Raymond J. Fonash Radnor High School, 2011-2012. May the FORCE be with you . . . Newton’s Laws of Motion F O R C E S. Use the force, you will. Forces are: Pushes or Pulls on an object. Forces cause objects to: START, STOP, or Change Direction
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Radnor High School, 2011-2012
May the FORCE be with you . . .
Use the force, you will.
Forces are: Pushes or Pulls on an object.
Forces cause objects to: START, STOP, or Change Direction
Two forces we are familiar with: Gravity
The symbol for Force is a capital F:F
Forces are vectors – they have both magnitude and direction.
Newton came to the Dark Side. I know more about the Force than he does.
An example of this is the Waiter and
the tablecloth trick. When you yank
out the tablecloth, the dinnerware
Remains in place. WHY????
When we push an object across the floor, we see that it does NOT stay in motion. It will eventually stop. Was Newton wrong ??????
Newton looked at the relationship between the force exerted on an object and what happened to the object.
Here’s what he saw:
If you exert a force on an object with a small mass, it begins to move quickly
If you exert the same force on an object with a large mass, it doesn’t move very quickly.
and he saw another relationship between the force exerted on an
object and what happened to the object.
Here’s what he saw:
Most people accelerate at the same rate in shopping aisles.
It takes a little Force
to push an empty cart
It takes a great Force
To push a full cart.
Forces do NOT cause objects to MOVE.
Forces cause objects to ACCELERATE !
F = 100N
You push a box with a horizontal force. How fast does it accelerate?
F = 100N
m = 50kg
m = 50kg
F = m*a
100 = 50 (a)
a = 2 m/s2
We need to find the Net Force.
- The Net Force is the vector sum of all the forces
acting on the object.
you need to add up both the magnitude AND the direction
of every force to make ONE Resultant force.
The forces add up as vectors. The net force Fnet = 5
The Fnetmakes the box accelerate to the right.
The Forces cancel each other out. Fnet = 0
The box will not accelerate.
If the Net Force = o then neither team accelerates.
Click for answer:
3.6N @ 56.3 degrees to the right of up
F = m ∙ a
F = 80kg ∙ 9.81m/s2
F = 784.8 kg∙m/s2
F ≈ 800 N
My mass is 80 kg
My weight is 800 N
1st Law – Non-Accelerating Bodies
Off-setting forces in
a straight line
Fnet = 0
(@ rest or in motion)
2nd Law – Accelerating Bodies
Forces in a straight line Fnet = m ∙ a
Newton’s 2nd Law of Motion for accelerating bodies in 1 Dimension (all forces acting along a straight line)
When forces are put on an object with mass, (remember that ALL objects have mass)
the object will accelerate. (This means it will start, stop, or change direction.)
Remember, when there are two or more forces, you must compute the net force.
Newton’s Law should be more precisely stated as: FNet = m ∙ a
Newton’s 1st Law – Non-Accelerating Bodies @ rest in 1 Dimension (all forces acting along a straight line)
Consider an object hanging from a single rope suspended from a ceiling . . . (@ rest)
You go mountain climbing. You bring a
very long rope (Be careful, you bought it
at the dollar store!) and 20kg of climbing
equipment. Your mass is 80kg. The rope
has a breaking force of 400N.
How can you safely climb the mountain?
Image two window cleaners on a scaffold as shown:
What is the tension in each rope?
What is the tension in each rope if
Painter #2 moves to the center of
What is the tension in each rope if
Painter #2 stands next to Painter #1?
What would happen in each case if the rope’s breaking force is 1600N ?
- You can try this @ home with two bathroom scales . . . .
Newton’s 1st Law – Non-Accelerating Bodies in Motion in 1 Dimension (all forces acting along a straight line)
Remember it was believed by Aristotle that if an object was in motion, there was a net force on it. Galileo and Newton said he was WRONG !!!
If an object is in motion, but traveling with constantvelocity, there is NO acceleration and hence NO Net Force !
