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Exercises on Forces
Newton’s laws of motion
Law 1:
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Law 2:
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Law 3:
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Demonstrate Experiment
Title:
To demonstrate Newton’s laws of motion
Diagram:
Method:
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Results:
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Calculations:
Conclusion
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Errors
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From the second law it is stated that a ____________________ is proportional to the
change in a body’s _________________.
Derivation of
f
=
ma
Line one: For a body of mass m, changing from a velocity v to one of u:
From Newton’s second law
As this is the formula used to define the unit of force, the Newton, a value of 1 can be
given to k.
A force is anything which causes or tends to cause an ________________________.
F
=
ma
The momentum of a body is the product of a body’s __________ and ____________.
P
=
mv
STS
Newton’s first law:
In the absence of air a spacecraft travelling in space will
do so without engines until a force is applied to stop it.
An apple falling from an apple tree will continue to fall
until
____________________________________________.
Newton’s second law:
Is seen in the use of seat belts. When a person is thrown
forward the belt expands slightly. This extends the time
over which the person is slowing down. This decreases
the _____________________ and with it reduces the
____________ on the person.
Newton’s third law:
With rockets the large force created towards the rear of
the rockets creates an ____________, but
_____________, __________ force causing the rocket
to move _____________.
Friction is a force which tends to oppose ______________. It occurs when a body
slides or tries to slide along another. It occurs in all states of matter. It is both
beneficial and problematic: e.g.
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Questions
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Conservation of momentum
The conservation of momentum states that
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The momentum of a body is the product of a body’s __________ and ____________.
P
=
mv
STS
The above law applies to situations which involve collisions. It applies in situations
such as the take off of a rocket. Before take off, the total momentum is ________.
Afterwards the momentum of the rocket ________________ is matched by the
momentum of the ________________________________.
It does not apply to systems that are not closed e.g. tennis where some momentum is
absorbed by the player’s arms.
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Questions
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Moments
The moment of a force about an _________ is equal to the magnitude of the
______________ multiplied by the perpendicular _____________ from the axis to the
line of action of the force.
Formulae:
_________________________
Unit:
_________________________
It is a ______________________ quantity.
The moment of a force tells us about its ability to make something _____________.
e.g. when we use a door, the handle is as far from the hinges as is reasonable.
Therefore if you apply a small force a large distance from the fulcrum (the turning
point) it results in a large moment or turning effect.
A lever is a rigid body which is free to ___________ about a fixed point.
A crow bar is an example of a lever that makes maximum use of a force to move a
heavy body.
Diagram
A couple is created by two parallel forces with the same magnitude acting in opposite
directions.
An obvious example is in the handlebars of a bike. One hand moves forward, the
other back, and the couple created turns the handlebar, and with it the bike.
Diagram
In a simple d.c. electric motor and in a moving coil galvanometer a pair of parallel
forces on a coil of wire exert a couple on the coil, resulting in the coil rotating.
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Conditions for equilibrium
1. _______________________________________________________________
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2. _______________________________________________________________
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Consider the below metre stick in equilibrium.
As the stick is in equilibrium we know that the total ____________ must equal the
________________________.
Find W, the weight of the stick.
Show that the sum of the moments about any point is zero.
1. about 0 cm
2. about 50 cm
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Questions
Density and pressure
The density of an object is its mass per unit volume.
Formulae:
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Unit:
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Symbol:
__________________ rho
It is a ______________________ quantity.
Pressure is defined as forces per unit area.
Formulae:
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Unit:
__________________________________
Symbol:
__________________________________
It is a ______________________ quantity.
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It can be proved that the pressure due to a liquid at a depth h in a liquid of density
ρ where acceleration due to gravity g, the pressure P due to the liquid is given by:
Demonstration of atmospheric pressure:
Heat a can with air in it and then seal it. As it cools the walls are drawn in. Why?
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STS
The Bends:
As divers move down through water the pressure _____________.
This _________________ is sufficient to ‘push’ more nitrogen into the
blood stream where it is dissolved. AS the diver rises and the pressure
_______________ this nitrogen will pass back out of the blood as
small bubbles which can cause great damage as they pass through the
brain.
Boyle’s law states that at _________________ temperature the pressure on a fixed
mass of gas is inversely proportional to its volume
i.e.
This law is seen in effect in a bicycle pump. How?
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Archmedes' principle states that a body immersed in a fluid will experience an
upthrust equal to the weight of fluid that it has displaced.
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The law of flotation states that when a body floats, it is displacing exactly its own
weight if fluid.
The objects float as the upthrust it experiences is equal to its weight. If the two forces
were not equal the object would either ______________ or __________________.
Diagram
STS
Archimedes’ Principle underlies the use of hydrometers. These are devices for
measuring the density of liquids. In a dense liquid, the hydrometer can displace its
own weight by displacing only a small quantity of liquid. In a less dense liquid, it
must sink deeper in order to displace the same weight.
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