It is very rare, especially on Earth, to have an object that is experiencing force from only one direction. A bicycle rider has the force of air friction pushing against him. He has to fight against the friction between the gears and the wheels. He has gravity pulling down on him. His muscles are pushing and pulling inside him and so on and so on.
Even as you sit there, you have at least two forces pushing and pulling on you. The force of gravity is pulling you to the center of the Earth. The chair is pushing up on you so you don’t go to the center of the Earth. So with all these forces pushing and pulling, how do you keep track of them all? That’s where net force comes in.
The net force is when you add up all of the forces on something and see what direction the overall force pushes in. The word “net”, in this case, is like net worth or net income. It’s a mathematical concept of what is left after everything that applies is added and subtracted. The next activity will make this clearer.
Here’s what you need:
[am4show have=’p8;p9;p11;p38;p92;’ guest_error=’Guest error message’ user_error=’User error message’ ]
- a rope (at least 3 feet long is good)
- a friend
- a sense of caution
(Be careful with this. Don’t pull too hard and please don’t let go of the rope. This is fun but you can get hurt if you get silly.)
Download Student Worksheet Exercises
1. You and your friend each grab an opposite end of the rope.
2. Both of you pull just a bit on the rope.
3. Have your friend pull a bit harder than you. Notice the direction that you both move.
4. Now you pull harder than your friend. Now which way do you go?
5. Lastly, both of you pull with the same strength on the rope. Even though you are both pulling, neither one of you should move.
In this experiment, there were always at least three forces pulling on the rope. Can you think of the three? They are you, your friend and gravity. You were pulling in one direction. Your friend was pulling in another direction and gravity was pulling down.
When one of you pulled harder (put more force on the rope) than the other person, there was a net force in the direction of the stronger pull. The rope and you guys went in that direction.
When both of you were pulling the same amount, there was an equal force pulling the rope one way and another equal force pulling the rope the other way. Since there were two equal forces acting in two opposite directions, the net force equaled zero, so there was no movement in either direction. No net force in this case means no movement.
As you’ll see when you learn about Newton’s second law, no net force means no acceleration.
Let’s take another look at the bicycle rider we talked about earlier. To make things easier, we’ll call him Billy. For Billy to speed up, he needs to win the tug of war between all of the forces involved in riding a bicycle. In other words, his muscles need to put more force on the forward motion of the bike than all of the forces of friction that are pushing against him.
If he wants to slow down, he needs to allow the forces of friction to win the tug of war so that they will cancel out his forward motion and slow down the bike. If he wants to ride at a steady speed, he wants the tug of war to be tied. His muscles need to exert the same amount of force pushing forward as the friction forces pulling in other directions.
Advanced Students: Download your Net Forces Lab here.
Exercises:
- For scenario 1, in which direction did you both move? Draw the free body diagram below
- For scenario 2, in which direction did you both move? Draw the free body diagram below
- For scenario 3, in which direction did you both move? Draw the free body diagram below
- For scenario 4, in which direction did the rope move? Draw the free body diagram below
- What was the same about question 1 and question 4? What was different?
- Even though the forces were less in question 1 than question 4, what was the net force for both?
- There were always at least 3 forces acting on the rope, what were they? Did you include the third force in your free body diagram?
- If the rope wasn’t moving, but you had only one force moving down, what does that tell you about the force you and your friend exerted?
[/am4show]