# Newton's laws

1. Jun 23, 2015

### J-dizzal

1. The problem statement, all variables and given/known data
An electron with a speed of 1.1 × 107 m/s moves horizontally into a region where a constant vertical force of 3.2 × 10-16 N acts on it. The mass of the electron is 9.11 × 10-31 kg. Determine the vertical distance the electron is deflected during the time it has moved 34 mm horizontally.

2. Relevant equations
F=ma

3. The attempt at a solution

2. Jun 23, 2015

### Matternot

Try starting by using your "relevant equation"?

When calculating the distance travelled, what information do we need to calculate initially?

3. Jun 23, 2015

### J-dizzal

distance = vt?

4. Jun 23, 2015

### Matternot

That's not a bad idea. We will need that equation. The problem is the velocity isn't constant because there is a force. How do we calculate the time it is travelling for though? The time it's travelling for is important.

Last edited: Jun 23, 2015
5. Jun 23, 2015

### J-dizzal

v = v0 + at and solve for t? this eqate to zero, nevermind

6. Jun 23, 2015

### Matternot

It's very important to treat the vertical and horizontal velocities separately. The vertical force does not affect the horizontal speed. The horizontal speed does not affect the vertical speed. Maybe try using $s=vt$?

7. Jun 23, 2015

### J-dizzal

would i use a motion equation(constant accerleration)?

8. Jun 23, 2015

### Matternot

Yeah, The motion equation for constant acceleration sounds like a really good idea to describe the vertical motion. The problem is we need to calculate the time it is travelling for to use that. The horizontal motion might help us here.

9. Jun 23, 2015

### J-dizzal

I dont see how a vertical force would'nt affect the horizontal speed. for example if the vertical force was enough to change the direction of the particle to a vertical path then its horizontal speed would be zero. and the horizontal speed would be changing all the way.

10. Jun 23, 2015

### Matternot

But what if the force can never be strong enough to change the direction of the particle to a vertical path? What if the horizontal speed can never be zero under the influence of this vertical force?

11. Jun 23, 2015

### J-dizzal

well then the vertical force is not strong enough, but even a very small vertical force acting on a horizontal particle would accelerate the particle vertically therefore changing its horizontal velocity. the speed of particle would not change, just the horizontal velocity dependent on its angle with respect to horizontal.

12. Jun 23, 2015

### J-dizzal

so, distance = vt and solving for t would not work yet because the time isnt known. and distance is not known

13. Jun 23, 2015

### Matternot

Why is the speed of the particle constant?

14. Jun 23, 2015

### J-dizzal

speed is constant assuming this is in a vacuum, sorry should of noted that

15. Jun 23, 2015

### Matternot

This will make sense when the separation of x and y directions makes sense. I think we should postpone this until that is understood.

I assume you're trying to use the idea of constant energy. Why do you think energy here might not be constant?

16. Jun 23, 2015

### J-dizzal

i do not know about energy, i dont thing we have got to that chapter in the book yet.

17. Jun 23, 2015

### Matternot

So what makes you think the speed is constant?

18. Jun 23, 2015

### J-dizzal

so would the particle be accelerating after it enters the force field?

19. Jun 23, 2015

### J-dizzal

wouldnt speed be constant because there are no other forces acting on the particle that are past 90deg

20. Jun 23, 2015

### Matternot

Where there is a net force on an object, there is always an acceleration. Newton's first.

The idea that horizontal and vertical motion can be treated separately is crucial.

Imagine throwing a ball to someone. When it is thrown it is given a horizontal velocity and a vertical velocity. The gravitational force will affect it's vertical velocity, but not the horizontal. As it's vertical velocity is being affected, the direction and speed of the ball is being affected.