Forces

Forces

1. Forces between objects (key points)

   • A force can change the shape of an object or change its motion or its state of rest.
   • The unit of force is the newton (N).
   • When two objects interact, they always exert equal and opposite forces on each other.

2. Resultant force (key points)

   • The resultant force is a single force that has the same effect as all the forces acting on an object
   • If two forces act on an object along the same line the resultant force is:
     • 1) their sum if the forces act in the same direction
     • 2) their difference if the forced act in opposite directions
   • If the resultant force on an object is zero, the object stays at rest or at constant velocity. If the resultant fore on an object is zero, the velocity of the object will change

3. Forces and acceleration (key points)

   • The bigger the resultant force on an object is, the greater its acceleration is.
   • The greater the mass of an object is, the smaller its acceleration is for a given force
   • Resultant force(newtons, N) =
     • mass(kilograms) x acceleration(metre/second/second)

4. On the road (key points)

   • Friction and air resistance oppose the driving force of a car.
   • High speed, poor weather conditions and poor maintenance all increase the braking distance.
   • Poor reaction time and high speed both increase the thinking distance.
   • The stopping distance of a car depends on the thinking distance and the braking distance.

5. Falling objects (key points)

   • The terminal velocity of a falling object is the velocity it reaches when it is falling in a fluid.
     • The weight is then equal to the drag force on the object
   • An object acted on only by gravity accelerates at about 10m/s/s
   • The weight of an object is the force of gravity on it. Its mass is the quantity of matter in it.

6. Forces and speed issues (key points)

   • Ant-skid surfaces increase the friction between a car tyre and the road surfaces. This reduces skids, or even prevents skids altogether.
   • Fuel economy of road vehicles can be improved by reducing the speed or fitting a wind deflector.
   • Average speed cameras are linked in pairs and they measure the average speed of a vehicle.

7. Stretching and squashing (key points)

   • The spring constant of a spring is the force per unit extension needed to stretch it.
   • The extension of a spring is directly proportional to the force applied to it, provided the limit of proportionality is not exceeded.
   • The extension is the difference between the length of the spring and its original length.