Forces

• Weight is the force acting on an object due to gravity. The force of gravity close to the Earth is due to the gravitational field around the Earth
• The weight of an object depends on the gravitational field strength at the point where the object is.
• weight  = mass  × gravitational field strength
• The weight of an object may be considered to act at a single point referred to as the object’s centre of mass
• The weight of an object and the mass of an object are directly proportional.
• Weight is measured using a calibrated spring-balance (a newtonmeter).
• A number of forces acting on an object may be replaced by a single force that has the same effect as all the original forces acting together. This single force is called the resultant force.
• A single force can be resolved into two components acting at right angles to each other. The two component forces together have the same effect as the single force
• When a force causes an object to move through a distance work is done on the object. So a force does work on an object when the force causes a displacement of the object.
• work done = force × distance
• One joule of work is done when a force of one newton causes a displacement of one metre.
• Work done against the frictional forces acting on an object causes a rise in the temperature of the object.
• A force or a system of forces may cause an object to rotate.
• The turning effect of a force is called the moment of the force.
• distance, d, is the perpendicular distance from the pivot to the line of action of the force, in metres, m.
• If an object is balanced, the total clockwise moment about a pivot equals the total anticlockwise moment about that pivot.
• A simple lever and a simple gear system can both be used to transmit the rotational effects of forces.
• The pressure in fluids causes a force normal (at right angles) to any surface.
• The water at the bottom of a container is at a higher pressure than the water at the top. Because as the depth increases there is a greater weight of liquid acting downwards. Pressure also increases with the density of a liquid. Liquids with a greater density have a greater weight acting downwards
• For an object to float the upthrust must be equal to the objects weight. If the upthrust is less than the objects weight then the object sinks
• The size of the upthrust acting on the object is the same as the weight of water displaced by the object
• The atmosphere is a thin layer (relative to the size of the Earth) of air round the Earth. The atmosphere gets less dense with increasing altitude.
• Air molecules colliding with a surface create atmospheric pressure. The number of air molecules (and so the weight of air) above a surface decreases as the height of the surface above ground level increases. So as height increases there is always less air above a surface than there is at a lower height. So atmospheric pressure decreases with an increase in height.
• walking ̴ 1.5 m/s running ̴ 3 m/s cycling ̴ 6 m/s.
• The speed of a moving object is rarely constant. When people walk, run or travel in a car their speed is constantly changing.
• It is not only moving objects that have varying speed. The speed of sound and the speed of the wind also vary. A typical value for the speed of sound in air is 330 m/s.
• Car on a main road 13 m/s , Fast train in UK 50 m/s , Aeroplane 250 m/s
• If an object is moving at a constant speed in a circle then its velocity is constantly changing even though its speed is constant.
• The acceleration of an object is its change in velocity in a given time.
• The gradient of a velocity time graph is the acceleration of the object
• A straight line on a velocity time graph means the object is moving at a constant velocity
• The total area under a velocity time graph is the distance travelled in a specific direction - displacement
• Near the Earth’s surface any object falling freely under gravity has an acceleration of about 9.8 m/s2 .
• As a skydiver falls they experience an upward force of friction with the air particles - air resistance. After some time the force of air resistance balances the force due to gravity. The object stops accelerating and moves at a constant velocity. This is called terminal velocity. This applies to any object falling through a fluid
• The terminal velocity that the object reaches depends on the object. Some objects experience a greater force of friction due to their shape so they will have a lower terminal velocity
• Newton’s First Law:

  If the resultant force acting on an object is zero and:

  • the object is stationary, the object remains stationary

  • the object is moving, the object continues to move at the same speed and in the same direction. So the object continues to move at the same velocity.
• So, when a vehicle travels at a steady speed the resistive forces balance the driving force.
• So, the velocity (speed and/or direction) of an object will only change if a resultant force is acting on the object.
• The tendency of objects to continue in their state of rest or of uniform motion is called inertia.
• 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.