of time
Equation for Velocity
Velocity (ms^-1) = Displacement (m) / Time (s)
Acceleration Definition
Acceleration tells us how rapidly something is changing velocity - for instance, the change in velocity in unit time. Deceleration is the same thing, but has a negative sign as the velocity is decreasing.
What does the area underneath a Velocity-Time Graph give
It gives the displacement
Equation for Acceleration
Acceleration (ms^-2) = Velocity (ms^-1) / Time (s)
Parabola Definition
The characteristic path followed by an object moving under gravity in a uniform gravitational field.
Projectile Definition
An object is given an initial force and then allowed to move freely under gravity.
Range Definition
The horizontal distance from its starting position at which a projectile will land. This can be predicted from the initial conditions of the flight.
Trajectory Definition
The path that an object follows through space, due to the forces that act on it. Under the influence of gravity, projectiles in free fall will follow a parabolic trajectory.
The 4 SUVAT Equations
v = u + at
s = ut + 1/2at^2
v^2 = u^2 + 2as
s = 1/2(u + v) x t
How do you get the Acceleration from a Velocity-Time Graph
You find the gradient. This is done by:
Change in V / Change in t
How can we measure acceleration?
- Set up time gates to measure the acceleration of a truck
- Draw a suitable table to take results
- Measure acceleration over the length of the ramp
- Measure acceleration 10cm from the top of the ramp
- Measure acceleration 10cm from the top of the ramp
How do you derive the SUVAT equation without s
From a velocity-time graph:
a = Δv/Δt = v-u/t
This can be rearranged to give:
v = u + at
How do you derive the SUVAT equation without v
The area under the velocity-time graph is equal to the displacement s.
- the rectangular area = ut
- the triangular area = 1/2 x (v-u) x t
From equation 1, (v-u) = at. If you substitute this into the expression for the area of the triangle, you get 1/2 x at x t. With ut for the area of the triangle, this gives the total area s.
s = ut + 1/2at^2
How do you derive the SUVAT equation without a
If you treat the area under a velocity time graph as the area of a trapezium (with u and v as the parallel sides , and t as the perpendicular length), this becomes:
s = 1/2 (u+v)t
In other words, the displacement s is the average velocity (u+v/2) multiplied by the time t.
How do you derive the SUVAT equation without t
According to Equation 1 (a = (v-u) / a):
t = (v-u) / a
This equation can be substituted into Equation 3 (s = 1/2 (u+v)t):
s = 1/2 (u+v) x (v-u)/a
Rearranging this gives:
(u+v)(v-u) = 2as
v^2-u^2 = 2as
v^2 = u^2 + 2as
How does a Skydiver reach their Terminal Velocity
- At the start of his jump, the air resistance is zero so he accelerates downwards.
- As his speed increases his air resistance will increase.
- Eventually the air resistance will be big enough to balance the skydiver’s weight. At this point the forces are balanced so his speed becomes constant - this is called Terminal Velocity.
- When he opens his parachute the air resistance suddenly increase, causing him to start slowing down.
- Because he is slowing down his air resistance will decrease again until it balances his weight. The skydiver has now reached a new, lower terminal velocity.
What value is gravity in Physics
In Physics, gravity is 9.81ms^-2
(However, in Maths it is 9.8ms^-1)
What is Free Fall
When an object is accelerating under gravity, with no other force acting on it, it is said to be in free fall. The acceleration of free fall is denoted by the label g. Since g is an acceleration, it has the unit ms^-2
What are the 3 main methods that can be used to determine g
- Electromagnet and Trapdoor
- Light Gates
- Taking Pictures
How can you find g by using the Electromagnet and Trapdoor experiment?
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 of g is calculated from the height of the ball and the time taken.
How can you find g by using Taking Pictures?
A small, metal ball is dropped from rest next to a meter rule, and it’s fall is recorded on video or with a camera in rapid-fire repeating mode. Alternatively, a stroboscope illuminates the scene with rapid multiple images of the falling ball. The position of the ball at regular intervals is then determined by examining the recording.
How can you find g by using Light Gates?
The electromagnet and trapdoor introduce tiny delays into the timing. Instead we can use ‘light gates’, two 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.
