Download 1.2.1 Force - StowmarketPhysics

Force
Objective
Use the equation:
net force = mass x acceleration
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Types of force
 Gravitational force

between two objects with mass
 Magnetic force

between two magnetic objects
 Electrical force

between charged objects
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Force, mass and acceleration
F = ma applet
 Newton established that:

acceleration produced by a force depends on
the mass of the object
 Also that if an object has no resultant force on
it, it will not accelerate

eg a car travelling at a constant velocity
(balanced force) has no resultant force, and
therefore no acceleration
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Force, mass and acceleration
 A resultant force always causes acceleration
 Zero resultant force implies constant velocity
(which may be zero)
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Force, mass and acceleration
 Newton established that:


acceleration is proportional to force
acceleration is inversely proportional to mass, if
the force is constant
Newton’s Second Law:
“The acceleration produced by a force when it
acts on a body is proportional to the force and
takes place in the direction of the force”
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Force, mass and acceleration
F = ma
where
F = force in Newtons (N)
m = mass in kilograms (kg)
a = acceleration (ms-2)
Objective
F = ma practical
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F = ma Practical
mask size 1 cm
trolley
light gates
washers
(resourcefulphysics.org)
accelerating washers
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F = ma Practical
 Plot a graph of acceleration (y – axis) against force (x-
axis).
 Use the graph to answer the following questions:
1 From Newton’s 2nd Law you would expect the graph of
acceleration against force to be a straight line which
passes through the origin.
2 Explain why you would expect this to be the case.
3 Explain what the gradient of the graph represents.
4 Calculate the gradient of the graph and explain whether
it agrees with your answer to question (3).
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F = ma Practical
 5
Your results may show some scatter. Explain why
this might be and the causes of any inaccuracy.
Does the experimental result support Newton’s
Second Law of Motion?
 6
Objective
Define the Newton
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Define the Newton
 One Newton is the force that will give a mass
of one kilogram an acceleration of one metre
per second squared
1 N = 1 kg x 1 ms-2
Objective
Explain why F = ma cannot be used for a
particle travelling at very high speeds
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Explain why F = ma cannot be used for
a particle travelling at very high speeds
 According to Einstein’s Theory of Special
Relativity, at speeds approaching the speed of
light, the mass of a body increases
 This means that at very high speeds,
Newton’s 2nd Law is invalid
 This is unlikely to affect the majority of
calculations!
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Example
 A car of mass 500kg is travelling at 20 ms-1.
The driver sees a red traffic light ahead, and
slows to a halt in 10 s. What force must the
brakes provide to do this?
a =v–u/t
= 0 ms-1 – 20 ms-1 / 10 s
= 2 ms-2
F = ma
= 500 kg x 2 ms-2
= 1000 N
Questions
Physics 1
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