High School Physical Mechanics Problems, Senior 1 Physical Mechanics Problems

1) The center of gravity is to treat an object as a particle, which can be easily analyzed.

Putting the force in the center of gravity is equivalent to seeing the object as a particle. The force acts on the particle.

2) Force balance.

The object is subjected to gravity, support (= pressure), and tension.

Force Analysis: n=mgsin f=mgcos n increase f decrease (your answer is wrong).

3) The synthesis of forces must conform to the law of triangles, a triangle, knowing the direction and direction of the hypotenuse, the size and direction of an edge, isn't this triangle established? Even the other side is established.

The triangle rule is derived from the parallelogram rule, so if you don't know it, ask again.

The first question: you have to recognize the three conditions of friction. (Analyze the concept of friction yourself).

Second question: This is an analysis of the force on the ball.

1.First of all, the problem has already stated that the ball volume is ignored and the state of the ball is a slow pull. This shows that the ball can be used as an object moving at a uniform velocity.

It can be concluded that the net external force exerted on the ball is zero (including gravity) and the equation for the net external force is zero according to this. Note that the surface is smooth and there is no friction! (1 pull of the rope, 2 gravity, 3 support (pressure)).

Question 3: When you have listed the equation for which the resultant external force is zero, analyze it yourself according to the conditions given in the question.

1) Friction Force Both support force and gravity force are forces exerted on an object When analyzing the force on an object, it is generally based on the center of gravity of the object Regarding the non-common point force, you can't use the parallelogram rule, right? Yes, you can only synthesize and decompose forces if you have a common point force, and parallelograms are synthesized and decomposed (you don't really have to think about it that much).

2) Look at this question, it's almost the same, and if you have a question, ask me again.

3) It may change In this problem, the direction of the pulling force is changing when the object is pulled up.

As far as this question is concerned, Ling's analysis method is also correct, but this kind of problem is often solved with similar triangles, why? Because it is relatively simple and clear, it is often easy to analyze errors by using the explanation method to solve the problem.

Categories: Education Science >> Entrance Examination >> College Entrance Examination.

Problem description: The total mass of the parachutist and the parachute is 80kg, and they fall at a constant speed in the direction of 30 degrees from the vertical direction, what is the air force on the parachute and the athlete, and what is the direction.

g takes 10 n kg

Analysis: First of all, look for the key sentence, the "falling at a uniform speed" in the title determines the overall force balance between the athlete and the umbrella, and the balance condition should be satisfied, that is, the combined external force is zero. Note that this conclusion has nothing to do with the speed direction of the athlete's movement, as long as it is "falling at a uniform speed", the above conclusion must be satisfied.

Secondly, the force analysis of the athlete and the umbrella as a whole is carried out. In addition to the gravity of the athlete and the umbrella as a whole, only air is in contact with the research object, so the air has a force on the athlete and the umbrella, that is, the athlete and the umbrella are balanced by the action of two forces. Combined with the above discussion, the two forces are equilibrium and the two forces are equal in magnitude, opposite in direction, and act in a straight line.

Therefore, the air force on the parachute and the athlete is the magnitude of gravity, i.e. 800N, and the direction is vertically upward.

In this question, it is important to distinguish between the concepts of "resistance of air" and "force of air". The total speed of the "resistance of the air" and the speed of the movement are reversed, and this question falls at a uniform speed of 30 degrees from the vertical direction, indicating that there must be a "force of wind". The "air force" in the title is the resultant force of "air resistance" and "wind force", that is, the total net force of air to the subject of study.

Question 1:

Solution: This can be measured.

With a spring, it is possible to measure the weight of attraction f of the moon

According to the law of gravitation, gmm r = f find the mass of the moon m=fr (gm) The moon is a sphere, so the volume v=4 r 3

So the density p=m v=3f (4 rgm)g is the gravitational constant.

