Mechanics and Materials (2): On the move Flashcards
What is the equation for an object moving in a circle at a constant speed?
v = 2(pi)R/T
What are the two types of speeds you can calculate when an object has a changing speed?
Instantaneous velocity
Average velocity
What is the equation for instantaneous velocity?
v = s/t at that point
What is the equation for average velocity?
v = (delta)s/(delta)t
where:
delta s - total distance
delta t - total time
What does the distance-time graph look like for an object travelling at a constant speed?
A straight line graph through the origin
How do you find the speed of an object (moving at constant speed) on a displacement/distance-time graph?
Find the gradient of the straight line
How do you find the instantaneous speed of an object (moving at a changing speed) on a displacement/distance-time graph?
Draw a tangent at the point that you want to know the speed for and calculate the gradient of that line
What is the definition of constant velocity?
When an object moves with the same speed without changing its direction of motion
Does an object moving in a circular path have a constant velocity?
NO, as its direction is constantly changing
Can displacement have both a +ve and -ve sign?
Yes
What is the definition of acceleration?
The change in velocity per unit time
Is acceleration a scalar or vector quantity?
Vector (they can have a negative or positive value for deceleration or acceleration)
What is the definition of uniform acceleration?
When the rate of change of velocity of an object is constant
What is the equation for uniform acceleration?
a = v-u/t
What type of graphs display acceleration as the gradient?
Velocity-time graphs
What is the definition of non-uniform acceleration?
When the rate of change of velocity of an object is not constant
How do you find the acceleration at a point on a velocity-time graph if the acceleration is not constant?
Draw a tangent at that point and find the gradient of it
In what scenario can you use suvat equations?
When the acceleration is constant
How do you find the displacement from a velocity-time graph?
displacement = area under the graph
What is theoretical way that you can determine the acceleration due to gravity?
Make a multi flash photo or video clip of the balls flight as it falls after being released from rest.
- The flashing light needs to flash at a known constant rate
- After you have collected the data you can slow the video down and record the time taken to travel each distance
What is acceleration of free fall?
When acceleration of an object is due to no other factor apart from the weight of an object
What is the value of acceleration of free fall?
9.81 ms^-1
Why is it unusual to get acceleration of free fall?
Due to air resistance acting on the body that is falling
What is the practical way we used to determine the value of g?
EQUIPMENT:
- Stand and Clamp
- Electromagnet
- Steel ball bearing
- Light gate connected to stopwatch
METHOD
- Set up the apparatus with the electromagnet at the highest point on the clamp and then two light gates one higher and one lower (the position of the lower light gate should be adjusted to a known height)
- Determine the height between the two light gates
- Turn on the electromagnet and attach the steel ball bearing to it
- Turn off the electromagnet, releasing the ball bearing and record the time taken for the ball to fall between the 2 light gates
- Reduce the height between the 2 light gates by 50 cm each time and doing three repeats for each height
- Determine the average time taken for the steel ball to fall between the two light gates
ANALYSIS
- h = ut + 1/2gt^2
- Plot a graph of h against t^2 against height and your gradient will = 2g
SAFETY
- A suitable container should be placed under the trapdoor to catch the ball bearing to prevent it from falling on the floor and creating a trip hazard
- Use a counterweight or clamp on the stand to avoid it falling over and causing injury
If a ball is dropped into sand describe what values change as the ball hits the sand
- Initial acceleration is g downwards
- When the ball reaches the sand it decelerates and so the value of acceleration is no longer g
NB: The link between the two stages is that the final velocity before the ball hits the sand is its initial velocity when the ball hits the sand
When doing a question involving projectiles and movement what is the first thing that you should do?
Define which way is positive and negative
- Normally up and right is defined as positive
What are the signs of the values for velocity and acceleration when an object is moving forwards and accelerating?
Velocity = +ve
Acceleration = +ve
What are the signs of the values for velocity and acceleration when an object is moving forwards and decelerating?
Velocity = +ve
Acceleration = -ve
What are the signs of the values for velocity and acceleration when an object is moving backwards and decelerating?
Velocity = -ve
Acceleration = +ve
What are the signs of the values for velocity and acceleration when an object is moving backwards and accelerating?
Velocity = -ve
Acceleration = -ve
The vertical and horizontal components of a projectile are…?
Independent
What is the definition of a projectile?
An object acted upon only by the force of gravity
In projectiles what is the vertical acceleration of the object equal to?
g (always negative as it is acting downwards)
The horizontal velocity of a projectile is…?
Constant
Describe the velocity-time graph for an object projected vertically.
- You have both negative and positive velocity as the value is a vector
The graph begins at a positive maximum as it is thrown up into the air. The velocity decreases at a constant rate (STRAIGHT LINE) until it reaches its maximum height where velocity = 0. The velocity then goes into the negative portion of the graph increasing at a constant rate again (STRAIGHT LINE) until it reaches its maximum velocity again
For an object projected horizontally, how does its vertical path change as time progresses?
The vertical distance the object travels in a given time increases as time progresses
For an object projected horizontally, how will changing the speed that it is projected at change its motion?
The object will be projected further
Does projecting an object faster horizontally change how long it takes for the object to reach the ground?
No
What apparatus can you use to measure compare horizontal versus vertical projection?
A stroboscope and camera with a slow shutter - this will capture photos of the two balls’ motions as they are projected
For a component projected horizontally, what is its horizontal displacement at time, t, after being projected with initial velocity, u?
sx = ut
(as the object does not accelerate horizontally)
For a component projected horizontally, what is its vertical displacement at time, t, after being projected with initial velocity, u?
sy = 1/2gt^2
(as the object has no initial vertical velocity)
What are two examples of projectile motion that aren’t to do with projecting a ball into air?
- Path of a ball rolled across an inclined plane
- Path of an electron between two charged plates -> Its motion parallel to the plates is at 0 acceleration but the acceleration of each electron is the same and constant towards the positively charged plate
What is the definition of friction?
A resistive force that opposes the motion of an object turning the kinetic energy of objects into other energy sources such as heat or sound
What is the name of the resistive force that opposes the motion of projectiles?
Drag/air resistance
What characteristic increases the value of drag/air resistance?
- Speed of the object
What is the name of the upwards force that acts opposite to air resistance?
Lift
How does air resistance effect the horizontal components of a projectile?
Air resistance has a horizontal component that reduces both the horizontal speed of the projectile and its range
How does air resistance effect the vertical components of a projectile?
If the initial projection of the projectile is above the horizontal then the maximum height it reaches will be lower due to air resistance. Also its descent will be steeper.
How can the shape of a projectile affect whether it experiences a life force due to air resistance e.g shape of an aircraft wing?
- This happens if the shape of the projectile causes air to flow faster over the top and slower under the bottom
- This causes a the pressure on the top surface to be less than the pressure on the bottom surface
- The pressure difference causes a lift force
