Physics questions for junior high school! About the force! A problem for calculating physical forces

Question 1 b Question 2: Object to object.

Question 3: Ceiling.

Question 4: No, because the force is the action of the object on the object, and the force can only occur between two objects.

Question 5: Within the range of elastic deformation, the original shape can be restored, but if the force is too large, the shape will be changed and cannot be restored.

1.I choose ぐRelying on inertia, the shot put is not forced.

2.Force is the action of an object on an object (I personally find this sentence funny)3Rope. 4.No. The force must have an object to which the force is applied, and there must be another object to which the force is applied. An object cannot produce the action of force.

5.Possibly reverted to the original state. Pinch the balloon and let go.

b is correct. The force is reciprocal, and when the foot pushes the ground, the ground is also powerful against the foot.

A false. After the shot put, the hand has no strength against the shot put. The shot put continues to go because of inertia.

2. Between objects.

Rope. No. The object subjected to force and the object applied by force are two objects and cannot be one.

The shape of the object is restored. For example, if you squash a ball with both hands and remove the other hand, the ball will regain its shape.

b mutual.

Chandelier. I can't, I can't change back to the way it was, spring, I can't change it back to the way it was, table tennis.

Question 1: Select b The force is between objects.

Rope. No, because the force is reciprocal, he stops in the air, and there is no object exerting force on him, so it can't.

Some of them are restored, some of them are kept in shape, and they are used to pinch the plasticine, plastic bottles, and so on.

Question 1 The b-shot put continues to fly because it has inertia.

Question 2: Between objects.

Question 3 Chandeliers.

Question 4 Inability The effect of force is mutual.

Question 5: It is possible to revert to the original state or to maintain the status quo, such as squeezing balloons and pinching plasticine.

b. Between objects.

Rope. No, the action of the force is reciprocal, and no object exerts a reaction force on her.

Some will recover, some won't, plasticine, springs.

Gravity is in the center of the rod.

Set the rod length to l and balance it according to the moment.

g*1/2l=f*l

f=1/2g=

Support force, the pull of a rope with a heavy object hanging on the rope.

As long as the force caused by deformation is elastic force, spring compression and elongation.

From the uniform thickness, it can be seen that the center of gravity of the wooden stick is located in the middle of the wooden stick, which can be obtained from the similarity, l2 l1=2 1

One end is raised, which is equivalent to a lever that is pointed at one end of the stick.

f1×l1=f2×l2

f1=g=500n

f2=250n

Ejection force Collision force Support force.

A wooden stick of uniform thickness weighs 500 N, and it takes 250 N to lift the other end when one end hits the ground.

Ejection force Bounce force The force that restores the object when it is deformed is the force of spring.

Friction is the force that hinders relative motion and is always opposite to the direction of relative motion, or opposite to the direction of the relative trend of motion.

If the force on the object in the horizontal direction is only frictional force, it is not subject to any other force

When an object moves in a straight line at a uniform speed, it is either not subjected to a force, or it is subjected to a balanced force.

The conveyor belt is moving at a constant speed to the right, when the rice bag is just put on, it changes from stationary to right, and there is no other tension or thrust in the horizontal direction, so it is only subject to the friction to the right;

When the object moves in a straight line at a uniform speed, it is either not subject to force, or it is subject to a balanced force When the rice bag moves at a uniform speed to the right, if it is subjected to friction in the horizontal direction, there must be another force to balance it, because there is no other force, it is not subject to force

When the conveyor belt is braked, the rice bag moves from right to stationary, and only the direction of friction to the left can prevent the rice bag from continuing to move to the right

At the beginning, the belt moves to the right, while the rice has no movement situation, once the rice is placed on it, the rice moves to the left relative to the belt, and the friction is opposite to the relative movement trend so it is to the right.

