Junior high school physics problems, the master comes to 20

The mass of copper divided by the density of copper (equal to the volume of copper (). And the volume of the copper block is (so the copper block is hollow.

Handkerchief. 2.The copper nuggets are submerged in the water. Drainage volume 1 liter. Buoyancy 10n

3.Mass divided by volume to get density: less than pure copper. So it's empty.

1) p = f s = 4450 Pa.

2) =m v=4450kg m3 sinking in water f=10n

3) Hollowness can be seen from (2).

Solution:1Analysis:

When an object is subjected to a balanced force, it will remain in a state of uniform linear motion or a state of rest. The direction of motion and the speed of motion are completely determined by the direction in which the motion begins, and have nothing to do with the direction of the force. You know:

When an object is subjected to a balanced force, the net force of the force experienced by it is zero. That is, the object is equivalent to not being acted on by any force at this time. Therefore, the direction of its motion and the speed of its motion can only be determined by the direction in which it starts to move.

Answer: A2Answer: 1n

The reason is that no matter how fast the object maintains a uniform linear motion, its force is aAffected by the force of equilibrium.

That's it. Because the magnitude of sliding friction is independent of velocity, when the velocity increases, the magnitude of friction is still 1n, and because it is still moving in a straight line at a uniform speed, the tensile force and friction force are still balanced forces. Therefore, the size is equal.

i.e. the pulling force is still 1n.

3.Wrong. The reason is that these two do not meet the conditions of the equilibrium of the two forces, so they cannot maintain a uniform linear motion state. On the other hand, the resultant force of these two forces is equal to the direction of the thrust. So the object will change its state of motion in the direction of the thrust!

Question 1: Choose A

Willingness is simple, to understand that the force is not the cause of the motion of the object, but the cause of the change of the force motion.

The object is balanced by force, i.e. it may be stationary or it may move in a straight line at a uniform speed.

The second question is equal to 1 ox.

The principle is the same as in the first question. Uniform linear motion means force balance.

Question 3 Incorrect.

It's still the first question.

Force is what changes the motion of an object. The thrust is greater than the drag, the force on the object is unbalanced, and it is impossible to move at a uniform speed.

Secondly, does the direction of the thrust change over time? Wood has a clear.

Question 1: Choose A. The reason is that the original motion of the object is not told, the object may be stationary or it may be in motion, while under the action of the equilibrium force, the state of motion remains the same.

Question 3: The answer should be equal to. Because the trolley still moves in a straight line at a uniform speed, the thrust and friction force are balanced forces, and the magnitude of sliding friction is only related to the magnitude of pressure and the roughness of the contact surface.

1. According to Newton's laws of motion, an object subject to a balanced force is either at rest or moving in a uniform linear motion in the direction of the force.

Question 2 is equal to, because both times are uniform linear motion, both on a horizontal table, the resistance is the same, and it is affected by the balance force, so the thrust is the same.

Question 3 is definitely wrong, when the thrust is greater than the resistance, the object is affected by the combined force of the thrust and the resistance, and it is definitely wrong to say that it is no longer subjected to any force...

, objects that are only subject to the equilibrium force are either stationary or do uniform linear motion 2Equal, as long as the linear motion at a uniform speed is done, the thrust does not change 3Wrong, but also by drag and thrust.

The circuit can be simplified by the equivalence method: there are only wires or switches between two points in the circuit, and there is no crossing the electrical or power supply.

It can be seen that when both S1 and S2 are closed, the three lamps are connected in parallel and the ammeter measures the dry current.

Only close S2 L1 and L2 are short-circuited The current is expressed as L1 and L2 are not lit and L3 is lit.

When both S1 and S2 are closed, L2 and L3 are connected in parallel; The ammeter measures the total current, and the indication is; When only S2 is closed, L3 emits its own light, and the indication is.

When switches S1 and S2 are closed, the three lights are connected (in parallel). If the current passing through each lamp is at this time, then the indication of the ammeter in the circuit is ( )a; If only the switch S2 is closed, then the indication of the ammeter in the circuit is ( ) A, then L1 (will not be on), L2 (will not) be on, and L3 (will be) on.

In a circuit, there are only wires or switches between two points, and there is no crossing the electrical appliance or power supply, and these two points can be regarded as one point. When S1 and S2 are closed, as shown in the figure, we can see that the three lights are connected between A and B, so they are connected in parallel. The ammeter measures the trunk current, which is equal to the sum of the currents of each branch, which is equal to.

Only close S2, L1 and L2 are short-circuited, the ammeter measures the current through L3, because the resistance is unchanged, the voltage at both ends is unchanged (also equal to the power supply voltage), so the current through L3 is also unchanged, and the current is expressed as . So L1 and L2 don't light up, L3 does.

c The battery is burned out...

Because once S is closed, there is almost no resistance to the branch passing through S, and according to the shunt of current, there is basically no current passing through L1 and L2, so L1 and L2 do not emit light, and they will not be burned out.

Since u is certain and r tends to be close to 0, the current i through the power supply tends to be infinite, and the power supply will be burned.

c This is the positive and negative poles of the power supply directly connected.

ABC instantaneous high current, burned out by burned out CThe battery was burned out.

Choose D, because when the switch is closed, L1 is not affected, while L2 will be short-circuited, and the battery itself will not be burned out.

The electrons emitted by the filament at the end of the TV display tube are negative electrons, and the direction of the negative charge is opposite to the direction of the current, so the direction of the current is from the fluorescent screen to the filament, so choose B, A and B are very different, and it is necessary to master how to judge the direction of the current.

The electrons are negatively charged, and because as long as the charge moves, there will be an electric current, and the direction of the load movement is opposite to the direction of the current, so B is chosen. You don't have to remember anything about the TV, just what electricity (positive, negative) and direction of movement you have.