3.1: Motion

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41 Terms

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Speed

A scalar quantity
Total ground covered

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Displacement

Vector quantity

Measures distance travelled in a particular direction

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Speed equation

Distance travelled/time

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Displacement equation

Total displacement/time

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Distance (compare and contrast)

How much ground is covered in total

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Displacement (compare and contrast)

The shortest possible distance between two points

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Point of instantaneous velocity

To overcome the fact that average speed doesn’t tell you whether speed changes over the total time period

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Instantaneous Velocity/Speed

Velocity/Speed of an object measured over a very short period of time

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How instantaneous velocity works

V=Δs/Δt

If it keeps getting smaller you get speed at an instant

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How to find instantaneous velocity

Use a tangent to find the gradient of a graph at a point

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Acceleration Definition

The rate of change of velocity

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What type of quantity is acceleration

Acceleration is a vector quantity

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Units of acceleration

ms-2

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Equation for acceleration

(final velocity - initial velocity) / time

Δv / Δt

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Objects with high acceleration

  • Rockets

  • F1 cars

    • Falling objects

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<p>Distance-time graph</p>

Distance-time graph

  • Distance on y-axis

  • Time on x-axis

  • Gradient is speed

  • Stationary objects represented with horizontal line

    • Objects moving with a constant speed are represented with a straight line with a constant gradient

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<p>Displacement-time graph</p>

Displacement-time graph

  • Displacement on y axis

  • Time on x-axis

  • Gradient is velocity

  • Stationary objects represented by a horizontal line

  • Moving objects represented by a sloping line

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Velocity-time graph

  • Time on x-axis

  • Velocity on y-axis

  • Gradient is acceleration

  • Area under graph is displacement

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S.U.V.A.T equation without displacement

v=u+at

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S.U.V.A.T equation without final velocity

s=ut + 1 / 2at2

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S.U.V.A.T equation without acceleration

s=1 / 2(u+v)t

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S.U.V.A.T equation without time

v2=u2+2as

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When do we use S.U.V.A.T equations?

When objects are moving at a constant acceleration with a changing velocity

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When is an object in free fall?

When an object is accelerating under gravity with no other forces acting on it

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What is the acceleration of free fall?

9.81ms-2

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What is an electromagnet

  • Electromagnet holds a small steel ball above a trap door

  • When current is switched off, timer is triggered and electromagnet demagnetises causing the ball to drop

  • When the ball hits the trap door, electrical contact is broken and timer stops

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Method for determining the acceleration of free fall (electromagnet)

  1. Measure height of fall with a ruler

  2. Record time of fall using timer

  3. Use s=ut+1 / 2at2 where s=h, v=0ms-1 and a=g to calculate acceleration of free fall

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What is a light gate?

Two light beams one above another, with detectors connected to a timer. When ball falls through the first beam, it interrupts the signal and the timer starts. When the ball falls through the second beam, a known distance below, the timer stops

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Method for determining the acceleration of free fall (light gates)

  1. Measure the height of fall with a ruler

  2. Record the time of fall using the stop clock

  3. Use s=ut+1 / 2at2 where s=h, u=0ms-1 and a=g to calculate acceleration of free fall

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Picture method for determining the acceleration of free fall

  • Drop a small metal ball from rest next to a metre ruler

  • Its fall is recorded on video with a camera in rapid fire, repeating mode

  • Camera shutter is held open, which produces a photograph with multiple images of the falling ball

  • The position of the ball at regular intervals is then determined by examining the recording

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Graphical method for determining the acceleration of free fall

  1. Measure the height of fall with a ruler

  2. Record the time of fall with a stop clock

  3. Plot a graph with height on y-axis and t2 on x-axis

  4. Use y=mx+c analysis to find g

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Thinking Distance

  • Distance vehicle travels in the time that it takes for a driver to react to a situation

  • Thinking distance=speed x reaction time

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Factors affecting thinking distance

Condition of the driver (which affect reaction time like alcohol levels):

  • Alcohol

  • Tiredness

  • Drugs

  • Distractions

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Braking distance

  • The distance a vehicle travels whilst the brakes are applied and the vehicle is decelerating to a stop

  • Braking distance= (speed)-2/-2 x deceleration

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Factors affecting braking distance

Condition of vehicle and road:

  • Road surface conditions

  • Worn tyres

  • Brakes conditions

  • Larger mass vehicles

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Stopping distance

The sum of the thinking and braking distances

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Horizontal and vertical forces in projectile motion

The vertical and horizontal components are independent of each other

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Projectile motion, assuming no air resistance

  • Vertical velocity changes due to the acceleration of freefall

  • Vertical displacement and time of flight can be calculated with SUVAT equations

  • Horizontal velocity remains constant

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Key points on projectiles fired at an angle to the vertical

  • Only Vy is affected by acceleration of freefall so this continually changes

  • Vv= maximum at take off and just before it lands= -maximum

  • Vv= 0 at maximum height

  • The time taken to reach max height is half the total time of flight

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Vy and Vx components of velocity (projectiles fired at an angle to the vertical)

  • Vertical component= Vy = Vsinθ

  • Horizontal component= Vx = Vcosθ

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Steps to solve distance travelled (projectiles fired at angle to the vertical)

  1. Draw vector triangle, find vx and vy components

  2. Calculate time for max height

  3. Multiply by 2 to get total time

  4. Use s=d/t to find distance travelled