Answer:
If the track is horizontal and the string is pulled horizontally, the friction on the cart would be 
.
Explanation:
Let 
denote the mass of this cart, and let 
denote the acceleration of this cart.
.
.
Apply Newton's Second Law to find the net force on this cart.
.
The following forces act upon this cart:
- (downward) gravitational attraction from the earth,
- (upward) normal force from the track,
- (forward) tension from the string, and
- (backward) friction from the track.
Assume that the track is horizontal, and that the string was pulled horizontally. The normal force from the track would exactly balance the downward gravitational attraction from the earth. Hence, the 
net force on this cart would be equal (in size) to the size of the tension from the string (
) minus the size of the friction from the track.
In other words:
.
.
.
Personally, I use a thermometer.
The correct answer is A, electrons enter orbitals of lowest energy first. The Aufbau principle states that electrons orbiting atoms fill the lowest energy levels available before filling higher levels. Following this, molecules can go into the most stable electron configuration.
The particles that carry charge through wires in a circuit are mobile electrons. The electric field direction within a circuit is by definition the direction that positive test charges are pushed. Thus, these negatively charged electrons move in the direction opposite the electric field.
