# Homework Help: Why doesn’t the motor start right away?

1. Mar 2, 2016

### Marcin H

1. The problem statement, all variables and given/known data
Not sure if this is the right place to ask, but why doesn’t a motor start right away as the voltage is increased above zero? Here is the circuit and some other info.

2. Relevant equations
KVL/KCL
V=IR

3. The attempt at a solution
Does this have anything to do with the load current? I'm not really sure why the motor doesn't start. I would think that it should start at a small voltage, but move very slowly. Does the load resistance have anything to do with this?

2. Mar 2, 2016

### haruspex

If you push very gently on the side of a box on the floor, does it move straight away?

3. Mar 2, 2016

### Marcin H

Friction? It doesn't move because of friction? I haven't thought about that, but it makes sense. Is that the only reason though?

4. Mar 2, 2016

### haruspex

Before worrying about any other possibilities, can you decide whether friction could explain all the observations?

5. Mar 2, 2016

### Marcin H

I want to say no, but I'm not sure. Wouldn't the sense resistor also affect the motor? You would have a voltage drop across it, but the current would be the same since they are in series. Also the motor isn't turning anything like a wheel or something, so I don't know if friction would be the main reason.

6. Mar 2, 2016

### haruspex

It's only there to detect the current, so it should be a small resistor. Besides, its only effect would be to lower the applied voltage a little, it would not make it zero.
The motor has to be made to turn. That implies an applied torque. There would certainly be some frictional torque in the motor axle.
Can you see a way to explain the two different threshold voltages in terms of friction?

7. Mar 2, 2016

### Marcin H

Not sure what you mean here. I guess once you get to 2.9V you overcome the force of friction and the motor starts moving. The turn off voltage is lower because of momentum, I think.

8. Mar 3, 2016

### haruspex

Think back through what you have been taught about friction. What are the key facts?

9. Mar 3, 2016

### Marcin H

Static frictional forces from the interlocking of the irregularities of two surfaces will increase to prevent any relative motion up until some limit where motion occurs. It is that threshold of motion which is characterized by the coefficient of static friction. The coefficient of static friction is typically larger than the coefficient of kinetic friction.

10. Mar 3, 2016

### CWatters

Not due to Momentum but..