We can think of the conveyor belt as a reference, then this is the conveyor belt is stationary, and the rice bag moves to the left relative to the conveyor belt, according to the definition of friction, the direction of friction on the object is opposite to the direction of movement of the object, so the direction of friction is to the left! (Give points!) ）

Because the moment when the rice bag is just put on the conveyor belt, due to the inertia to remain stationary, and the conveyor belt is moving to the right, the relative direction of movement between the rice bag and the conveyor belt is to the left, and the direction of friction is opposite to the relative direction of movement, so it is to the right.

Because the rice bag moves to the right with the conveyor belt at the beginning, and the rice bag has a rightward acceleration, which is generated by the rightward friction of the rice bag.

The direction of friction on the rice bag is opposite to the direction of its movement.

Looking at the relative movement trend, the speed of the rice bag sliding when it goes up is less than the speed of the transmission belt, so it moves to the left relative to the conveyor belt, so it is subject to the friction force of the direction to the right. And you think he's moving to the right, which is relative to the ground, and he's not in contact with the ground, it's not pure friction.

Quite simply, remember that the object will maintain inertia, and when you first put it on, the bag v=0At this time, the conveyor belt speed is horizontal to the right. The friction force is not opposite to the direction of motion, but to the opposite direction of motion (relative to the contact surface), then the bag is to the left with respect to the conveyor belt (because the bag is to the right), so the conveyor belt has a force to the right to prevent the bag from going to the left.

You should be able to understand this explanation!!

When considering the force of an object, the object is used as a reference frame, and when the conveyor belt is placed, it accelerates to the right, proving that it is subjected to a force to the right, and this force is the frictional force.

When the bag is just put up, the bag will move to the left, and the friction has the force to stop the movement of the object, so it is subjected to the friction in the direction to the right.

When the rice bag is placed on the conveyor belt, it will move to the right, and in the horizontal direction, it will only move when it is subjected to friction to the right.

Note the frame of reference, you are talking about the conveyor belt as the frame of reference, that is, the space that serves as the reference. Imagine if you stand on a conveyor belt and you will see the rice bag moving to the left. If you are standing on the ground, that is, using the ground as a frame of reference, then you will see the rice bag moving to the right, so the rice bag force is to the right.

In the horizontal direction, the initial velocity of the rice bag is 0 and it is subject to only one horizontal friction force, so the direction of motion is consistent with the direction of the force. Be imaginative, and do these questions to help you understand.

The rice bag is stationary at first, and then moves to the right, which is naturally subject to a force to the right, and the gravity and support forces are both vertical, so naturally it can only be subjected to the friction of the conveyor belt to its right. Friction is used here as "power". You can also understand that the rice bag moves to the left relative to the conveyor belt, so that the friction it receives from the conveyor belt is to the right.

The rice bag is just put on there is no speed, the conveyor belt and the rice bag friction, so that the rice bag moves forward, so the direction of the force to the right, is a uniform acceleration movement, and then wait until the rice bag accelerates to the same speed as the conveyor belt, then it is not subject to force in the horizontal direction, and it moves at a uniform speed.

The direction of sliding friction is opposite to the direction of movement of the relative contact surface, and if the contact point is used as a reference, the rice bag is moving to the left, so the direction of sliding friction is to the right.

When the conveyor belt is first placed, the rice bag tends to move to the left, so it is subject to friction to the right.

If the friction is to the left, how can the bag move to the right?

Standard format.

1) Understand the time when they met:

t=s/vs/(v1+v2)

75km (50km h+100km h) The distance from the original departure point is.

s=vt100km/h*

50km A...

Idea: The first question is a simple encounter problem, and this lz should understand that distance is equal to velocity and multiplication time.

The second question is the distance traveled by taxi during this period, the simplest itinerary problem, speed multiplication time lz, you give me a share.

1) Solution: The time is the same for both of them, and the time is x.

The distance of the bus is 50x and the distance of taxi is 100x.

100x+50x=75

x=hour) 2) s=vt=100=100 x

1) Solution: Let them meet in x hours.

