Forces and Motion

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Terminal Velocity

Explain why a falling object reaches terminal velocity.

As an object travels through the air, initially, weight is the only force acting upon it meaning it accelerates downwards at 10m/s^2. However as the speed of the object increases, so does the drag force. This process repeats until both the forces are balanced and the resultant force is zero. Due to the resultant force being zero, the acceleration is also zero. This means that the object is travelling at a constant speed known as terminal velocity.

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Resultant Force

The resultant force is the overall force acting on an object after all the forces have been taken away from each other.

E.g. Applied forces 20N drag and 40N acceleration = resultant force of 20N

Resultant force = difference between the forces acting on an object

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Stopping distances

Stopping distance = Thinking distance + Braking distance

> Thinking distance = reaction time x initial speed

Drinking alcohol affects thinking distance - Increases reaction time

Road conditions affect braking distance - It takes longer for the car to stop if slippy. If uneven it decreases the braking time.

Drugs affect thinking distance - Increases reaction time

Tiredness affects thinking distance - " "

Initial speed affects braking distance - Higher speed = increased braking distance

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Graphs

Distance time graphs:

> Gradient gives speed (zero gradient means object is stationary)

> Constant gradient means constant speed

> Changing gradient indicates acceleration

> Gradient levelling off means deceleration

Velocity time graphs:

> Gradient gives acceleration (zero gradient means constant speed, no acceleration)

> Staight diagonal line means constant acceleration

> The area under the graphs gives the distance travelled

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Acceleration

Acceleration = change in velocity/time

Caused by a force e.g gravity or friction acting on an object

When forces are balanced then acceleration is impossible. It will either be stationary or travel at a constant velocity

How much an object accelerates depends on the force acting on it and the mass of the object

Acceleration = force/mass

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Equations

Weight (N) = mass (kg) x gravity (m/s^2)

Average speed (m/s) = distance (m) / time (s)

Acceleration (m/s^2) = change in velocity/time

Acceleration = force/mass (therefore Force = mass x acceleration)

Resultant force = difference between forces acting on an object

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

Stopping distance = thinking distance + braking distance

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