Explaining Motion

P4

Speed

• In real life, it’s pretty rare for an object to go at exactly the same speed for a long period of time

• Objects usually start off moving 0 m/s but accelerate up to speed

• It’s more useful to know the average speed, which is the same formula as for normal speed

Speed, Distance and Velocity• Gradient = Speed• Curves

– Acceleration– Deceleration

• Displacement can be positive or negative – The object can be going in one direction

or the opposite direction

• A distance given in a particular direction is called displacement

• Speed is just a number, but velocity has direction too

• Velocity is sometimes more useful when measuring motion

• Instantaneous velocity is its speed and direction at a given moment in time

• Can be negative velocities

Distance time graph:

Acceleration and Velocity

• Acceleration is the change in velocity or speed

• Deceleration is negative acceleration

• The units for acceleration are m/s2

• Gradient = Acceleration• Speed-time graphs ignore

direction but velocity-time graphs don’t

Forces and Friction

Forces• Forces occur when two

objects interact • Interaction pair

– When two forces interact

• An object resting on a surface experiences a reaction force – The upward force on the

object, response

Friction• Moving objects normally

experience friction • Friction opposes a movement • A reaction force • A frictional force will match

the size of the force trying to move an object, up to a maximum point. After this point the friction will become less than the other force allowing the object to move

Types of Friction

• Friction between solid surfaces which are gripping – This friction allows people to walk – If there was no friction people would just slip and slide

• Friction between solid surfaces which are sliding past each other – This kind of friction can be reduced but putting a lubricant between

the surfaces • Resistance or ‘drag’ from fluids, liquids or gasses

– Drag is basically the same as the other types of friction – An object pushing through molecules, causes friction – Drag only happens if an object is moving through a fluid – There is no drag in space

Forces• Arrows show the size and direction of forces

– The length of the arrow shows the size of the force – The direction of the arrow shows the direction of the force

• If the forces are opposite and equal then the forces are balanced • The reaction on a surface – Balanced forces

– If an object is resting on a surface, it’s weight causes a downward force – This causes an equal reaction force to push up on the object– The two forces are opposite and the same size so they are equal interaction pairs, meaning the

object won’t move • Steady speed – Balanced forces

– If an object is moving at a steady speed then the forces must be balanced – Just because something is moving doesn’t mean that there’s an overall force acting on it – unless it’s

changing speed or direction, the overall (resultant) force is zero• A resultant force is the difference between the two forces in an interaction pair

– Overall force acting on an object – Decides the motion of the object – Accelerate, decelerate, stay at a steady speed – If there is a resultant force acting on an object, its speed / direction (or Both) changes

Forces and Momentum• Acceleration – Unbalanced forces • The bigger the resultant fore, the greater

the acceleration • The grater the mass of an object, or the

greater it’s velocity, the more momentum the object has

• Momentum is a vector quantity – It has size and direction (like velocity but

not speed)

• A resultant force of zero means that a stationary object will stay still

• If the object was moving, it stays at a constant velocity and constant momentum

• If the resultant force on an object is not zero, its momentum changes in the direction of the force

Change in Momentum and Force• The change in momentum

depends on the force – When a resultant force acts on an

object, it causes a change in momentum in the direction of the force

– The change of momentum it causes is proportional to the size of the force and the time it acts for

• The bigger the force and the longer it acts for, the bigger the change in momentum

• Car safety features increase time to reduce forces in collisions

Work• Work = a measure of the

amount of energy being transferred from one place to another

• Doing work = when you push or pull something to make it work

• When lifting something vertically…– Force = weight of the object lifted

• Some energy is lost as heat or noise so the object doesn’t have the full energy transferred

Kinetic Energy (K.E.)• Is energy of movement • Any moving object has K.E.• Depends on mass and speed

– The greater its mass and the aster it’s going, the bigger its kinetic energy

• To increase K.E., the velocity must be increased– So force must be applied

• Thus work is being done on the object

• K.E. is just movement energy – If work is done on an object and it doesn’t

accelerate then the kinetic energy is not increased

• The increase in an object’s K.E. is normally a bit less that the amount of work done on it, because some energy is wasted as heat – If there is no friction or air resistance acting on

an object, OR you’re told to ignore it, then the increase in an object’s K.G. is equal to the amount of work done on it

Gravitational Potential Energy (G.P.E.)

• The energy stored in an object when it is raised to a height against the force of gravity– When an object is lifted, its

G.P.E. increases– When an object falls its G.P.E.

decreases

• The increase in G.P.D. is equal to the amount of work done by the lifting force in order to raise the height

• K.E. gained = G.P.D. lost– If friction and air resistance is

ignored