"Balance of Forces" Force and Motion PPT Courseware 3

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"Balance of Forces" Force and Motion PPT Courseware 3

Review the past

1. What is the content of Newton’s first law?

2. Is Newton's first law directly derived through experiments?

Why can't we get it directly through experiments?

Because in our actual life we ​​cannot be immune to external forces.

Observation: Is the inflatable at rest? Is it affected by external forces at this time?

Observation: The airplane can maintain uniform linear motion despite being acted upon by external forces. Why?

Thinking: In real life, objects can remain at rest or move in a straight line at a uniform speed when subjected to external forces. Does this contradict Newton's first law?

1. Equilibrium state: When an object is acted upon by two (or more) forces, it can remain stationary or move in a straight line at a uniform speed. We say that the object is in a state of equilibrium.

Balanced state

Stationary state

Uniform linear motion state

2. Balance force: Two forces (or forces) that keep an object in equilibrium.

3. Two-force balance: If an object is in equilibrium due to only two forces, this situation is called two-force balance.

Equilibrium state: a state of uniform linear motion or rest

Is the following situation an equilibrium state:

child swing

Raindrops fall vertically at a constant speed

Pencil box stationary on the desk

Running second hand

Plane flying horizontally and at constant speed

Experimental exploration

Ask the question: What kind of force does an object have to be in equilibrium?

Conjecture, hypothesis:

A. The two forces are equal in magnitude and opposite in direction.

B. The two forces are on the same straight line, and the resultant force is zero.

C. Two forces act on the same object.

How do we know if our conjecture is correct?

conduct experiment

What are the conditions required for the balance of two forces?

Experimental conclusion: Conditions for the balance of two forces:

1. The two forces are equal in magnitude.

2. Opposite direction

3. Act on the same straight line

4. Act on the same object

The relationship between force and motion

1. Force is what changes the state of motion of an object.

2. The motion of an object does not require force to maintain it.

3. The object is at rest or moving in a straight line with uniform speed.

4. The object is subject to a balancing force.

5. The object is subject to unbalanced forces.

6. When the state of motion changes, a force must act on the object, and it is an unbalanced force.

7. When a force acts on an object, the state of motion does not necessarily change.

8. The direction of the resultant force is the same as the direction of movement of the object, and the object accelerates

The direction of the resultant force is opposite to the direction of motion of the object, and the object decelerates.

The direction of the resultant force is not in a straight line with the direction of motion of the object, and the object moves in a curve

Summary of this lesson

1. Balance of two forces

When an object remains at rest or moves in a straight line at a constant speed under the action of two forces, we say that the two forces balance each other or the two forces are balanced.

2. Conditions for the balance of two forces (same object, equal magnitude, opposite direction, collinear)

Two forces acting on the same object must be equal in magnitude, opposite in direction, and act on the same straight line.

3. An object remains in motion under the action of a balancing force, that is, it always remains stationary or moves in a straight line at a uniform speed.

4. When an object is acted upon by an unbalanced force, its state of motion changes.

1. Use a spring dynamometer to pull an object weighing G twice to move vertically upward. The image of S-t during the two movements is as shown on the left. The corresponding spring dynamometer indications are F1\F2 and F1, respectively. F2, the size relationship is ( )

A F1

B F1=F2

C F1>F2

D All three of the above situations are possible

2. There are two objects A and B. They move in a straight line along the horizontal plane under the action of force. Its V-t image is shown in the figure. It can be seen from the figure that the force in the horizontal direction during the motion of the two objects is ( )

A. Both objects are acted upon by a balancing force

B. Neither object is acted upon by a balancing force

C. A is acted upon by a balanced force, and B is acted upon by an unbalanced force.

D. A is not affected by the balancing force, and B is affected by the balancing force.

3. As shown in the figure, a car is stationary on a horizontal road. An iron ball is fixed on the pole of its bracket. Regarding the direction of the force exerted by the pole on the iron ball, which of the following statements is correct ( )

A. Along the pole

B is perpendicular to the pole

C.Vertically upward

D. Along a certain direction between the pole and the vertical direction

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Update Time: 2024-11-20

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