Teacher Reference PageTeacher Reference PageThis powerpoint is a presentation of Forces and This powerpoint is a presentation of Forces and Newton’s Laws. It probably goes through more Newton’s Laws. It probably goes through more than is necessary for students to know on the than is necessary for students to know on the proficiency test, but is not as detailed as what proficiency test, but is not as detailed as what would be discussed in a physics class.would be discussed in a physics class.

I made no effort to make this super-fancy because I made no effort to make this super-fancy because I just don’t know how. Sorry!!I just don’t know how. Sorry!!

There are links to the RPDP site at the end of this There are links to the RPDP site at the end of this presentation. There are also a few links to sites presentation. There are also a few links to sites that show animations of various principles that show animations of various principles described in this unit. described in this unit.

Please use your physics peers for help in Please use your physics peers for help in understanding, presentation, or resources. That’s understanding, presentation, or resources. That’s what we’re here for.what we’re here for.

Forces and Newton’s Forces and Newton’s Laws of MotionLaws of Motion

What is a Force?What is a Force?

A force is defined as a push or a pull.A force is defined as a push or a pull. A force is something that CHANGES A force is something that CHANGES

the motion of an object. This could the motion of an object. This could mean that the object changes its mean that the object changes its speed by slowing down or speeding speed by slowing down or speeding up. It might also mean that the up. It might also mean that the object changes its direction of travel object changes its direction of travel after a force has been applied.after a force has been applied.

Can Objects Move Without a Can Objects Move Without a Force?Force?

Yes, a force only CHANGES the way Yes, a force only CHANGES the way an object moves. Therefore, if an an object moves. Therefore, if an object is already moving, it will object is already moving, it will continue to move at the same speed continue to move at the same speed in the same direction.in the same direction.

Changes in speed or direction are Changes in speed or direction are known as ACCELERATIONSknown as ACCELERATIONS

Therefore, FORCES cause objects to Therefore, FORCES cause objects to ACCELERATE.ACCELERATE.

Newton’s First LawNewton’s First Law

An object at rest will stay at rest, and An object at rest will stay at rest, and an object in motion will stay in an object in motion will stay in motion in a straight line, UNLESS motion in a straight line, UNLESS acted upon by an outside force.acted upon by an outside force.

Again, it takes a force to get an Again, it takes a force to get an object moving (change in motion) or object moving (change in motion) or a force to change the speed or a force to change the speed or direction of motion (also a change in direction of motion (also a change in motion)motion)

Why don’t objects keep moving Why don’t objects keep moving when I throw them on Earth?when I throw them on Earth?

Because of Newton’s 1Because of Newton’s 1stst Law. Gravity Law. Gravity is a force (a push or pull acting is a force (a push or pull acting between all objects with mass). If between all objects with mass). If you throw an object through the air, you throw an object through the air, gravity is a force that can change of gravity is a force that can change of the motion of the object.the motion of the object.

Why do rolling objects slow down?Why do rolling objects slow down?

Because of Newton’s 1Because of Newton’s 1stst Law. Law. Friction is a force that impedes the Friction is a force that impedes the motion of objects when two surfaces motion of objects when two surfaces are in contact with one another.are in contact with one another.

If we lived in a world without friction, If we lived in a world without friction, then objects would actually keep then objects would actually keep moving at the same speed. Because moving at the same speed. Because friction is a force, it changes the friction is a force, it changes the motion of the object.motion of the object.

More on FrictionMore on Friction

Friction always acts in a direction Friction always acts in a direction opposite to motion.opposite to motion.

Objects that weigh more have a Objects that weigh more have a greater frictional force acting on greater frictional force acting on them.them.

Friction even acts when objects are Friction even acts when objects are at rest. To move an object, you must at rest. To move an object, you must provide enough force to overcome provide enough force to overcome friction.friction.

Mass and InertiaMass and Inertia

Mass is the amount of matter in an Mass is the amount of matter in an object.object.

