Module 3.1 - Motion Flashcards
Define speed
Distance per unit time
Define displacement
Distance moved in a particular direction from a reference point
Define velocity
Displacement per unit time
Define acceleration
Change in velocity per unit time
Difference between average and instantaneous speed
Average speed is total distance travelled / total journey time
Instantaneous speed is speed at an actual point in time of the journey
Average speed has a fixed value
Instantaneous speed is constantly changing
What does a displacement-time graph show?
How the displacement of a moving object varies with time
What is the gradient on a displacement-time graph equal to?
Instantaneous velocity of the moving object
What do different lines on a displacement-time graph represent?
Straight diagonal line - constant velocity
Horizontal line - stationary
Curved line - non-uniform velocity (gradient gives instantaneous velocity)
Curved increasing positive gradient - acceleration
Curved decreasing positive gradient - deceleration
What do different lines on a velocity-time graph represent?
Horizontal - constant velocity
Diagonal line with positive gradient - constant acceleration
Diagonal line with negative gradient - constant deceleration
Curve - non-uniform velocity change (gradient gives acceleration)
What is the area under a velocity-time graph?
Displacement
Why do objects of different masses fall at the same rate?
Even if one object has a larger force as it falls (due to F=ma as its mass is larger), its inertia will also be larger
This is because acceleration of an object is directly proportional to the force acting on it, but inversely proportional to its mass
What are the two types of approach that can be used to measure the acceleration of free fall?
Direct approaches - e.g. timing a falling ball and working out acceleration due to gravity using equations of motion
Indirect approaches - e.g. measuring time taken for a pendulum to complete a full swing because the motion of the pendulum is dependent on the value of g
Describe the trap door and electromagnet method for determining g
Electromagnet supports steel ball
When current through electromagnet is switched off, ball starts to fall and electronic timer is triggered simultaneously
Ball falls through trap door and electronic timer is stopped
s is distance ball falls, t is time taken
Repeat for and find mean value of t and do more readings at different values of s
Find value of g by rearranging s = ut + 0.5at^2 to g = 2s / t^2
Find value of g by plotting a graph of s against t^2 and the gradient will equal g / 2
Sources of uncertainty for the trap door and electromagnet method
If electromagnet current is too high there will be a delay in releasing the ball after the current is switched off and the timer is triggered - current must only just support the ball
If distance of fall is too large or ball is too small Sir resistance may have a noticeable effect on the speed
Measure distance from bottom of ball to top of trap door accurately
Describe the light gates method for determining g
Use a light gate, data logger and piece of card
Add two pieces of blu-tack to the lower corners of the card to improve its stability as it falls
Average velocity of the card is found using v = L / t, where L is the length of the card and t is the time recorded by digital timer for the card to pass through the light gate (may be calculated automatically by data logger)
Use a ruler to measure vertical height of card above light gate (s)
Hold card vertically above light gate and release
Find value of g using v^2 = u^2 + 2as, where u = 0
Vary the height from which the card is released to obtain a range of values for v and s
Find value of g by plotting a graph of v^2 against s and the gradient is 2g
