Answer :
1. A car that increases its speed from 20 km/h to 100 km/h undergoes positive acceleration.
2. It usually takes more force to start an object sliding than it does to keep an object sliding because static friction is usually greater than sliding friction.
3. In a closed system, the loss of momentum of one object is equal to the gain in momentum of another object.
4. The universal force that is effective over the longest distances is gravitational force.
5. As an object’s temperature increases, the rate at which it radiates energy increases.
6. When a person plucks a guitar string, the number of half wavelengths that fit into the length of the string determines the pitch of the sound produced.
7. The law of reflection states that the angle of reflection is equal to the angle of incidence.
8. The region where a magnet's force is strongest is at the pole.
9. The magnetic field lines of a bar magnet begin near the magnet’s north pole.
10. The energy source used to produce most of the electrical energy in the United States is fossil fuels.
2. It usually takes more force to start an object sliding than it does to keep an object sliding because static friction is usually greater than sliding friction.
3. In a closed system, the loss of momentum of one object is equal to the gain in momentum of another object.
4. The universal force that is effective over the longest distances is gravitational force.
5. As an object’s temperature increases, the rate at which it radiates energy increases.
6. When a person plucks a guitar string, the number of half wavelengths that fit into the length of the string determines the pitch of the sound produced.
7. The law of reflection states that the angle of reflection is equal to the angle of incidence.
8. The region where a magnet's force is strongest is at the pole.
9. The magnetic field lines of a bar magnet begin near the magnet’s north pole.
10. The energy source used to produce most of the electrical energy in the United States is fossil fuels.
1. A car that increases its speed from 20 km/h to 100 km/h undergoes positive acceleration.
In fact, acceleration is defined as:
[tex]a=\frac{v-u}{t}[/tex]
where v is the final velocity, u is the initial velocity, and t is the time taken. t has always positive sign, and in this case, the final velocity (v=100 km/h) is greater than the initial velocity (u=20 km/h), so the numerator is positive and therefore the acceleration is positive.
2. It usually takes more force to start an object sliding than it does to keep an object sliding because static friction is usually greater than sliding friction.
When sliding an object over a horizontal surface, the magnitude of the frictional force that acts agains the motion of the object is given by:
[tex]F_f = \mu m g[/tex]
where [tex]\mu[/tex] is the coefficient of friction, m is the mass of the object and g is the gravitational acceleration. The value of [tex]\mu[/tex] changes if the object is at rest or if it is moving: in fact, when the object is at rest, the value of this coefficient is larger ( in this case we have static friction), while when the object is already moving, the value of this coefficient is smaller (in this case we have kinetic friction).
3. In a closed system, the loss of momentum of one object is equal to the gain in momentum of another object.
In a closed system, the total momentum must be conserved. For instance, if we have two objects, the total momentum is
[tex]p=p_1 +p_2[/tex]
and since p must be conserved, if one object has a loss in momentum (for example, [tex]p_1[/tex] decreases), the other object must gain a momentum equal to the loss in momentum of the first object (so, [tex]p_2[/tex] increases).
4. The universal force that is effective over the longest distances is gravity.
Gravity is the fundamental force that acts between every object with mass, and its magnitude is given by:
[tex]F=G\frac{m_1 m_2}{r^2}[/tex]
where G is the gravitational constant, m1 and m2 are the two masses, r is the distance between the two objects. Gravity has an infinite range, therefore it acts between all the objects with mass of the universe.
5. As an object’s temperature increases, the rate at which it radiates energy increases.
The temperature of an object is directly related to the average kinetic energy of the molecules inside the object, and also to the thermal energy possessed by the object. The rate at which an object radiates energy to the surroudings depends on its temperature: the higher the temperature, the faster it will radiates energy to the surroundings.
6. When a person plucks a guitar string, the number of half wavelengths that fit into the length of the string determines the harmonic of the sound produced.
When a guitar string is plucked, standing waves are produced in the string. The characteristics of these standing waves is that only certain wavelengths/frequencies are allowed in the string: these modes of oscillations are called harmonics, and the number of wavelengths that fit inside the string determines the order of the harmonic.
7. The law of reflection states that the angle of reflection is equal to the angle of incidence
When a light ray hits a surface, it can undergo reflection: the angle of incidence, which is measured as the angle between the incident ray and the normal to the surface, is equal to the reflection angle, which is the angle between the reflected ray and the normal to the surface.
8. The region where a magnet's force is strongest is at the poles.
The pole of a magnet is the region where there is a larger density of magnetic field lines: the concentration of field lines gives the strength of the field, so the greater this concentration, the stronger the field, and this occurs at the poles of the magnet.
9. The magnetic field lines of a bar magnet begin near the magnet’s poles.
In a bar magnet, the magnetic field lines start from the north pole and end at the south pole of the magnet.
10. The energy source used to produce most of the electrical energy in the United States is coal, natural gas and nuclear energy.