50x+100x=75

x=2）s=vt=100km/h*

10 bricks are 1000N, which is useful work, and the efficiency is 80%, so the total weight is 1000 80% = 1250 N, so the movable pulley is 50N.

When a person is on the ground, the more bricks they pull, the more efficient they are. The maximum pulling force of the rope is 700N, and no matter how big it is, people will fly, which is impossible.

The total weight on the left is 1400n, so it's a maximum of 11 bricks!

1100n is the gravitational force of useful work, and the total weight is 1100 + 200 + 50 = 1350 n maximum efficiency = 1100 1350 * 100% =

It can be seen from the question that when the worker lifts 10 bricks at a uniform speed, the mechanical efficiency of this device is 80%. i.e. 10*100

So the pulley weight is 1250N-100*10-200=50N, and because: the worker mass is 70kg, so the maximum pulling force is 700N, this device is equipped with a movable pulley, so the maximum lifting is 1400N, 1400N-250N=1150N is the maximum lifting weight of the object, and because: The weight of a single object is 100N, so the maximum lifting is 11 pieces, so the mechanical efficiency is:

I'm on my phone now, and it's hard to answer! Wait for me to get on the computer in the afternoon to solve it for you!

There should be another option for the first question: it is easy to fall the graduated cylinder and break it, which is not advisable.

Question 1: The measured quality is inaccurate due to the water absorption of the wooden stopper; Objects that cannot be weighed wet on the scale.

2: (This is the best answer I know) Since it is a volume, you can weigh it first, and then make it soft.

After the cork absorbs enough water, it is then thrown into the water to measure the volume. This also prevents the plug from floating up in the water and having to be poked down with a wire.

3: grams of cubic centimeters.

The fourth step is problematic, wet objects cannot be weighed directly on the balance.

2) It is possible to put a layer of plastic film on the outside of the object (this question should have a better answer).

3: grams of cubic centimeters.

The quality of the cork should be measured first, so as to avoid the water absorption of the cork and the inaccurate quality of the cork; Wet objects cannot be weighed directly on the balance; The indication corresponding to the water surface should not be read out after a period of time, but should be read out as soon as possible.

2) Sand drainage.

3: grams of cubic centimeters.

b. The quality of the cork should be measured first, because the cork will absorb water after being immersed in water, which will affect the quality, and can not be read again after a while, the cork stopper has water absorption, which will reduce the liquid level, and fine sand can be used instead of water.

grams cubic centimeters.

If the distance to the 2nd floor is s, the speed of the escalator can be calculated as v1=s and the speed of 30 people walking is s 90.

When a person walks up the escalator, the speed is the sum of the two, and the time taken should be:

t=s (s 30+s 90)=seconds.

Escalator speed v1, human speed v2, escalator length s:

t1=s/v1=30 s

t2=s/v2=90 s

The person walks up the escalator of movement:

t=s/(v1+v2)=s/(s/t1+s/t2)=t1t2/(t1+t2)= s

Escalator length l

The speed of the ladder v1 = l 30, the speed of the person v2 = l 90 the speed of the person walking on the ladder v = v1 + v2 = 4l the time it takes for a person to walk on the ladder t = l (4l 90) = physical wolf pack.

Staircase length s, escalator speed v1, person speed v2 The time for people to delay the escalator going up is t

Known: s v1 = 30 i.e. v1 = s 30

s v2=90 i.e. v2=s 90

t=s/(v1+v2)=s/(s/30+s/90)=90/4= s

Let the distance be L, the escalator is V1, and the relative speed of the person is V2 when the ladder moves and does not move:

l=t1*v1=30v1

v1=l/30

When the ladder does not move:

l=v2*t2=90v2

v2=l/90

When a person walks up a moving escalator.

t=l/(v1+v2)=l/[(l/30)+(l/90)]=30*90/(30+90)=