Download Science 10th grade LEARNING OBJECT What does “resultant force

Science 10th grade
LEARNING UNIT
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Language
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Curricular axis
Standard competencies
Background Knowledge
English Review topic
Vocabulary box
LEARNING OBJECT
What does “resultant force” mean and what is it used
for?
 SKILL 1: Verify Newton’s second law.
 SKILL 2: Represent the external forces that act on a
body in a free-body diagram.
 SKILL 3: Determine the resultant force that acts on a
body, through its components.
 SKILL 4: Apply Newton’s second law to solve
exercises.
 SKILL 5: Explain, in quantitative terms, the effect of the
point of application of a force on the movement of an
object.
English
Colombia
Physical environment.
Explain the forces between objects as interactions due
to electric charge and mass.
Knowledge of Newton’s second law.
Zero conditional.
Kinetic (adjective): of motion.
Slide (verb): to (cause to) move or pass along
smoothly.
Mass (noun): (a) measure of the quantity of matter
in an object.
Body (noun): a mass.
Exert (verb): to bring forcefully into use or action.
Spring (noun): a twisted or coiled piece of metal
that returns to its original shape when it is pressed
down or stretched.
Smooth (adjective): having a flat, even surface:
not rough: not having any bumps, ridges, or uneven
parts.
Thread (noun): a long, thin piece of cotton, silk,
etc., used for sewing.
Sources:
http://www.merriam-webster.com
http://dictionary.cambridge.org
NAME: _________________________________________________
GRADE: ________________________________________________
Introduction
A soccer player places the ball in the free kick zone, gathers momentum to kick the
ball, and generates a change in the velocity of the ball, given by its acceleration.
What happened when the player kicked the ball? How did its velocity change? Did
the ball exert a force contrary to that of the kick? Why?
Objectives
-
To verify and explain Newton’s second law.
To represent the forces exerted on an object in a free-body diagram, and
deduce the resultant force.
Activity 1
SKILL 1: Verify Newton’s second law.
Verify Newton’s second law
Newton’s second law states that a force is capable of producing a change in the
velocity of a body, that is, a change in its acceleration. When there is only one
force acting on the body, the direction of the force coincides with the direction of
the acceleration it generates. The magnitude of the force is the product of the mass
of the body times the magnitude of the produced acceleration. (TIPLER, 1995)
𝐹⃗ = 𝑚𝑎⃗
Figure. Newton’s second law.
Learning activity
Watch the following video:
[Vladimir Albiter]. (2009, mayo 18). Segunda Ley de Newton [Archivo de video].
Retrieved from: https://www.youtube.com/watch?v=ZvPrn3aBQG8
Based on the video, make a concept map of the moments of our daily lives in
which we can observe Newton’s second law.
Activity 2
SKILL 2: Represent the external forces that act on a body in a free-body diagram.
SKILL 3: Determine the resultant force that acts on a body, through its
components.
Figure.
Based on the figure, we can say:
Free-body diagram m1
Σf(y)=0
N-m1g=0
N=m1g
Σf(x)=T
T=m1a
Free-body diagram m2
Σf(y)=T-m2g
Σf(x)=0
Did you know that…?
There is a force called “friction force”, that can be kinetic (it slides) or static (it does
not slide), and that is opposite to the force of acceleration.
Learning activity
In the following figure, place the forces that are acting on the object: resistance
force Fr, net force Fn, force of gravity g, and normal force N.
Take an object with a known weight (2 kg or more) and slide it on a smooth surface
approximately one meter long. Write down for how long (in seconds) the object
moved and find the net or resultant force.
Activity 3
SKILL 4: Apply Newton’s second law to solve exercises.
SKILL 5: Explain, in quantitative terms, the effect of the point of application of a
force on the movement of an object.
Watch the animation.
Learning activity:
Based on the animation, solve the following exercises:
a. A given force, applied to a mass m1, gives it an acceleration of 20
m/s2. The force, applied to m2, gives it an acceleration of 30 m/s2. The
two masses are joined, and the same force exerted on them before is
applied on the combination. Find the resultant acceleration.
b. A 5 kg body is pulled on a horizontal surface without friction, by a
horizontal force of 10 N. (a) If the object is at rest at t=0, what is its
velocity after 3 seconds? (b) What distance does it travel from t=0 to
t=3 s?
The point of application of a force is the specific point at which the force is applied.
Observe the following exercise, related to the point of application:
Exercise:
Two parallel forces that act in the same direction, F1 = 12N and F2 = 9N, are
separated by 14 cm. Calculate the resultant force and its point of application.
Solution:
1) The intensity of the resultant force (R) is the sum of the intensities of its
components.
So: R = F1 + F2 = 12N + 9N = 21N, in the same direction as the components.
