Motion Flashcards
What is speed? Measurement? How to work out your average speed? Triangle What is your instantaneous speed?
Speed is a measure of how fast an object is travelling
Units for speed in the equation: Speed is m/s, distance is m, time is seconds
Speed = distance travelled / time taken
Triangle has the distance at the top, on the left speed and on the right time.
Your instantaneous speed is the speed that you are moving at any one moment in time
Velocity The two directions What is displacement Equation Tricks in questions
Velocity tells us the speed and the direction that is is moving
In most applications, we only need to worry about 2 opposite directions, so we will have a positive and a negative direction. For example, if a car is driving at 30m/s northwards we say that because it is northwards, it is the positive direction and so the velocity would be +30m/s. However, if the car is driving southwards, then the velocity would be -30m/s
A car might drive a constant and steady speed but it is constantly changing direction whilst turning and so it’s velocity is constantly changing.
Another way to think about velocity is to consider how much an object’s displacement has changed since it started to move. Displacement is a measure of the distance and direction that an object is from its starting point.
Velocity would be calculate - Velocity = displacement from start point / time taken
However you may be tricked when answering questions like this. If they asked you for example what was the average velocity this person has over the whole race which was run over an oval track, it would be 0 m/s because he is back where he started so there is no displacement.
Plotting and explaining distance-time graphs
A distance time graph is a way of graphically representing the motion of an object. By seeing the gradient, you can see the speed the object:
A steady speed is represented by a straight line graph showing the same change in distance each second.
The steeper the gradient, the faster the object is moving ( the more closer it is to be standing straight up)
If it is a line going straight down, it shows a stationary object, it is an object moving at a steady speed of 0 m/s because the gradient is 0.
When is asks you to work out the speed of the object, you work out the gradient using change in y / change in x
If the line is not straight but is curved then the speed will be changing.
A line getting steeper means it is accelerating
A line getting less steep means it’s decelerating
Acceleration
Acceleration means the rate of change in velocity. As velocity describes an object’s speed and direction then any that object that is accelerating could be:
Speeding up
Slowing down
Changing direction
Equation - Acceleration = Change in velocity (Final - Initial) / Time taken to change
Acceleration - m/s ^2
Triangle - velocity at the top, acceleration in bottom left and time in bottom right
Be careful if it’s returning then it’s negative! The minus sign in the answer shows is that is is slowing down or decelerating.
Velocity - Time Graphs
Another way of graphically representing the motion of an object.
However this type of a graph shows more. For example if it’s a straight line, you can read the y axis to see what the velocity is.
A steady velocity is represented by a straight flat line which shows the velocity doesn’t change.
A graph where it’s line is a 0 on the y axis shows a stationary object and it’s velocity is constantly 0
A graph where it is accelerating will have a straight line going diagonally upwards. It has a constant acceleration because the line is straight.
A graph where it is decelerating will have a straight line going diagonally downwards. It had a constant deceleration because the line is straight.
Calculating the gradient of a velocity - time graph will tell us the acceleration of the object. The steeper the line, the bigger the acceleration or deceleration.
They can also tell us the distance that an object travels. We know that to work out Distance = velocity x time. If the graph shows that the velocity is 8 for example and the time is 6 then you do 8 x 6 = 48m
If the shape is irregular, there is another method. To work out the distance, you divide the shape into easy to manage shapes. For example a rectangle and a triangle. You would work out the area for the two shapes. So for example, the rectangle goes up to 15 and goes across to 1.4, the area would be those two numbers x. You would do the same for the triangle and would add them up to find the total distance.
It can also tell us the direction that an object is travelling bu using positive and negative values on the y axis. The graph might go up and down and up and down and you would have to calculate the acceleration between two specific points, you would use the same method and equation for acceleration and work out the change in velocity and time. Or you might have to work out the distance where you would have to split it up into easy to manage shapes and work out the area.
Practical - Investigate the motion of everyday objects such as toy cars or tennis balls.
You can use a toy car and slope and we would investigate the motion of a toy Car. You could use this to measure the average speed of the car for different values of height.
You need to measure the height of the raised end of the wooden track. You measure the distance from the top of the slope to the bottom, and measure how long the car will take to move down. Do it three times as a human error might occur when measuring the time and then work out the average time.
You may be asked to conclude the experiment in which if it passes through the origin and through the plotted points as a straight line you can conclude that there is a proportional relationship between the quantities you have plotted on the graph.
Method for working out the speed of someone e.g. a person on a bike going around a track
- Measure the length of one circuit of the track as accurately as possible… perhaps by using a trundle wheel would might easier than a tape measure.
- Repeat the measurement twice more, and find the mean length of one lap of the track.
- set the athlete/car/cyclist going round the track for multiple laps.
- Time how long it takes them to complete a fixed (or counted) number of laps/circuits, eg 10 or 20 laps using a stop watch or
timing system. - Calculate the total distance travelled = length of track x number of laps completed
- Calculate their average speed by using: 𝑨𝒗𝒆𝒓𝒂𝒈𝒆 𝒔𝒑𝒆𝒆𝒅 = 𝑻𝒐𝒕𝒂𝒍 𝒅𝒊𝒔𝒕𝒂𝒏𝒄𝒆 𝒕𝒓𝒂𝒗𝒆𝒍𝒍𝒆𝒅 /𝑻𝒐𝒕𝒂𝒍 𝒕𝒊𝒎𝒆 𝒕𝒂𝒌𝒆𝒏
