Physics 8 Lesson 4: Performance of force
1. Theoretical summary
1.1. Force concept
Force is the cause of change in velocity. When the velocity of an object changes, we can conclude that there is a force acting on the object.

Example: People push a car, under the action of thrust, the speed of the car increases gradually from 0 to a certain value.
– Under the action of force, in addition to changing the speed of the object, the force can also cause the object to deform.
The unit of force is the newton (symbol N).
1.2. Force show
– Force is a vector quantity represented by an arrow with:

The origin is the point at which the force is applied (the point at which the force acts on the object).

The direction and direction coincide with the direction and direction of the force.

Length represents the magnitude (magnitude) of the force on a given scale.
The force vector is denoted by . \(\overrightarrow F \), the magnitude (magnitude) of the force is denoted F.
1.3. Solution method
a) How to represent force on the figure
– To represent force with an arrow, we need to determine the correct factors:

The point where the force is placed on the object to determine the origin of the arrow.

The direction and direction of the force to determine the direction and direction of the arrow.

The intensity (magnitude) of the force to choose the appropriate scale.
b) Describe the elements of the force shown in the figure
– To describe the elements of force, we need to define:

Where is the origin of the arrow? That is the set point of the force.

What is the direction and direction of the arrow? That is the direction and direction of the force. (Especially if the direction of the arrow does not coincide with the vertical or horizontal, then see how many degrees that direction makes with the vertical or with the horizontal).

How many intervals are there on the arrow and each interval corresponds to the chosen scale to determine the correct strength of the force.
2. Illustrated exercise
2.1. Form 1: Representing gravity acting on an object
Express the force of gravity acting on an object of mass 50 kg on a scale of 1 cm corresponding to 100 N.
Solution guide
Gravitation \(\overrightarrow P \) effects on objects that have:
Set point: at G (center of gravity of the object).
Direction: vertical, direction from top to bottom.
Magnitude: P = 50.10 = 500 N (corresponding to 5 cm).
2.2. Form 2: Determine the factors of force
Describe the elements of the drawing force in the following figure.
Solution guide
Force \(\overrightarrow F \) effects on objects that have:
Set point: at A.
Direction: make an angle of 30 . with the horizontal^{o} (with anticlockwise rotation),
Direction: upward.
Magnitude: F = 3.15 = 45 N
3. Practice
3.1. Essay exercises
Question 1: Is a waterfall falling from above moving due to the action of gravity?
Verse 2: What factors do we need to know to represent a force vector?
Question 3: Objects 1 and 2 are moving with velocities v1 and v2 when they experience forces as shown in the figure. Do objects 1 and 2 have an increasing or decreasing velocity?
Question 4: An object is moving in a straight line with velocity v. In order for an object to move in the same direction and speed up, what force must be applied to the object?
3.2. Multiple choice exercises
Question 1: When there is only one force acting on an object, what is the velocity of the object?
A. Velocity does not change
B. Increasing speed
C. Velocity is decreasing
D. Can be ascending and also descending.
Verse 2: Use suitable phrases to fill in the blanks: ….. is the cause of the change in the velocity of the motion.
A. Vector B. Change
C. Velocity D. Force
Question 3: The following conclusions are not correct?
A. Force is what sustains motion.
B. Force is what causes an object to change its motion.
C. Force is what causes an object to change its velocity.
D. An object is deformed due to a force acting on it.
Question 4: Which of the following situations tells us that when subjected to a force, an object is both deformed and changed in motion?
A. The wind blows the swaying branches and leaves.
B. After hitting the racket, the tennis ball bounced back.
C. An object is falling from above.
D. When braking, the bicycle slows down.
4. Conclusion
Through this lecture, students need to complete some of the objectives given by the lesson, such as:
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