Motion

The s-t and v-t graph flashcards are about what their shape means

Method to answer projectiles questions

• Sketch the situation and add any values known.

• Split into vertical and horizontal motion.

• Use suvat for vertical.

• Decide which direction you are taking as positive.

• Velocity remains constant in the horizontal directions

• For max height problems velocity at top is zero.

Define thinking distance

The distance travelled between the moment when you first see a reason to stop, to the moment when you use the brake

Define breaking distance

The distance travelled from the time the brake is applied until the vehicle stops.

Define stopping distance

• Thinking distance + breaking distance

• Total distance travelled from when the driver first sees a reason to stop to when the vehicle stops

Factors that affect thinking distance

• Speed of the vehicle

• Alertness of driver - are they tired? are they under the influence of alcohol or drugs?

Factors that affect breaking distance

• Speed of the vehicle

• Condition of brakes

• Condition of tyres

Factors that affect stopping distance

• Speed of the vehicle

• Alertness of driver - are they tired? are they under the influence of alcohol or drugs?

• Condition of brakes

• Condition of tyres

Average speed (definition & equation)

• The rate of change in distance calculated over a complete journey

• Equation: ___ = distance travelled / time taken

Instantaneous speed

The speed at at the moment it is measured; the speed over an infinitesimal interval of time

Displacement

• The direct distance between an object’s starting and ending positions in a particular direction

• Is a vector quantity

Velocity

• The ratge of change of displacement

• Is a vector quantity

Acceleration

• The rate of change of velocity

• Is a vector quantity

What does the gradient of a displacement-time graph equal?

Equals the velocity (of the object)

What does the gradient of a velocity-time graph equal?

Equals the acceleration (of the object)

What does area under a velocity-time graph equal?

Equals the displacement (of the object)

Straight line of constant, positive gradient on a v-t graph means…

Constant positive acceleration

A straight line fo zero gradient on a v-t graph means…

Constant velocity / zero acceleration

A straight line of constant, negative gradient on a v-t graph means…

Constant negative acceleration (deceleration)

A curve of changing gradient on a v-t graph means…

Acceleration is changing

A straight line of constant, positive gradient on a s-t graph means…

Constant positive velocity

A straight line of zero gradient on a s-t graph means…

Constant displacement / zero velocity

A straight line of constant, negative gradient on a s-t graph means…

Constant negative velocity

A curve of changing gradient on a s-t graph means…

Velocity is changing (object is accelerating)

Equipment for determining g using manual timing

• Ruler

• Spherical object

• Stopwatch

Method & analysis for determining g using manual timing (graphical analysis)

Method:

• Drop the object from a height measured with the ruler

• Use the stopwatch to time how long it takes the object to reach the ground and record it along with the height

• Repeat this for a number of heights and record the results

Analysis:

• Plot a graph or height against time², and draw a line of best fit.

• Calculate the gradient (m) of this line

• s = ut + ½at² and innitial velocity u = 0, so s = ½at²

• As such the gradient m is ½a, so a = 2m

Method & analysis for determining g using manual timing (non-graphical analysis)

Method:

• Drop the object from a height measured with the ruler

• Use the stopwatch to time how long it takes the object to reach the ground

Analysis:

• s = ut + 1/2at² and innitial velocity u = 0, so s = ½ at².

• a=g, so s = ½ gt²

• Rearrange this to get g = 2s/t², which we can be used to calculate our value of g.

Equipment for determing g using 2 light gates

• object to drop, e.g. a piece of plasticine

• stop clock

• data logging system

• 2 light-gates

• interrupt card

• ruler

Method to determine g using 2 light gates

• Set up the data logger with two light gates vertically above each other.

• Measure distance between light gates using ruler

• Drop the interupt card from just above the top light gate and record time shown on data logger

• Repeat a number of times and record time to fall for each

Analysis for determing g using 2 light gates

• Calculate the average time taken to fall between light gates

• s = ut + 1/2at² and innitial velocity u = 0, so s = ½ at².

• a=g, so s = ½ gt²

• Rearrange this to get g = 2s/t², which we can be used to calculate our value of g.

Diagram & method for determining g using an electromagnet and timer

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. Record the height of fall (h) using the ruler.

Analysis for determining g using an electromagnet and timer

• s = ut + 1/2at² and innitial velocity u = 0, so s = ½ at².

• a=g and s=h, so h = ½ gt²

• Rearrange this to get g = 2h/t², which we can be used to calculate our value of g.