Any forces acting on it will cancel each other out. ∑ Forces = 0
A 10,000kg plane travels at a constantvelocity through the air. It’s engines put out 1200N of thrust.
I Love to cut grass !
Consider this . . .
Q: what is the resisting horizontal force of
friction on the lawnmower by the grass?
A: the exact same as the horizontal force applied to
the mower, just in the opposite direction.
The Law of Action / Reaction Pairs
Let’s start with the easy part . . . To every action there is always an equal
and opposite reaction.
Very simply stated:
If you want to go forward, Newton said you have to push backward.
If you want to go up, you need to push down.
Forces always come in pairs - known as "action-reaction force pairs." Identifying and describing action-reaction force pairs is a simple matter of identifying the two interacting objects and making two statements describing who is pushing on whom and in what direction. For example, consider the interaction between a baseball bat and a baseball.
The baseball forces the bat to the left; the bat forces the ball to the right. Together, these two forces form the action-reaction force pair.
The action is the force (weight) of the book pushing down on the table.
The reaction is the force of the table pushing back up on the book.
FYI: This upward reaction force perpendicular to the plane of contact
between two bodies is called the Normal Force. FN
You pull with 500 N on a rope tied to a brick wall.
The action is the pulling force of 500N on the wall.
The reaction is the pulling force of 500N back on you.
(Yes, the wall actually pulls back on you!)
The forces of two bodies on each other are always equal and are directed in opposite directions.
Imagine two ice skaters pushing off from each other.
The force exerted on each other is identical except
for direction. This means that each skater can only
push the other with the same magnitude of force.
When you fire a rifle, the blast pushes the bullet forward with the same amount of force as the recoil pushes you back. The bullet just accelerates at a faster rate than you because of its smaller mass. Don’t forget Newton’s 2nd Law: F = m∙a
So then how does anything ever move if there are always equal and opposite forces?
Only forces onyou accelerate you.
Forces you exert on other objects accelerate the other objects.
Let’s consider a classic Physics example:
1. A hammer hits a nail into a block of wood. (Let’s say with a force of 50 N)
the action force: the hammer exerts a force on the nail.
the reaction force: the nail exerts an equal and opposite force on the hammer.
Let’s look at the forces ON each object involved . . .
5N Force on nail50N Force on nail
by wood. by hammer.
Fnet(nail) = 45N causes nail to accelerate into wood.
50N Force on hammer by nail.
Fnet (hammer) = 50N causes hammer to stop.
5N Force on wood by nail.
Fnet(wood) = 5N causes wood to accelerate.
Here’s another classic Physics Example of Newton’s 3rd Law of Motion:
If the horse pulls on the cart with the same force that the cart pulls on the horse,
How does they EVER move?
Again, look at each object alone and identify the forces ON that object.
Action Force: the horse pulls on the cart with 100N of Force.
Reaction Force: the cart pulls on the horse with 100N of Force
Action Force: the horse pushes on the earth with 150N of Force.
Reaction Force: the earth pushes on the horse with 150N of Force.
Earth Horse Cart
150N on earth by horse. 150N on horse by ground 100N on cart by horse.
100N on horse by cart
Fnet = 150N Fnet = 50N Fnet = 100N
Effect: earth accelerates Effect: horse accelerates Effect: cart accelerates
Aristotle believed that space travel was NOT possible because there were no air molecules to push against. Understanding Newton’s 3rd Law of Motion and the idea of equal and opposite forces, state how rockets can travel through space.
You are standing at the middle of an ice rink (very slippery) and cannot walk to the edge of the rink. You are holding a fire extinguisher. How could you get to the edge of the rink.
What happens when you punch a brick wall? How do you know?
How does a starting block work for track runners?
What happens when you hit an empty box of tissues. Describe the action/reaction force pairs.
Look at the picture below. Describe in your own words how the
puller and toboggan can accelerate
even with action/reaction pairs of forces.
Force Diagrams are also called Free Body Diagrams. They replace drawings with concise coordinate vectors.