Question 2: 1) Reach point b velocity v, then v -v. =2AL where A= - ug

Yields: v=2m s

After that, the ball is thrown flat, and the falling time t is t, then there is t=of GT 2=H, then the horizontal distance is s=v t=

2) Conveyor belt linear speed v(1)=w r =6m s if it reaches 6m s before slipping b

To slide from 8m s to 6m s then (8m s) -6m s) = 2as of s=7 3, so the horizontal distance s=vt=6m s x m(3) is set to have a large angular velocity.

Then this is when friction becomes the driving force. Let the end velocity be v

Then v -v. ²=2al 。a= ug

This yields v=root number (124)m s

Arrive at s=vt and figure out the answer.

When I went to school, this kind of question is a simple question, as if there is a formula for celestial motion, put it in, bring the known number in, see if you can find the answer you want, the conveyor belt problem is the most common, nothing more than two situations, known to strive for motion or known motion to seek force, it is recommended to draw a force diagram first, this kind of problem must first draw a force diagram, and then analyze the movement, motion and force must be right, you draw it yourself, after five years of graduation, I have forgotten it.

1. Yes.

2. Use the law of conservation of energy to calculate the initial velocity to the B end, and then solve it according to the flat throwing motion.

3. Look at the direction of movement of the conveyor belt, if it is leading, then the friction force is doing work, which is also solved by the law of conservation of energy, and if it is reversed, the friction force does negative work, which can also be solved by the law of conservation of energy.

1): Yes on Earth g1=g constant m1m r1 2 on the moon g2=gm2m r2 2

g1 g2 r1 r2 is known to thirst m2, i.e. the mass density of the moon = m v=3m 4 r 3 thirst for the density of the moon 2): 1: a=ug=6 s=1 2 at 2 thirst.

I think these questions were all captured back then, but now that I'm old, I've almost forgotten them!

Take advantage of the laws of gravitation and free fall.

The second question is not read.

Let the highest point of the circumference be d, the triangle ADC is a right-angled triangle, let the radius of the circumference be r, the angle between ac and the horizontal direction is a, ac = 2rsina, and the acceleration of the ball as it slides along ac is gsina, according to the displacement formula, 2rsina=gsinat1 2 2t1 2 =4r g

Descending along the arc, it is equivalent to a simple harmonic motion of a single pendulum, t2 2 = r (2g) falling vertically and freely from the height r, t3 2 = 2r gt1>t3>t2

"If there is friction, there must be elasticity, and if there is no elasticity, there must be no friction".

The following two sentences are wrong, "If there is elasticity, there must be friction."

If there is no friction, there must be no elasticity".

Yes, because friction is defined as the tangential resistance that prevents two objects from moving relative to each other when there is pressure between them.

According to the definition, it can be known that there are three conditions for friction to occur: 1. there is pressure (elastic force) and 2. there is tangential relative motion or tendency. 3. The contact surface is rough, so there must be elasticity if there is friction; There is no elastic force, there must be no friction.

The elastic deformation of the object is due to the return to its original shape, and the force on the object in contact with it is called elastic force. By definition, it can be judged that the second condition for the generation of elastic force does not need to be met to produce frictional force. So having elasticity does not necessarily mean having friction; No friction is not necessarily no elasticity.

You can imagine a spring pressing vertically on the ground, without the action of the horizontal force, he only has elastic force and no friction. If there is a vertical pressure (elastic force) and a horizontal force, there must be friction. If it only acts with force in the horizontal direction and there is no pressure (elastic force) between it and the ground, then there must be no friction.

There are two conditions that produce friction: 1Stretch 2There is a tendency to slide relative or slide relatively3The contact surface is rough, and the three conditions are indispensable, so let's look at the problem.

If there is friction, there must be elasticity Correct.

If there is elasticity, there must be friction False, there is also a second condition, such as a football placed on a smooth floor that has elasticity but no friction.

If there is no friction, there must be no elastic force Error ditto.

If there is no elastic force, there must be no friction Correct elastic force is one of the conditions for the occurrence of frictional force.

