# Forces

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## Newton's Laws

1st:

• An object remains at rest or continues at a constant velocity unless an unbalanced force acts
• No force = No change

2nd:

• F= ma
• Resultant Force = mass x acceleration (See Equations)

3rd:

• Forces exist in pairs
• Every force has an equal, opposite force
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## Equations

F=ma:

• Resultant force (N) = mass (kg) x acceleration(m/s^2)

W=mg:

• Weight (N) = mass (kg) x grav. field strength (N/kg)

a=(v-u)/t:

• Acceleration(m/s^2) = (change in velocity (End velocity - start velocity)(m/s))/ time taken (secs)

F=ke:

Force (N) = spring constant (N/m) x extension (m)

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## Stopping & Starting

• When a car is moving at a constant velocity on a flat road, the force of the engine is qual to the resistive forces on the car
• When a car is decelerating on a flat road, the force of the engine is less than the resistive forces on the car
• When a car is accelerating on a flat road, the force of the engine is greater than the resistive forces on the car
• Thinking distance = the distance travelled in the time it takes the driver to react
• Braking distance = the distance travelled by the vehicle in the time in which the driver is pressing the brakes
• Factors:
• Greater speed
• Alcohol/ Drugs
• Distrations
• Thinking distance = speed
• Braking distance = speed^2
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## Resultant forces // Falling Objects (Moon)

• The leftover force in any particular direction
• This can cause a change in motion (Start moving or accelerate)
• No resultant force means that a stationary object remains stationary or a moving object remains moving at the same speed in the same direction
• The greater the mass of an object, the smaller its acceleration for a given force

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• Weight (kg) = The force of the gravity on the object
• Mass (N) = Quantity of the matter (Never changes)
• Terminal velocity = When the resultant force acting on a falling object reaches 0, so the object moves at a constant speed
• If two objects fall without air resistance, both object will hit the ground at the same time as they have the same acceleration.
• Although one has a bigger weight, it has to accelerate a bigger mass, so it cancels out
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## Falling objects (Earth)

Parachutist:

1.

• Parachutist pulled down by weight
• No speed, so no drag
• Resultant force downwards causes acceleration

2.

• Drag increases
• Still resultant downwards, but smaller
• Smaller acceleration downwards

3.

• Drag increases till equal to weight
• No resultant, no acceleration
• Terminal velocity
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## Falling objects (Earth) Part 2

4.

• Parachute opened
• More surface area to cause more drag
• Resultant force upwards causes deceleration
• Drag is smaller

5.

• Drag reduces till the forces are balanced
• No resultant, no acceleration
• Terminal velocity
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## Spring Constant (k)

• The amount of force needed to produce a certain extension (usually 1m)
• eg. Apply 20N to a spring which stretches it 37cm
• k = F/ e
• k = (20/37) X 100
• k = 54 N/m
• The spring constant indicates the stiffness of the spring
• A higher 'k' means stiffer spring
• eg. Apply 12N to a spring stretches it 15cm
• k = 12/0.15
• k = 80 N/m
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