Physics - Calculating Acceleration Due To Gravity

OCR A-Level Physics 2022

Created: October 13, 2020 | Read markdown | About these notes | View in context | Study these flashcards |

Weight and the equation for g

What is the _ word _ equation for weight?

weight-word-equation
\[\text{weight} = \text{mass} \times \text{gravitational field strength}\]

What is the _ symbol _ equation for weight?

weight-symbol-equation
\[W = mg\]

The equation $W=mg$ is a special case of which more general formula?

weight-special-case-of-newton-second-law
\[F = ma\]

What are the two different units for $g$, the acceleration due to gravity?

units-of-g
\[\frac{N}{kg} \\\\ ms^{-2}\]

Choosing and rearranging the SUVAT equation

Which SUVAT equation would be the most useful for calculating $g$?

suvat-equation-for-g
\[s = ut + \frac{1}{2}at^2\]

When using $s = ut + \frac{1}{2}at^2$ to calculate $g$, how can you simplify the equation?

suvat-simplified-zero-initial-velocity
\[s = \frac{1}{2}at^2\]

How could you rearrange $s = \frac{1}{2}gt^2$ for $g$?

rearrange-for-g
\[\frac{2s}{t^2}\]

What’s the simplest experiment you could perform to measure $g$ using $s = \frac{1}{2}at^2$?

simplest-drop-experiment

Drop an object from a known height and measure the time it takes to fall.

If an object takes $0.1s$ to fall $1m$, how could you calculate $g$?

worked-drop-calculation
\[\frac{2 * 1m}{0.1s^2}\]

Improving the drop experiment

How could you improve the inaccurate timing of a simple drop experiment for calculating $g$?

improving-timing-accuracy
  • Use light gates
  • Use a trapdoor
  • Use a phone camera

How could you reduce the effect of air resistance in a drop experiment for calculating $g$?

reducing-air-resistance

Use a streamlined object.

Plotting a graph to find g

Instead of doing repeats for a single height, what are the two main reasons it is better to do varying heights?

varying-heights-reasons
  • You can plot a graph:
    • Can identify outliers
    • Drawing a line of best fit is like taking an average

If you wished to plot a series of displacements ($s$) and times ($t$) for a drop experiment for calculating $g$, what would your axies be?

drop-experiment-graph-axes
  • $s$, along the x-axis
  • $t^2$, along the y-axis

Why is it better to plot for $t^2$ rather than $t$ when doing calculating $g$?

why-plot-t-squared

Because it means the graph produced will be a straight line and you can calculate $g$ by finding the gradient.

Sources of error and accuracy

When using the equation $s = \frac{1}{2}at^2$ to calculate error, what are the two sources of error?

two-sources-of-error
  • Displacement
  • Time

How can you ensure that the displacement is measured accurately when calculating $g$ using a drop experiment?

accurate-displacement-measurement
  • Ensure the ruler is level
  • Check for zero errors

How can you ensure that the time is measured accurately when calculating $g$ using a drop experiment?

accurate-time-measurement
  • Use light gates
  • Use metal trapdoor

What is more accurate, using a metal trapdoor or light gates for calculating $g$?

trapdoor-vs-light-gates-accuracy

Using light gates.

Light gate method

What three SUVAT variables do you know when calculating $g$ using light gates?

light-gate-known-suvat-variables
  • $s$, displacement
  • $u$, initial velocity
  • $v$, final velocity

What SUVAT equation do you use when calculating $g$ using light gates?

light-gate-suvat-equation
\[v^2 = u^2 + 2as\]

Why might an electromagnetic trapdoor method for calculating $g$ be inaccurate?

electromagnetic-trapdoor-inaccuracy

Because there may be some delay in the switch being toggled and the ball dropping.

What are the three methods for calculating $g$, in worst-to-best order?

three-methods-ranked
  1. Simple use of a timer and ruler
  2. Using an electromagnetic trapdoor
  3. Using light gates and a timer card


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