# Forces

What forces are there and what is their effect on objects.

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## Resultant forces

The resultant force is the sum of the forces acting on an object.

If the resultant force is zero, the object is stationary or continues moving at a constant speed. If the resultant force does not equal zero, the object accelerates in the direction of force. This can cause the object to speed up, slow down or change direction.

The bigger the resultant force, the bigger the acceleration. Double the resultant force, double the acceleration.

The bigger the mass, the slower the acceleration. Double the mass, halve the acceleration.

F = m x a

F = resultant force (N)

m = mass (kg)

a = acceleration (m/s2)

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## Forces and motion

Forces and motion can be represented in distance-time graphs and velocity-time graphs.

Distance-time graphs - gradient shows speed. Steeper gradient = faster. Horizontal line = stationary.

Velocity-time graphs - gradient shows acceleration. Area under shows distance. Horizontal line shows constant velocity.

a = (v-u) / t

a = acceleration (m/s2)

v = velocity (m/s)

u = initial velocity (m/s)

t = time (sec)

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## Forces and braking

Forces acting on a vehicle:

• Weight
• Reaction force (road pushing up)
• Driving force
• Friction
• Air resistance

Stopping distance = thinking distance + braking distance

Thinking distance can be effected if the diver's tired, distracted, drunk, taken drugs.

Braking distance can be effected if the brakes are worn, the road is poor, the weather is poor.

Acceleration - bigger driving force

Deceleration - smaller driving force and/or larger resistant forces

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## Terminal velocity

Terminal velocity is when the resultant force acting on a falling object is zero so it falls at a constant speed.

Forces acting on a falling obect are weight and air resistance.

Three stages of falling:

• acceleration - no air resistance
• gains speed - air resistance increases - resultant force downwards
• terminal velocity - weight and air resistance balance - resultant force zero

Terminal velocity is affected by the objects mass. The larger the mass, the longer it takes to acheive terminal velocity.

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## Forces and elasticity

When an elastic object is stretched or squashed, it stores elastic potential energy.

Work is done when an elastic object changes shape.

Hooke's law says that the extention of an elastic object is directly proportional to the force applied to it.

F = k x e

F = force (N)

k = spring constant (N/m)

e = extension (m)

Spring constant = length of extended spring - original length of spring

Hooke's law works until the spring reaches its elastic limit (limit of proportionality).

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