P2.3 - Forces In Action Flashcards
What are the three types of forces that change the shape of a body?
- Compression
- Tension
- Bending
What is the difference between elastic and plastic bodies?
Elastic bodies are objects that return to their original shape when the deforming force is removed whereas plastic bodies are objects that do not return to their original shape when the deforming force is removed.
Explain springs?
- Springs are designed to return to their original
shape when the deforming force is removed. - Hence, springs are elastic bodies.
- However, if you apply too great a force and stretch
a spring too far, it will suffer plastic deformation
(will break and not return to its original shape).
What is Hooke’s Law?
“The extension of a body is directly proportional to the force applied, as long as the limit of proportionality is not exceeded.”
How can you calculate the spring constant of a spring?
The spring constant, k of a spring/body can be calculated from the gradient of its force-extension graph.
What is the elastic limit?
- The elastic limit, (marked X on a graph) of a body
is the point after which the body is permanently
deformed when the deforming force is removed. - Below the elastic limit, if you remove the force,
the spring returns to its original length. - Above the elastic limit, it does not return to its
original length - it is permanently deformed. - This point comes slightly after the limit of
proportionality.
What is the limit of proportionality?
The point at where a spring stops obeying Hooke’s Law and extension is no longer proportional to force.
What is elastic potential energy?
- EPE is the energy stored by an elastic body,
that has been stretched (or squashed). - The greater the extension (or compression) of a
body, and the stiffer it is, the more EPE it has. - A body that has EPE can do work.
- A stretched rubber band will transfer its EPE to
KE when it is released - Work done is the
transfer of energy - Therefore, a stretched
rubber band does work when it is released.
What is the spring constant of a spring?
- The spring constant, k, is a measure of how stiff a
material is. - The bigger k, the stiffer the material, the more
force it needs to bring about the same extension,
the more EPE it has.
What is a gravitational field?
- A region where a mass experiences an
attractive force. - The force is bigger if, the mass of the object(s)
are bigger or is the distance between them are
smaller. - Gravity is a force field that applies a certain
amount of force on a body, per unit mass.
What is weight?
Weight is the pull force down acting on a body, due to the gravitational field of a planet.
What is gravitational potential energy?
- GPE is the energy stored by a body, whenever it is
raised through a height in a planet’s magnetic
field. - This is because in order to lift a body, a force must
be applied up to counter its weight. - Thus work is done on the body (energy to
transferred to it) where the final energy is in the
form of GPE.
What is the moment of a force?
The moment of a force is a measure of its turning effect, about a pivot.
What is the moment determined by?
The moment, M of a force, F about a pivot is determined by the perpendicular distance,d from the pivot to the line of action of the force.
How can you increase the moment of a force?
- Increasing the size of the force.
- Increasing the perpendicular distance from the
pivot.
Are moments scalars?
- Moments are vectors.
- They have a size and direction.
What does it mean by a body in equilibrium?
- A body in equilibrium isn’t accelerating - There is
no resultant force acting on it. - A body in equilibrium also isn’t turning - There is
no resultant moment acting about any pivot.
What is the principle of moments?
- Any body in equilibrium obeys the principle of
moments. - The sum of the clockwise moments about any
point = The sum of the anticlockwise moments
about any point. - When the principle of moments holds true; nothing
is turning!
What is a lever?
- A lever is a force multiplier.
- A small effort (applied force) on one side of the
lever, leads to a large load (force exerted) on a
body on the opposite side of the pivot. - The closer the load to the pivot, the smaller the
effort to lift it.
What is the equation for mechanical advantage (levers)?
- Mechanical advantage = Load/Effort.
- Load is the output force.
- Effort is the input force.
What are gears?
- Gears act as moment multipliers.
- The force applied between gears is the same,
but as the perpendicular distance changes, the
moment of the force changes.
What is the equation for mechanical advantage (gears)?
- Mechanical Advantage = Output cog radius / Input
cog radius. - Mechanical advantage < 1 : Speed multiplier
- Mechanical advantage > 1 : Torque multiplier
What happens to a car starting from rest or a cycle cycling uphill in terms of gears?
- Small gear to big gear.
- Moment increases, larger acceleration.
- Output rotation speed low.
What happens to a car travelling at high speed or a cycle cycling on level ground in terms of gears?
- Big gear to small gear.
- Moment decreases.
- Smaller acceleration.
- Output rotation high
What angle is produced by liquid pressure?
The pressure inside fluids (liquids and gases) produces a force that always acts perpendicular to any surface.
What are hydraulic machines and how do they work?
- Hydraulic machines are force multipliers.
- Two pistons are connected by a pipe full of fluid.
- A force is applied to piston 1, acts over area 1, and
creates a pressure within the fluid. - The same pressure acts at piston 2, over an area 2 - Area 2 > Area 1
- Output force increased
- Area 2 < 1 - Output force decreased
