P4 Explaining Motion

P4 for the science 4's exam

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Speed is just the distance travelled in a certain time.

The find speed you need to know the distance and the time.

Speed = Distance/Time

Speed is measured in metres per second.

The speed of an object normally changes.

Instantaneous speed is still an average, just an average over a really short time period.

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Speed and Velocity

One way to look at the movement of an object is on a distance time graph.

Distance time graph:

  • The gradient of the graph = the speed (gradient = vertical/horizontal)
  • The flat sections are where the moving object has stopped
  • The steeper the graph, the faster its travelling
  • 'downhill' section means the object is going back to the starting point
  • Curves represent acceleration or deceleration
  • Steepening curve = speeding up
  • Levelling off curve = slowing down.

Speed is only a size- scalar quantity. Velocity has direction as well as speed- vector quantity.

Distances can be positive or negative (going in one direction +, the other -)

Speed is how fast the object is travelling- direction isn't important. Velocity describes both speed and direction.

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Velocity has direction which can be positive or negative.

If 2 objects are heading in opposite directions, you can say that one has a positive velocity and the other a negative velocity.

Velocity time graphs:

  • The gradient on the graph represents acceleration.
  • Flat sections represent steady speed.
  • The steeper the graph, the greater the acceleration or deceleration.
  • Uphill sections (/) acceleration, downhill section (\) deceleration.

The area under any section of the graph is equal to the distance travelled in that time interval.

A curve means changing acceleration.

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Forces occur when 2 objects interact.

When an object exerts a force on an object, it always experiences a force in return. These 2 forces are called an interaction pair.

Eg. You push on a wall, the wall will push back against you just as hard, you stop pushing, so will the wall. If the interaction force was not there, you and the wall would fall over.

An object resting on a surface experiences a reaction force.

Eg. If a book is on a table, the weight of the book acts downwards on the table and the table exerts an equal and opposite force upwards on the book. This upwards force is the reaction force.  

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Moving objects normally experience friction.

When an object is moving relative to another, both objects experience a force in the direction that opposes the movement. This force is friction.

3 types of friction:

Friction between solid surfaces which are gripping (static friction). The objects are trying to slide past each other but friction force is making them stay put.

Friction between solid surfaces that are sliding past each other. This can be reduced by putting a lubricant between the surfaces eg oil.

Resistance or drag from fluids (liquids and gases) eg air.

There is no friction in space as there is no fluid to move through.

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Forces and Motion

Arrows show the size and direction of forces, the length of the arrow shows the size of the force and the direction of the arrow shows the direction of the force.

The reaction of a surface, the forces are balanced (the arrows are equal lengths).

Steady speed represents balanced forces. Just because somethings moving doesnt mean there is an overall force acting on the object- unless its changing speed or direction.

Resultant force is the overall force acting on an object. Its the force that decides the motion of the object. The resultant force is the sum of all the different forces acting on the car. If a pair of forces are equal, then the resultant force is zero (eg the weight and reaction of a car on a road).

If the driving force of the car is bigger than the air resistance and friction from the tyres, then you take them away from the driving force and you get the resultant force.

Acceleration just means change in velocity.

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Forces and Motion Cont.

Acceleration = unbalanced forces.

If a car engine exerts a bigger force forwards than the drag backwards, the car will accelerate. However, the forces in other directions are still balanced eg the weight and reaction.

Momentum is a vector quality as it has size and direction (like velocity but not speed).

Momentum kg m/s = Mass kg x Velocity m/s

The heavier the object, the faster its moving, the harder it is to stop.

The greater the mass of an object, the greater its velocity, the more momentum the object has.

A resultant force of zero means a stationary object will stay still, but a moving object with a resultant force of zero will stay at the same speed in the same direction.

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Forces and Motion Cont.

The change in momentum depends on the force.

When a resultant force acts on an object, it causes a change in momentum. The change it causes depends on the size of the force and the time it acts for.

Change in momentum(Kgm/s) = Resultant force(N) x Time for which the force acts(s)

If momentum changes very quickly, the forces will be very large.

Car safety features reduce the rorces.

These safety features in a car increase the collision time to reduce the forces on the passengers.

The greater the time = The smaller the force.

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"Work done" is just "energy transferred".

When a force moves an object, energy is transferred and work is done.

Work done (J) = Force (N) x Distance (M)

The thing putting the effort in needs a supply of energy (fuel, food, electricity) it then does work by moving the object. The energy is transferred usefully or wasted (as heat or noise), but "work is done".

If energy is transferred, then the object doing the work loses energy:

Change in energy (J) = Work done (J)

Applying a force to something, your doing work on it.

Work done on an object, then energy is transferred. Energy is always conserved, you cant create or destroy it- it gets transformed from one kind of energy to another.

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Kinetic Energy

Kinetic energy is energy of movement (anything that's moving has kinetic energy).

The kinetic energy of something depends both on its mass and speed.

The greater its mass, the faster its going, the bigger its kinetic energy.

Kinetic energy = 1/2 x Mass x Velocity squared

To increase somethings kinetic energy, you need to increase its speed, only way to increase this is to apply a force.

Kinetic energy is movement energy, so if you do do work on an object, but it doesn't accelerate, then you haven't increased its kinetic energy.  

The increase in an objects kinetic energy is normally a bit less than the amount of work done on it. This is because some energy is "wasted" due to friction and air resistance.

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Gravitational Potential Energy

Gravitational Potential Energy (GPE) is 'height energy'.

GPE is stored in an object.

Gravitational Potential Energy = Kinetic Energy

Stored Kinetic Energy (KE) is released when an object falls as falling objects convert GPE into KE.

The further an object falls, the faster it will go.

Change in GPE (J) = Weight (N) x Change in height (M)

Some of the GPE will be displaced as heat due to air resistance (but this isn't considered in the equation).

Kinetic energy gained = potential energy lost

If something gains height again from being dropped (GPE to KE), some KE will convert back to GPE again (KE to GPE) eg like a roller coaster.

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