P2.1 Forces and Their Effects

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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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