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Newton's Laws

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									                         Forces and Newton’s Laws P2
                            In 1687, Newton revolutionised how we thought about the way
                            things move. Even now, his three famous laws of motion can
                            seem surprising and counter-intuitive, yet they remain incredibly
                            effective, over three hundred years after their conception.

                            Newton’s 1st Law
                            “A particle will remain           An Air Hockey table is great for
                            stationary, or continue at a      demonstrating Newton’s 1st Law.
                            constant velocity, unless
                            acted upon by a net force”.

Note that net force refers to the sum of all forces acting.
In other words, if the sum of all forces acting on
something is zero, then that something must move at
constant (or zero) velocity.

Hovercrafts make use of this fact, and can travel equally     As the puck moves, the only force
well over any terrain (land, ice or water), supported by a    acting on it is friction. This friction
cushion of air flow underneath. This reduces the friction     has been greatly reduced, and is
dramatically, making them very efficient.                     almost zero. This means the puck
                                                              is able to continue moving for a
Try making your own hovercraft by following these             very long time.
instructions.
                          Forces and Newton’s Laws P2
Newton’s 2nd Law
“The net force on an object is equal to the mass
                                                   Resultant Force
of the object multiplied by its acceleration”.
                                                   When dealing with Newton’s 2nd Law, we
Or, to put it more mathematically,                 are talking about the sum of all forces on an
                                                   object, usually called the resultant force.
                      =
This is an extremely famous equation in
Physics, perhaps as well known as Einstein’s
  =      .

This can be re-arranged to find:
                                                   This car has a “motive force” of 2000N
                       =                           (from the engine), but it also has a “resistive
                                                   force” of 425N (air resistance). What is the
Both force and acceleration are vectors, so        resultant force?
they both have size and direction.

F and a both have the same direction.

                                                   If the car’s mass is 1500kg, what will its
                                                   acceleration be?




Consider this situation: a husky is pulling a 50kg sledge along on the snow, with a force of
100N. Assuming there is no friction, how fast will they accelerate?




                                            Now there is another husky to help him, who
                                            also pulls with a 100N force.

                                            The resultant force is, therefore, 200N.

                                            What will the acceleration be?
                          Forces and Newton’s Laws P2




Finally, a particularly stupid driver has set two
packs of two dogs against each other. Each
group of two applies a 200N force, but in
opposite directions. The resultant force is

               = 200 − 200 = 0

What is the acceleration?



Newton’s 3rd Law
“For every action there is an equal and
opposite reaction.”

This law is illustrated well if you try to step
off a boat onto the bank of a lake. If you are
not careful the boat will slip in the opposite
direction, and you will fall into the water.

When a book is placed on the table, it has a certain weight. This action force must have an
equal and opposite reaction force. This is what stops the book from falling to the floor.

                                                  Whenever a force is exerted by one body on
                                                  another, there is an equal and opposite force
                                                  exerted the other way, by the second object
                                                  on the first.

                                                  A boat floats because the upthrust from the
                                                  water balances with the weight of the boat. If
                                                  there is not enough upthrust, then the boat
                                                  will sink.

If two people are stood on skateboards and they
pull on a rope, they will each feel the same force
towards each other.

If only one person pulls then, amazingly, the
same thing happens!
                            Forces and Newton’s Laws P2
         Use the words below to fill in the blanks.
                 proportional      force     reaction     stationary     accelerates      opposite
                                 constant     resultant     inversely     balanced

Newton's 1st law

If the forces acting on an object are ………..................…….. it's either ………..................…….. or

moving at a ………..................…….. speed.


Newton's 2nd law


If an object has a ………..................…….. force acting on it, it ………..................……..        in   the

direction of the ………..................…….. . The acceleration is ………..................…….. to the force

and ………..................……..   ………..................…….. to the mass.


Newton's 3rd law


For every action there is an equal and ………..................……..         ………..................……..

								
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