P2 Specification

Forces and their effects

Forces can cause changes to the shape or motion of an object.

Objects can move in a straight line at a constant speed.

They can also change their speed and/or direction (accelerate or decelerate)

Graphs can help us to describe the movement of an object ~ these may be distance - time graphs or velocity - time graphs

Candidates should use their skills knowledge and understanding to:

  • interpret data from tables and graphs relating to speed, velocity and accleration
  • evaluate the effects of alchohol and drugs on stopping distances
  • evaluate how the shape and power of vehicle can be altered to increase the vehicle's top speed
  • draw and interpret velocity-time graphs for ojects that reach terminal velocity, including a consideration of the forces acting on the object
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Resultant Forces

Resultant Force - the single force that would have the same effect on an object as all the forces that are acting on the object

Whenever two objects interact, the forces they exert on each other are equal and opposite

A number of forces acting at a point may be replaced by a single force that has the same effect on the motion as the original forces acting together  ( resultant force)

A resultant force acting on an object may cause a change in its state of rest or motion  -Candidate should be able to determine the resultant of opposite or parrarel forces acting on a straight line

If the resultant force acting on a stationary object is:

  • Zero, the object will remain stationary
  • not zero, the object will accelerat in the direction of he resultant force

If the resultant for acting on a moving object is:

  • zero, the object will continue to move at the same speed in the same direction
  • not zero, the object will accelerate in the direction of the resultant force
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Forces and motion

The acceleration of an object is determined by the resultant force acting on the object and the mass of the object                                            a = F/m or F = m X a                                                                                                F is the resultant force in newtons, N

                                                                                               m is the mass in kilograms, kg

                                                                                               a is the acceleration in metres per second squared, m/s2 

The gradient of a distance-time graph represents speed Candidates should be able to construct distace-time graphs for an object moving in a  straight line when the body is stationary or moving with a constant speed

HT only: Candidates should be able to work out the speed of an object from the gradient of a distance-time graph

The Velocity of an object is its speed in a given direction

The acceleration of an object is given by the  equation:    v-u                                                                                        a = t

                                                                                                              a is the acceleration in metres per second suared, m/s2

                                                                                                              v is the fina l velocity in metres per second, m/s

                                                                                                              u is the initial velocity in metres per second, m/s

                                                                                                              t is the time taken in seconds, s

The gradient  of a velocity-time graph represents acceleration

HT only: candidates should be able to calculate:

  • The acceleration of an object from the gradient of a velocity-time graph  
  •  The distance travelled by an object from a velocity-time graph
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Forces and braking

When a vehicle travels at a steady speed the resistive forces balance the driving force - candidates should realise that most of the resistive forces are caused by air resistance

The greater the speed of a vehicle the greater the braking force needed to stop in a certain distance - candidates should understand that for a given brkaing force the greater the speed, the greater the stopping distance

The stopping distance of a vehicle is the sume of the distance the vehicle travels during the dirvers reaction time (thinking distance) and the distance it travels under the braking force (braking distance) 

A driver's reaction time can be affected by tiredness, drugs and alchohol - distractions may affect the driver's ability to react

When the brakes of a vehicle are applied, work done by the friction force between the brakes and the wheel reduces the kinetic energy of the vehicle and the tempurature of the brakes increase

A vehicle's braking distance can be affected by adverse road and weather conditions and poor conditions of the vehicle. - 'adverse road conditions' includes wet or icy conditions. Poor coditions of the car is limited to the car's brakes or tyres

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Forces and terminal velocity

The faster and object moves through a fluid the greater the frictional forces that acts on it

An object falling through a fluid will initially accelerate due to the force of gravity, Eventually the resultant force will be zero and the object will move at its terminal velocity (steady speed) - candidates should understand why the use of a parachute reduces the parachutists's terminal velocity

Candidates should be able to draw and interpret velocity-time graphs for objects that reach terminal velocity, including a consideration of the forces acting on the object

The calculation for the weight of an object using the foce exerted on it by a gravitational force is:  W= m X g  

                                                             W  is the weight in newtons, N

                                                             m  is the mass in kilograms, kg

                                                             g  is the gravitational field stranght in newtons per kilogram, N/Kg

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Forces and elasticity

A force acting on an object may cause such as a change in shape of the object

A force applied to an elastic object such as a spring will result in the object streching and storing elastic potential energy - calculation of the energy stored when streching an elastic material is not required

For an object that is able to recover its original shape, elastic potential energy is stored in the object when work is done on the object to change its shape

The extension of an elastic object is directly proportional to the force applied, provided that the limit of proportionality is not exceeded: F = k  X e          F is the force in newtons,N                                                                                 is the spring constant in newtons per metre, N/m                                                                                 e is the extension in metres, m

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The Kinetic energy of objects speeding up or slowi

When an object speeds up or slows down, its kinetic energy increases or decreases. The forces which cause the change in speed do so by doing work. The momentum of an object is the product of the objects mass and velocity

Candidates should use their skills, knowledge and understanding to:

  • evaluate the benefits of different tyes of braking systems, such as regenerative braking
  • evaluate the benefits of air bags, crumple zones, seat belts and side impact bars in cars - this should include ideas of both energy and momentum changes
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Forces and energy

When a force casues an object to move throgh a distance work is done

Work done, force and distance are related by the equation: W=Fxd

                                                               W is the work done in, J

                                                               F is the force apllied in newtons,N

                                                               d is the distance moved in he dorection of the force in metres, m

Energy is transfered when work is done - candidates should be able to discuss the transfer of kinetic energy in particular situations. Examples might includeshuttle re-entry or meteorites burning up in the atmosphere

Power is the work done or energy transfered in a given time     P=E/t

                                                                         P is the power in watts,W

                                                                         E is the energy transferred in joules, J

Gravitational potential energy is the energy that an object has by virtue of it position in a gravitational field - candidates should understand that when an object is raised vertically work is done against gravitational force and the object gains gravitational potential energy       Ep= m x g x h

                                                  Epis the charge in gravitational potential energy in joules, J

                                                                m is the mass in kilograms,kg

                                              gis the gravitational field strength in newtons per kilograms, N/Kg

                                                                his the change in height in metres, m

The kinetic energy of an object depends on its mass and its speed. Ek = 1/2 x m x v2

                                                              Ek is the kinetic energy in joules, J

                                                              m  is the mass in kilograms

                                                              vis the speed in metres per second, m/s

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