2) The point of application must follow this equation:
(1). F1 • d1 = F2 • d2
The two parts must follow the equation:
d1 + d2 = 14cm, so: d2 = 14 – d1
Substituting equation (1), we obtain:
F1 • d1 = F2 • d2 = 12N • d1 = 9N • (14 – d1)
12d1 = 126 – 9d1
12d1 + 9d1 = 126
21 d1 = 126
d1 = 126/21
d1 = 6 cm
Learning activity
1. Two parallel forces that act in the same direction, F1 = 6N and F2 = 3N, are
separated by 10 cm. Calculate the resultant force and its point of application.
Abstract
Newton’s second law makes references to the forces exerted on an object, which
are capable of producing a change in its velocity and, likewise, in its acceleration.
These forces (that act on a body) can be represented in a free-body diagram,
which shows all the external forces and their directions, indicated with arrows.
Furthermore, when several forces act on a body, they can be added up through a
vector addition to obtain the resultant force. When this force is equal to zero, there
are two possible cases: the body either is at rest, or has a uniform linear motion.
The movement of the object will depend on the point of application of the force.
Homework
1. Search and describe in a paragraph some of the applications of Newton’s
second law.
2. Explain the relationship between Newton’s second law and masses that are
constant and not constant.
3. Make a comparative table of static and kinetic friction.
4. In groups of three students, design the following objects, carry out the
described procedure, explain the results, and share your experience with your
classmates.
Materials:
 An object (eraser, rock, piece of wood, etc.).
 A spring.
 A piece of thread that is long enough to be tied around the object.
Procedure:
Case 1: Put the object on a horizontal table.
Case 2: Tie the object to the free end of the spring, in such a way that it hangs
vertically and at rest.
Case 3: Tie the thread around the object and put the object on the table.
Without moving the object, pull the thread until it is parallel to the table.
For each one of these cases:
a) Draw a free-body diagram.
b) Give values to each force, and find the resultant force.
5. Search ways of adding vectors and give an example for resultant force.
Evaluation
1. Match column A to column B, by placing the corresponding letters next to the
numbers.
Column A
1.
2.
3.
4.
__ Newton’s second law
__ Free-body diagram
__ Resultant force
__ Static friction
Column B
a)
b)
c)
d)
Sum of forces
Normal force
Change in velocity
Graphic representation
2. Label these forces on the diagram: applied external force, force of gravity on
the block, force of the table on the block.
Figure 3. Block moving to the right on a rough surface, due to an external force.
3. Answer true (T) or false (F) according to the case:
a. When a body is at rest, the sum of the forces equals zero.
b. The force applied on an object does not make it move.
c. The resultant force is found through vector addition.
( )
( )
( )
4. Multiple choice questions with one answer. Mark the correct answer with an x.
I.
a.
b.
c.
d.
It is opposite to the direction of the force.
Net force
Normal
Gravity
Friction
II. When the movement of a body is horizontal, the sum of the forces in Y is equal
to:
a. 100
b. 10
c. 1
d. 0
III.
a.
b.
c.
d.
Which forces act on an object hanging on a string?
Gravity and normal
Tension and gravity
Tension and friction
Normal and friction
Bibliography
1. Gabrielamate.Retrieved from:
http://gabrielamate.wikispaces.com/
2. [MateMovil]. (2015, mayo 5). Fuerza de Rozamiento Ejercicios
Resueltos Nivel
3. [Archivo de video]. Retrieved from:
https://www.youtube.com/watch?v=qr0QgMn1hbA=PL3KGq8pH1b
FS0lr5NfcXXgSxh4CGQqY-i=65=False
4. Profesor en línea. Sistema de fuerzas paralelas y en el mismo
sentido. Retrieved from:
http://www.profesorenlinea.cl/fisica/Fuerzas_paralelas.html
5. [Vladimir Albiter]. (2009, mayo 18). Segunda Ley de Newton
[Archivo de video]. Retrieved from:
https://www.youtube.com/watch?v=0Q45yZRYa5U
6. Tipler, P. A. (1995). Física tercera edición. Barcelona: Editorial
Reverté, S.A.
Glossary
Acceleration: a change in velocity within an instant.
Friction: a force opposite to the direction of the movement of an object across a
surface.
Normal force: a force opposite to gravity, when a body is on a surface.
Resultant force: the sum of all forces.
Point of application: exact place where the force is applied.
Vector: arrow that indicates the direction of a force.
Vocabulary box
Kinetic (adjective): of motion.
Slide (verb): to (cause to) move or pass along smoothly.
Mass (noun): (a) measure of the quantity of matter in an object.
Body (noun): a mass.
Exert (verb): to bring forcefully into use or action.
Spring (noun): a twisted or coiled piece of metal that returns to its original shape
when it is pressed down or stretched.
Smooth (adjective): having a flat, even surface: not rough: not having any bumps,
ridges, or uneven parts.
Thread (noun): a long, thin piece of cotton, silk, etc., used for sewing.
Sources:
http://www.merriam-webster.com
http://dictionary.cambridge.org
English review topic

Zero conditional