Forces and Motion
Contents
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Velocity and Acceleration
D-T Graph
S-T Graph
Newton’s Laws of Motion
Forces and Acceleration
Velocity and Acceleration
Speed = how fast something travels
 Velocity = how fast something travels in a direction
 Acceleration = change in speed in a given time
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d
s
v-u
t
a
t
v = final velocity, u = initial velocity
D-T Graph
Distance (m)
Decelerating
Stopped
Accelerating
Steady speed
Time (secs)
S-T Graph
Speed (m/s)
Decreased
acceleration
Steady speed
Increased
acceleration
Deceleration
Acceleration
Time (secs)
Newton’s Laws of Motion
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1st Law: Balanced forces = constant velocity
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Steady Forces:
- Vertical (e.g. parachutist at terminal velocity)
weight force = drag force
- Horizontal (e.g. car moving at steady speed)
thrust force = drag force
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2nd Law: Resultant force = acceleration in that direction
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Acceleration: starting, stopping, speeding up, slowing down,
changing direction
Newton’s Laws of Motion
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3rd Law: every action has an equal and opposite reaction
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E.g. a book on a table
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The book’s weight (mass x gravity) pushes down on the table
The table pushes back with a reaction force
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Size of action (weight) = size of reaction
Gravity on Earth, g = 10m/s2
Gravity acts towards the centre of the Earth
Forces and Acceleration
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Resultant Force = overall unbalanced force
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Any resultant force produces acceleration
F
m
a
F = force (N)
m = mass (kg)
a = acceleration (m/s2)
1) A car travels at a steady speed… the thrust force is
equal to the drag force. This does not necessarily mean
that the car is stationary, but it does mean that the car is
neither accelerating nor decelerating.
2) A car accelerates from rest.
The forward arrow (thrust) is larger than the backward
arrow (drag). The car will accelerate in the direction of
the bigger arrow
Summary
distance (m) = speed (m/s) x time (s)
 Velocity is speed with a direction e.g. 10m/s Northwards
 Acceleration (m/s2) = change in speed (m/s) / time taken (s)
 a = (v-u)/t
 Balanced forces = constant velocity (this can be stationary)
 Unbalanced forces (resultant) = change in velocity (+ve / –ve)
 Every action force has an equal and opposite reaction force
 Force (N) = mass (kg) x acceleration (m/s2)
 The difference between opposite forces = resultant force. An
object will accelerate / decelerate according to its size
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