# P2.1 Forces and Their Effects

## P2.1.1 Resultant Forces

• When two objects interact, the forces they exert on each other are equal and opposite.
• Resultant forces:
• A number of forces acting in the same straight line or are parallel are added
• Take note of direction (a backwards force would be minus)
• May cause a change in an object's state of rest or motion
• If a resultant force is zero:
• It will remain stationary (if already stationary)
• It will travel at a constant speed
• If a resultant force is not zero:
• The object will accelerate in the direction of the resultant force
• If already moving in that direction, will get faster
• If moving in opposite direction, will slow down and eventually move in that direction
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## P2.1.2 Forces and Motion

Acceleration: F= ma

• F = Resultant force acting on the object in N
• m = Mass of the object in kg
• a = Accelerationg in m/s(squared)
• Acceleration is in the same direction as the force
• If the resultant force = zero, the acceleration = zero
• A negative force means the object is accelerating backwards or slowing down

Distance-time graphs

• The gradient of the graph represents speed
• Can be measured by choosing two points on the line and calculating the difference between the points on the y-axis divided by the difference on the x-axis
• Speed = distance / time
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## P2.1.2 Acceleration and Velocity

Velocity - An object's speed in a given direction

Acceleration- The rate of change of velocity or speed

a = v-u / t   or    a = change in velocity / t

• a = acceleration in m/s(squared)
• v = final velocity in m/s
• u = initial velocity in m/s
• t = time taken for change in velocity to happen in s

Velocity-time Graphs

• Gradient represents acceleration
• Area under the graph represents distance
• Area of triangle = 1/2bh
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## P2.1.3 Forces and Braking

When a vehicle travels at a steady speed, the resistive forces (mainly air resistance) balance the driving force

Stopping distance = thinking distance + braking distance

• Thinking distance = distance travelled in the time it takes for the driver to react
• The greater the speed of the vehicle, the further it will travel while the driver is reacting
• Can be increased when a driver is tired, has taken drugs, or is distracted by a mobile phone
• Braking distance = distance travelled while the brakes are applying a force to slow the vehicle
• The greater the speed of the vehicle, the greater the braking force needed to stop it in a certain distance
• Can be increased by adverse road and weather conditions and poor vehicle conditions
• Brakes  produce a frictional force between the brake and wheel, so the kinetic energy of the vehicle is reduced. Energy is transferred to the brakes so the temperature increases.
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## P2.1.4 Forces and Terminal Velocity

W = mg

• W = weight of the object (force exerted on it by gravity) in N
• m = mass of the objects in kg
• g = gravitational field strength in N/kg
• 10N/kg on Earth
• 1.7N/kg on the Moon

The faster an object moves through a fluid, the greater the frictional force that acts on it

• Initially, the weight of the object makes it accelerate downwards
• The resistance of the fluid creates an upwards force
• As the object accelerates, the resistance forces increase
• When the resultant force is zero, the object moves at its terminal velocity
• Constant speed

Parachutes have large surface areas, which increases the air resistance so that is larger than the weight. The resultant force is upwards, so acceleration downwards is negative.

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## P2.1.5 Forces and Elasticity

A force acting on an object may cause it to change shape, which includes stretching, compressing, bending or twisting the object into a new shape.

Elastic objects

• These objects recover their original shape when the force is removed
• e.g a spring
• When a force is applied to an elastic object (when work is done on the object to change its shape), the object will stretch and store elastic potential energy

Forces and extension

• The extension is the change in length of the object when it is stretched
• The extension of an elastic object is directly proportional to the stretching force applied to it, as long as the force doesn't exceed the limit of proportionality

F = ke

• F = applied force in N
• k = spring constant for the object in N/m
• e = extension of the object in m
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