Forces

• Scalar quantities have magnitude only.

  Vector quantities have magnitude and direction.
• What is a force?

  A push or pull that acts on an object due to the interaction with another object.
• Contact forces: the objects are physically touching

  Non-contact forces: the objects are physically separated.
• Weight is the force acting on an object due to gravity. The weight of an object depends on the gravitational field strength at the point where the object is.
• What is centre of mass?

  The point where the entire mass of an object can be considered to be concentrated. This is where the weight of the object is considered to act.
• When a force causes an object to move through a distance, work is done on the object. A force does work on an object when the force causes a displacement of the object.
• Work done against the frictional forces acting on an object causes a rise in the temperature of the object.
• 1 joule = 1 newton metre
• The extension of an elastic object, such as a spring, is directly proportional to the force applied (if limit of proportionality is not exceeded)
• To stretch, bend or compress an object, more than one force has to be applied. If a single force is applied to an object, the forces are no longer balanced so the object will only move in the direction of the force - not change shape.
• A force that stretches (or compresses) a spring does work and elastic potential energy is stored in the spring. If an object is not inelastically deformed, the work done is equal to to elastic potential energy.
• What is inelastic deformation?

  Permanently changing an object's shape due to overstretching. The object exceeds the limit of proportionality.
• What is elastic deformation?

  An object returns to its original shape when the load/force has been removed.
• What is Hooke's Law?

  The extension of an elastic object is directionally proportional to the force applied.
• Describe the graph for Hooke's law.

  Linear graph that is straight and passes through the origin.

  Stops being linear after it exceeds the limit of proportionality.
• Distance is how far an object moves. Distance does not involve direction. Distance is a scalar quantity.
• Displacement includes both the distance an object moves (measured in a straight line from the start point to the finish point) and the direction of that straight line. Displacement is a vector quantity.
• The speed at which a person can walk, run or cycle depends on many factors including: age, terrain, fitness and distance travelled. Speed is a scalar quantity.
• If an object was travelling in a circular direction, the object is constantly changing direction. Therefore, the velocity of the object changes as velocity is a vector that measures movement and direction. However, the speed stays constant.
• How do you calculate the distance travelled on a velocity time graph?

  Calculate area under the graph.
• Near the Earth’s surface any object falling freely under gravity has an acceleration of about 9.8 m/s².
• An object falling through a fluid initially accelerates due to the force of gravity. Eventually the resultant force will be zero and the object will move at its terminal velocity.
• As an object falls, the air resistance increases so the resultant forces decrease. This means acceleration (a resultant force) decreases. Acceleration will not be constant.
• What is inertia?

  The tendency of objects to continue in uniform velocity or to stay at rest.
• What is Newton's First law?

  An object has a constant velocity unless acted on by a resultant force. So, the velocity (speed and/or direction) of an object will only change if a resultant force is acting on the object.
• If there is no resultant force on an object:

  And the object is stationary, it will stay stationary.

  If the object is moving, it will continue to move at the same velocity.
• What is Newton's Second law?

  The acceleration of an object is proportional to the resultant force acting on the object, and inversely proportional to the mass of the object.
• What is inertia mass?

  The ratio of force/acceleration.

  The measure of how difficult it is to change the velocity of an object.
• What is Newton's third law?

  Whenever two objects interact, the forces they exert on each other are equal and opposite.
• The stopping distance of a vehicle is the sum of the thinking distance and the braking distance.
• A greater speed means a greater braking distance is needed to stop the vehicle.
• Stopping distance = thinking distance + braking distance
• What factors can affect thinking distance?

  Tiredness

  Drugs

  Alcohols

  Distractions

  Reaction time
• What factors affect braking distance?

  Road conditions (icy/wet) - lead to decreased friction

  Weight (more passengers)

  Poor conditions of vehicle (tyres or brakes)

  Speed of car
• How can weight of passengers affect braking distance?

  More work needs to be done to stop the car due to increased weight, momentum increases. This increases the braking distance.
• The greater the braking force the greater the deceleration of the vehicle. Large decelerations may lead to brakes overheating and/or loss of control.
• How can high speeds affect braking distance and cause a loss of control?

  A greater speed means a greater braking force is needed to stop the car. A greater force means a greater acceleration. This means more force is applied to the brake. More work is done due to the friction of the brakes which reduces the kinetic energy of the vehicle - causing the temperature of the brake to increase. This can cause brakes to overheat which can cause a loss of control of a vehicle.
• When a force is applied to the brakes of a vehicle, work done by the friction force between the brakes and the wheel reduces the kinetic energy of the vehicle and the temperature of the brakes increases.
• What is conservation of momentum?

  In a closed system, the total momentum before an event is equal to the total momentum after the event.
• A train collides with a stationary carriage on the track. Explain why the velocity of the train after the collision is less than it was before the collision.

  The momentum is conserved during the collision. During the collision, the momentum of the carriage increases so the momentum of the train decreases. Because the mass is constant, the velocity of the train decreases (momentum = mass x velocity).