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AS PHYSICS – MODULE 3: FORCES AND MOTION
3.1 Motion
3.1.1 Kinematics
Quantity Definition S.I. unit Equation
Rate of change of distance → distance and
Speed ms-1
speed are scalars (magnitude only)
Distance moved in a particular direction from a
Displacement m s = vt
reference point → vector
Velocity Rate of change of displacement → vector ms-1
Acceleration Rate of change of velocity → vector ms-2
Motion graphs
• Displacement-time graph → displacement = y axis, time = x axis
• Instantaneous speed/velocity – speed/velocity of car
over very short time period
• Found by drawing the tangent to the d-t graph at that time
• Then work out the gradient of the tangent
• Greater gradient = greater instantaneous speed
1|T. Chaudhary
,2
• Velocity-time graph → velocity = y axis, time = x axis
The gradient of the
velocity time graph is
the acceleration.
The area under the
velocity time graph is
the displacement.
3.1.2 Linear Motion
• Suvat equations:
Investigating motion & collisions of objects:
• Apparatus → trolleys, tick timers, light gates, data-loggers, air-track gliders and
video techniques
A light gate
• Timer starts when the leading edge of the card breaks the light beam and it stops
when the trailing edge passes through
• Speed calculated by dividing the card length by the time taken for it to go through
the light gate
Trolley on a track with ticker-tape
• Track tilted until trolley will run down at a steady speed (occurs when dots on
ticker tape are evenly spread) → to compensate friction
• Different forces → using two or three identical pieces of elastic
• Different masses → stacking trollies on top of each other
• Analysing results: ticker-tape cut into 10-dot lengths → 50 dots produced every
sec ∴ average velocity every 0.2s calculated → acceleration calculated
• Even dot spacing = constant speed; increasing dot spacing = acceleration;
decreasing dot spacing = deceleration
2|T. Chaudhary
, 3
Stopping distance
• Stopping distance – thinking distance + braking distance
• Thinking distance – distance travelled by the vehicle from when the driver first
sees a reason to stop to when the brakes are applied
• Braking distance – distance travelled by the vehicle from when the brakes are
applied to when the vehicle stops
Factors that influence these:
• Thinking distance – CAST (concentration, alcohol, speed, tired)
• Braking distance – MMRST (mass, maintenance, road conditions, speed, tyres)
• Thinking distance = speed x reaction time
▪ Greater the speed or reaction time, the further a vehicle will travel before its
driver applies the brakes
▪ Thinking distance increases linearly with speed if reaction time is constant
(speed x2 → distance x2)
• When brakes are applied, the KE of the car is transferred into heat and sound
energy due to the force of friction
• K.E of car = work done by frictional force (force from road on car)
• ½ mv2 = Fx where x = braking distance (m)
▪ Braking distance increases as a squared relationship if mass and force are
constant (speed x 2 → distance x 22 = x 4)
Free fall and g
• Free fall – when an object is accelerating under gravity, with no other force
acting on it
• Acceleration of free fall (g) = 9.81ms-2
• Varies according to altitude, latitude and geology (i.e. density of rocks on surface)
3|T. Chaudhary
AS PHYSICS – MODULE 3: FORCES AND MOTION
3.1 Motion
3.1.1 Kinematics
Quantity Definition S.I. unit Equation
Rate of change of distance → distance and
Speed ms-1
speed are scalars (magnitude only)
Distance moved in a particular direction from a
Displacement m s = vt
reference point → vector
Velocity Rate of change of displacement → vector ms-1
Acceleration Rate of change of velocity → vector ms-2
Motion graphs
• Displacement-time graph → displacement = y axis, time = x axis
• Instantaneous speed/velocity – speed/velocity of car
over very short time period
• Found by drawing the tangent to the d-t graph at that time
• Then work out the gradient of the tangent
• Greater gradient = greater instantaneous speed
1|T. Chaudhary
,2
• Velocity-time graph → velocity = y axis, time = x axis
The gradient of the
velocity time graph is
the acceleration.
The area under the
velocity time graph is
the displacement.
3.1.2 Linear Motion
• Suvat equations:
Investigating motion & collisions of objects:
• Apparatus → trolleys, tick timers, light gates, data-loggers, air-track gliders and
video techniques
A light gate
• Timer starts when the leading edge of the card breaks the light beam and it stops
when the trailing edge passes through
• Speed calculated by dividing the card length by the time taken for it to go through
the light gate
Trolley on a track with ticker-tape
• Track tilted until trolley will run down at a steady speed (occurs when dots on
ticker tape are evenly spread) → to compensate friction
• Different forces → using two or three identical pieces of elastic
• Different masses → stacking trollies on top of each other
• Analysing results: ticker-tape cut into 10-dot lengths → 50 dots produced every
sec ∴ average velocity every 0.2s calculated → acceleration calculated
• Even dot spacing = constant speed; increasing dot spacing = acceleration;
decreasing dot spacing = deceleration
2|T. Chaudhary
, 3
Stopping distance
• Stopping distance – thinking distance + braking distance
• Thinking distance – distance travelled by the vehicle from when the driver first
sees a reason to stop to when the brakes are applied
• Braking distance – distance travelled by the vehicle from when the brakes are
applied to when the vehicle stops
Factors that influence these:
• Thinking distance – CAST (concentration, alcohol, speed, tired)
• Braking distance – MMRST (mass, maintenance, road conditions, speed, tyres)
• Thinking distance = speed x reaction time
▪ Greater the speed or reaction time, the further a vehicle will travel before its
driver applies the brakes
▪ Thinking distance increases linearly with speed if reaction time is constant
(speed x2 → distance x2)
• When brakes are applied, the KE of the car is transferred into heat and sound
energy due to the force of friction
• K.E of car = work done by frictional force (force from road on car)
• ½ mv2 = Fx where x = braking distance (m)
▪ Braking distance increases as a squared relationship if mass and force are
constant (speed x 2 → distance x 22 = x 4)
Free fall and g
• Free fall – when an object is accelerating under gravity, with no other force
acting on it
• Acceleration of free fall (g) = 9.81ms-2
• Varies according to altitude, latitude and geology (i.e. density of rocks on surface)
3|T. Chaudhary
