P4 explaining motion

OCR 21st century science

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Interactions and forces

When two objects interact they both experience a force, this force is called an interation force and the two objects are an interation pair.

These two forces are equal in size but opposite in direction. Each acts on a different object.

Vehicles and people move by pushing back on something. This interation causes a forward force to act on them.

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Friction and Normal Reaction

Friction is an interaction between two objects that are sliding against each other.

The friction force matches the applied force that is making the objects slide when the object which is moving is moving at a steady pace.

When an object sits on a surface, it distorts it slightly. The springiness of the surface then causes a reaction force on the object matching it's downward push on the surface.

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Describing Motion

The average speed of a moving object is:

  • Distance divided by time taken

The instantaneous speed of a moving object is its speed at a particular instant. To get a good estimate of the instantaneous speed of a moving object we must measure it's average speed over a very short period of time (or distance)

Velocity means speed in a particular direction.

The following graphs are useful for summarizing or analyzing the motion of an object:

  • Distance-time graphs,
  • Speed-time graphs,
  • velocity-time graphs
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Forces and Motion

When a force acts on an object, it causes a change in it's momentum. Momentum is:

  • Mass times velocity.

The change of momentum is equal to:

  • Force times time for which it acts.

Many vehicle safety features work by making the time of an event (such as collision) longer so that the average force is less, for the same change of momentum.

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Laws of Motion

If the resulatant force acting on an object is zero, the momentum of the object does not change. If it is stationary it does not move. If it is moving, it will keep moving at a constant speed in a straight line.

If the resulatant force acting on an object is not zero, this will cause a change in it's momentum, in the direction of the resultant force.

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Work and Energy

Work is doen when a force makes an object move. The amount of work is force times distance.

When something does work, its energy decreases by that amount. If work is done on it, its energy increases by that amount.

Doing work on an object can increase its gravitational potential energy or its kinetic energy, its gravitational potential energy is increased if you lift it up and its kinetic energy is increased if you make it move faster.

Change in gravitational potential energy is weight times vertical height difference. Vhange in kinetic energy is 0.5 times mass times velocity squared.

When an object drops to a lower level, it loses gravitational potential energy. If frictioncan be ignored, it gains the same amount of kinetic energy.

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