Elastic force generation conditions: objects contact each other and squeeze each other.

Conditions for the occurrence of friction: objects are in contact with each other, squeezing each other, and there is relative motion or relative motion tendency between two objects.

Therefore, if there is friction, there must be elasticity.

There is no friction if there is elastic force but no relative motion or relative tendency of motion There is no friction without friction not necessarily without elastic force (e.g. an object placed on a horizontal plane with no tendency to move and has elastic force (support force) but no friction force).

If there is no elasticity, there must be no friction. (Sliding friction formula can be used to help understand).

The difference between the displacement of the plank and the displacement of the block is = the length of the plank is 5m

In the first stage, A to C analyzes the small wooden blocks: f-U1mg=5G, that is, A1=5M S 2, at this time, the wooden board U1mg-2U0mg=3M A2=3, so the wooden board is used as the reference system: ac=1, 2(A1-A2)T1 2, T1=1S, and in the second stage, C to B, small wooden blocks, A3=10

Plank a4=-2

So stop after the plank. At this point, the small wooden block has reached the edge.

So s2=4m=5t2+1 2a3t 2-(3t+1 2a4t 2) gives t=2 3

So share 5 3s

I don't know if there's a mistake.

The title is not clearly stated, and the specific amount is not explained. . .

1.You are wrong, f and f friction are action and reaction forces, is a pair of forces that are always reversed, equal in size, and the force objects are different, when considering the bicycle movement, you should not put together, but should consider the resistance of the bicycle, such as wind resistance, front wheel resistance and rear wheel power (that is, you say f friction) and so on.

2.There is no reaction force, or the reaction force is 0

It feels like you don't understand enough about action and reaction forces.

zrunnerup You see the landlord's sentence "The friction of the rear wheel of the bicycle is forward, and the force of the rear wheel on the ground is backward" Now only talk about the rear wheel, do you analyze the front wheel?

You say, "Then f exists (assuming the friction of the front wheels) and f friction (assuming the friction of the rear wheels) will not exist." Let me ask, is this in line with the nature of "simultaneous production and disappearance" of force? ”

The answer is yes!! If you want to talk about the front wheels, yes, the front wheels also have a pair of action and reaction forces (the friction of the front wheels against the ground and the resistance of the ground to the front wheels backwards), and it is in line with the nature of the force "simultaneously producing and disappearing at the same time!!

When the rear wheel is lifted, the original force and reaction force of the rear wheel have disappeared at the same time, which is in line with the nature of "simultaneous production and simultaneous disappearance" of the force!! Got it???

1.The uniform movement of the bicycle is the whole of the bike, and you don't think about the front wheel of the car. The force f applied to the rear wheel is the friction force of the rear wheel on the ground, and the friction force of the ground on the rear wheel is a pair of interaction forces.

The friction force on the ground against the rear wheels is the forward momentum of the car, and this force is balanced by the backward friction of the front wheels on the ground.

2.If you push an object by hand on a smooth plane, there is no force interaction between the hand and the object without considering inertia. It's equivalent to your hand and has to push anything.

You need to force your hand to move, it's your body that uses the hand, but there's no force between the hand and the object being pushed. I don't know if you can figure it out.

It is true that action and reaction forces exist at the same time. What I mean by this is that in the case of 2, your hand has no force on that object, and that object has no force on your hand. Neither action nor reaction forces are available.

1。The earth is the reference, and the earth gives the rear wheel a force (direction of motion) relative to the earth, and the bicycle is of course forward. Think of the car as a piece of wood, man.

2。A reaction force exists, and the object moves at an accelerated pace.

Note: The force is generated by the interaction of two objects, and there is an action force and a reaction force relative to the two objects. You don't have a clear concept of mechanics.

The generation of force is the minimum of two objects, one acting on the other and the other having a reaction force on the object. For example, if the teacher hits you, you are in pain, but in fact he is also in pain.