Solving the Conceptual Fnet=ma Problem for Bill & Susan

  • Thread starter saralsaigh
  • Start date
  • Tags
    Conceptual
In summary, Bill and Susan are both standing on identical skateboards with Bill weighing three times as much as Susan. Bill pushes on Susan's back causing Susan to start moving away from him. After Bill stops pushing, the correct statement is that Susan and Bill are moving away from each other with Susan's speed being three times larger than that of Bill. This is due to the application of Newton's third law and the law of conservation of linear momentum.
  • #1
saralsaigh
19
0
sorry i got one more question, its conceptual and i have a little problem with it,
1. The problem:
Bill and Susan are both standing on identical skateboards (with really good ball bearings) initally at rest. Bill weighs three times as much as susan. Bill pushes horizontally on Susan's back, causing susan to start moving away from Bill. Immediately after Bill stops moving.
We are give many choices and asked to pick the correct statement, the two that i have a problem with are:
a) Susan is is moving away from Bill, and Bill is stationary
b) Susan and bill are moving away from each other, and susan's speed is three times larger than that of Bill.


Homework Equations


Ok i think Newton's thrid law applies here and Fnet=ma


The Attempt at a Solution



The correct answer is (b) however i have a problem with that, ok i know that the force Bill is exerting on susan is equal and opposite to the force susan is exerting on Bill however it is possible that susan starts moving while bill is stationary because the static friction of bill is larger than that of susan because his weight is more and therefore his normal force is larger. however i don't think its correct to say that if they both overcame their static friction that susan's SPEED would be three times larger shouldn't it be her ACCELERATION that is three times larger not her speed? because of Fnet=ma?
 
Physics news on Phys.org
  • #2
The wording of your problem statement must not be 100% accurate. What does this statement mean? "Immediately after Bill stops moving."

That makes no sense. Are you missing some words or punctuation marks in your problem statement? If Bill is not moving (v=0), then Susan is moving infinitely faster than Bill, not 3x.
 
  • #3
yea sry its "Immediately after Bill stops moving, "
 
  • #4
saralsaigh said:
1. The problem:
Bill and Susan are both standing on identical skateboards (with really good ball bearings) initally at rest. Bill weighs three times as much as susan. Bill pushes horizontally on Susan's back, causing susan to start moving away from Bill. Immediately after Bill stops moving.
I assume this last sentence is a typo, and should have read something like: Immediately after Bill stops pushing.
The correct answer is (b) however i have a problem with that, ok i know that the force Bill is exerting on susan is equal and opposite to the force susan is exerting on Bill
Right, they exert equal and opposite forces on each other.
however it is possible that susan starts moving while bill is stationary because the static friction of bill is larger than that of susan because his weight is more and therefore his normal force is larger.
I'm sure you are expected to ignore any effects of static friction. They have excellent, low-friction skateboards.

however i don't think its correct to say that if they both overcame their static friction that susan's SPEED would be three times larger shouldn't it be her ACCELERATION that is three times larger not her speed?
You are correct that her acceleration will be three times greater than Bill's, from Newton's 2nd law. But since the force acts for the same amount of time on each, her resultant speed will also be three times greater than Bill's.
 
  • #5
First, Newton's third law does not, at least explicitly, say Fnet = ma. It's the one with action and reaction, remember?

What you say regarding static friction is right. A greater force would be need to push Bill than Susan, but I think the problem assumes that there is no friction between the skateboards and the ground.

To the speed/acceleration: You need to use the Fnet = ma, which happens to be Newton's second law, here. A body is accelerated as long as there is a net force acting on it. Once Bill stops pushing Susan, she starts moving with the constant velocity to which she was accelerated to, and this velocity turns out to be three times Bill's velocity.(as measured by someone "fixed" to the ground) When you start learning a bit more in mechanics, you'd realize that this is a consequence of the law of conservation of linear momentum.
 
  • #6
Thanks guys for ur help i think i can sort of picture it now :biggrin:
 

1. How do I approach solving the Fnet=ma problem?

The first step in solving the Fnet=ma problem is to identify all the forces acting on the object. This includes both external forces, such as gravity or friction, and internal forces, such as tension in a string. Once all the forces are identified, use vector addition to find the net force acting on the object.

2. What is the significance of Fnet=ma in physics?

Fnet=ma is a fundamental equation in Newtonian mechanics, which describes the relationship between force, mass, and acceleration. It is derived from Newton's Second Law, which states that the net force acting on an object is directly proportional to its acceleration and inversely proportional to its mass.

3. Can you provide an example of solving the Fnet=ma problem?

Sure, let's say Bill and Susan are sitting on a seesaw, with Bill on the left side and Susan on the right. Bill has a mass of 50 kg and Susan has a mass of 40 kg. If Bill pushes down on his side with a force of 200 N, what is the net force and acceleration of the seesaw? First, we identify the forces: the normal force from the seesaw on Bill, the normal force from the seesaw on Susan, and the force from Bill pushing down. The net force is the vector sum of these forces, which in this case is 200 N to the right. Using Fnet=ma, we can calculate the acceleration of the seesaw to be 200 N / 90 kg = 2.22 m/s^2.

4. Are there any common mistakes to avoid when solving the Fnet=ma problem?

One common mistake is forgetting to consider all the forces acting on the object. It is important to identify both external and internal forces. Another mistake is mixing up the direction of forces and acceleration. Remember that forces and acceleration are vector quantities, so their direction matters.

5. How can I check if my solution to the Fnet=ma problem is correct?

To check your solution, you can use the equations of motion, such as s=ut+1/2at^2 or v=u+at, to find the displacement or velocity of the object. You can also use the principles of conservation of energy or momentum to see if your solution is consistent with these laws. Additionally, you can plug your values back into the original equation Fnet=ma to see if it balances out.

Similar threads

  • Introductory Physics Homework Help
Replies
2
Views
863
  • Introductory Physics Homework Help
Replies
4
Views
2K
  • Introductory Physics Homework Help
Replies
16
Views
5K
  • Introductory Physics Homework Help
3
Replies
90
Views
3K
  • Introductory Physics Homework Help
Replies
9
Views
2K
  • Introductory Physics Homework Help
Replies
7
Views
204
  • Introductory Physics Homework Help
Replies
10
Views
5K
  • Introductory Physics Homework Help
Replies
7
Views
702
  • Introductory Physics Homework Help
Replies
20
Views
1K
Replies
24
Views
1K
Back
Top