Download What Students Need to Know about

Work and Force Resources
Teaching Targets
 Review force, balanced and unbalanced forces, and simple machines.
 Force = push or pull (review examples of force, include examples of forces in
 plants
 Balanced forces are equal in size and opposite in direction. They do not
 cause a change in motion
 Unbalanced forces are not equal in size or direction and will cause a change in motion.
 Contrast work vs. no work (using various situations).
 Work = Force x Distance (perform simple calculations and understand the relationship
between work, force and distance).
 If there is no distance traveled, then there is no work done.
 If there is no force exerted on the object (push or pull), then there is no work
 done.
 Students will compare work done with different amounts of force in situations such as
moving an object without a ramp.
 Students will compare work done with different amounts of force in situations such as
moving an object with a ramp.
 Students will compare situations where no work is done such as standing still, pushing
on a wall, or walking while carrying books
 Simple machines do NOT do less work. They allow a person to use less force to do the
same amount of work. If less force is used, and work is constant, then distance must
increase. Each simple machine increases the distance over which the effort force is
applied.
 Plant cells are able to maintain a constant size and shape as water within the cytoplasm
exerts a force, or pressure, on the cell wall, pushing it outwards.
 The pressure within the cell is called turgor pressure, and the combined turgor pressure
of all the cells allow plants to stand upright and is strong enough to break items like
concrete.
 Plants are geotropic and grow in response to the force exerted upon them by gravity
 Even when turned, plants will recognize the force of gravity and grow their stems
upward and their roots downward.
 Any growth response that results in curvature of whole plant organs toward or away
from stimuli is called a tropism.
 Tropisms and turgor movements are two forms of plant movement that allow the plant
to react to stimuli
 Geotropism is the term applied to the consequent orientation response of growing plant
parts. Roots are positively geotropic, that is, they will bend and grow downwards,
towards the center of the Earth. In contrast, shoots are negatively geotropic, that is,
they will bend and grow upwards, or away, from the surface.
 Thigmotropism, a reaction to touch, causes parts of the plant to thicken or coil as they
touch or are touched by environmental entities. Tree trunks, for instance, grow thicker
when exposed to strong winds and vines tend to grow straight until they encounter a
substrate to wrap around.
 Gravitropism, a response to gravity, causes parts of a plant to grow either upward or
downward. If a plant is placed on its side, its shoot will begin to grow upward (against
gravity) and roots will follow the pull of gravity to grow downward.
7th Grade Science
Work and Force Resources
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Turgor pressure is the force exerted by water inside the cell on the cell walls. Changing
amounts of water within the cell cause the turgor pressure to change. Turgor
movements occur more rapidly than tropisms and are easily reversible. Tropisms rely
on changes in turgor pressure (exerted by water on cell walls) within certain plant cells.
Turgor movements are responsible for many plant responses, such as when leaves or
flowers droop and fold up at certain times of the day or night or in response to an
external touch. A Venus flytrap, for instance, depends on changes in turgor pressure to
close its "jaws" around insects when they land on the plant.
Tropisms are responses to stimuli that result in the long-term growth of the plant
toward or away from the stimulus.
Phototropism, a reaction to light, causes the plant to bend toward the light source.
Plants grow toward light, and if you rotate a plant, it will reorient its growth until its
leaves again face the light
Plants can also elicit a fight or flight response by releasing
7th Grade Science
Work and Force Resources
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