How and why things move

Revision Links: Revision Questions Set 1   Revision Questions Set 2     Formulae Summary

 

Ge3.1   How can we describe motion?

 

How can I measure how fast something moves?

 

I should be able to:

 

·         Use and apply the equation for average speed:

 

Speed= distance moved
             Time taken



 ·         distinguish between average speed and instantaneous speed (in effect, an average over a short time interval) for examples of motion where speed is changing.

·         interpret the motion of an object moving along a straight line from a graph showing how its distance from the starting point changes with time;

·         recognise that distances in one direction are positive, and in the other, negative;

·         draw and/or interpret the shape of a distance-time graph for an object that is:

-stationary;

-moving at constant speed;

-moving with increasing or decreasing speed;

 

·       interpret a steeper gradient of a distance-time graph as a higher speed;

 

·         calculate a speed from the gradient of a straight section of a distance-time graph.

 

How can I show which direction the object is moving in?

 

I should be able to:

 

·         appreciate that the velocity of an object at any instant is its speed plus an indication of the direction;

·         interpret the velocity of an object moving in a straight line as positive if it is moving in one direction and negative if it is moving in the opposite direction;

·         relate the motion of an object travelling in a straight line to its representation by a velocity-time graph;

·         draw and/or interpret the shape of a velocity-time graph for an object that is:

·         stationary;

·         moving in a straight line with constant speed;

·         moving in a straight line with steadily increasing or decreasing speed;

·         relate these ideas about recording motion to applications such as lorry tachographs.

 

Ge3.2           What are forces?

 

How do forces arise?

 

I should be able to:

 

·         describe a force as arising from an interaction between two objects;

·         appreciate that when one object exerts a force on another, it always experiences a force in return;

·        appreciate that the two forces in an interaction pair are equal in size and opposite in direction; they act on different objects.

 

How can I describe the different forces involved?

 

I should be able to:

 

·         recall that some forces arise in response to the action of an applied force;

·         discuss the interaction between an object and a surface it is resting on; the object pushes down on the surface; the surface pushes up on the object with an equal force; this is called the reaction of the surface;

·         discuss the interaction between two surfaces which slide (or tend to slide) relative to each other; each surface experiences a force in the direction which prevents (or tends to prevent) relative movement; this interaction is called friction.

 

How can I identify forces in a variety of situations and work out what will happen?

 

I should be able to:

 

·         identify forces arising from an interaction between two objects;

·         identify the ‘partner’ of a given force (i.e. the other force of the interaction pair);

·         specify, for each force, the object which exerts it, and the object on which it acts;

·         use the idea of equal and opposite pairs of forces to explain in outline how rockets and jet engines work.

 

Note : You  should always use arrows to show the directions of forces acting.

 

 

Ge3.3          What is the connection between forces and motion?

 

How exactly do forces change motion?

 

I should be able to:

 

·         discuss situations in which several forces act on an object;

·         identify, for everyday situations involving horizontal and vertical forces only, the forces on each individual object involved (e.g. an object sitting on a surface or hanging from a string; an object being pushed or pulled across a level surface);

·         relate the resultant force on an object to the sum of all the individual forces acting on it, taking their directions into account;

·         recognise that if a resultant force acts on an object, it causes a change of momentum in the direction of the force;

·         recall that the momentum of an object is defined as:

momentum (kilogram metre per second, kg m/s)   =  mass (kilogram, kg) × velocity (metre per second, m/s);

·         appreciate that the size of the change of momentum is related to the size of the resultant force and the time for which it acts in the following way:

change of momentum (kilogram metre per second, kg m/s)
= resultant force (newton, N) x time for which it acts (second, s)

 

How can I use these ideas to understand road safety measures?

 I  should understand that the horizontal motion of objects (like cars and bicycles) can be analysed in terms of a driving force (produced by the engine or the cyclist), and a counter force (due to friction and air resistance). 

 For a moving object, if the driving force is:

 greater than the counter force, the vehicle will speed up;

equal to the counter force, the vehicle will move at constant speed in a straight line;

smaller than the counter force, the vehicle will slow down.

 

 I should be able to:

 

·         appreciate that in situations involving a change in momentum (such as a collision), the longer the duration of the impact, the smaller the average resultant force (F) involved:

·         use this idea to discuss and explain the action of road safety measures, such as car seat-belts, crumple zones, air bags, cycle and motorcycle helmets.

·         appreciate that if the resultant force on an object is zero, its momentum does not change (if it is stationary, it stays at rest; if it is already moving, it continues at a steady speed in a straight line).

 


Ge3.4        How can we describe motion in terms of energy transfers?

 

How does work done on an object affect its energy?

I should know that the energy of a moving object is called kinetic energy and appreciate that as an object falls, its gravitational potential energy decreases.

 

I should be able to:

 

·         recall that when a force causes movement of an object, work is done;

·         recall and use the equation:

 

o        work done by a force =  force   ´   distance moved by the force
                        (joules, J)          (kilograms, kg)  (metres, m)

 

·         appreciate that when work is done on an object, the energy of the object increases and when work is done by an object, the energy of the object decreases according to the relationship:

·         change in energy = work done;

 

·         understand that when an object is lifted to a higher position above the ground, work is done by the lifting force against the gravitational force acting on the object (its weight); this increases the object’s gravitational potential energy (GPE);

 

·         recall and use the equation:

·         change in GPE  =  weight  ´  vertical height difference
            (joules, J)          (newtons, N)     (metres, m)

 

·         recognise that when a force acting on an object causes its velocity to increase, this increases the kinetic energy of the object; the force is said to do work on the object;

 

·         appreciate that the greater the mass of an object and the faster it is moving, the more kinetic energy it has;

 

·         recall and use the equation:

·          kinetic energy = ½  ´     mass     ´     [velocity]2
                             (joules, J)     (kilograms, kg)   (metres per second, m/s)2

 

·         explain that if friction and air resistance can be ignored, the object’s kinetic energy increases by an amount equal to the work done by the force;

 

·         appreciate that air resistance or friction may mean that the gain in kinetic energy is less than the work done, because some energy is dissipated through heating;

 

·         recognise that energy is always conserved;

 

·        calculate the gain in kinetic energy, and the speed, of an object that has fallen through a given height.