Inertia is the mass of an object.Inertia is the mass of an object. Inertia is also a measure of an Inertia is also a measure of an

object’s resistance to CHANGING its object’s resistance to CHANGING its motion.motion.

More Mass and InertiaMore Mass and Inertia

If something has a lot of mass, it has If something has a lot of mass, it has a lot of resistance to changing its a lot of resistance to changing its motion – it has a lot of inertia.motion – it has a lot of inertia.

Therefore, heavy objects are hard to Therefore, heavy objects are hard to get moving if they are at rest, OR get moving if they are at rest, OR they are hard to stop moving if they they are hard to stop moving if they are already in motion.are already in motion.

Newton’s 2Newton’s 2ndnd Law Law

The net force (all of the forces acting The net force (all of the forces acting on an object) cause the object to on an object) cause the object to have an acceleration.have an acceleration.

F = maF = ma F = total forceF = total force m = mass of the objectm = mass of the object a = accelerationa = acceleration

Newton’s 2Newton’s 2ndnd Law Law

It may be easier to explain Newton’s It may be easier to explain Newton’s 22ndnd Law in terms of acceleration (the Law in terms of acceleration (the CHANGE in the object’s motion)CHANGE in the object’s motion)

a = F/ma = F/m

a = F/ma = F/m

Because acceleration and Force are Because acceleration and Force are directly related in the equation, a big directly related in the equation, a big force results in a big acceleration. Or force results in a big acceleration. Or a small force results in a small a small force results in a small acceleration.acceleration.

This means a big push causes a big This means a big push causes a big CHANGE in motion like a drastic CHANGE in motion like a drastic speed increase or decrease, or a speed increase or decrease, or a sudden change in direction.sudden change in direction.

a = F/ma = F/m

Because acceleration and mass are Because acceleration and mass are indirectly related, applying a force to indirectly related, applying a force to a SMALL mass will result in a LARGE a SMALL mass will result in a LARGE acceleration. Applying a force to a acceleration. Applying a force to a LARGE mass will result in a SMALL LARGE mass will result in a SMALL acceleration.acceleration.

The units for ANY type of Force is the The units for ANY type of Force is the Newton.Newton.

ExampleExample

If I apply a 1 Newton force to a If I apply a 1 Newton force to a 1 kilogram object, what is its 1 kilogram object, what is its

acceleration?acceleration?

a = F/m = 1/1 = 1 m/sa = F/m = 1/1 = 1 m/s22

This is not a very large acceleration and This is not a very large acceleration and would result in only a minor change ofwould result in only a minor change of

motion.motion.

Another ExampleAnother Example

If a 500 N force is applied to a If a 500 N force is applied to a

1 kilogram object, what is the acceleration 1 kilogram object, what is the acceleration of the object?of the object?

a = F/m = 500/1 = 500 m/sa = F/m = 500/1 = 500 m/s22

This is a very large acceleration whichThis is a very large acceleration which

means that the object will have a drastic means that the object will have a drastic increase or decrease in motion.increase or decrease in motion.

USE COMMON SENSEUSE COMMON SENSE

Does it take big forces to move big Does it take big forces to move big objects? OF COURSE!!!objects? OF COURSE!!!

Does a bigger force (push or pull) Does a bigger force (push or pull) cause a greater change in motion?cause a greater change in motion?

YES!!YES!!

Use DiagramsUse Diagrams

A free body diagram shows all of the A free body diagram shows all of the forces acting on an object.forces acting on an object.

An object always accelerates in the An object always accelerates in the direction of the net force.direction of the net force.

ExampleExample

10 Newtons (The floor pushing up on the box)

10 Newtons (The object's weight acting downward)

25 Newtons (You pulling on the box to the right)

13 Newtons (Your friend pullingon the box to the left)

What the picture says…What the picture says…

The object clearly moves to the right The object clearly moves to the right because that is the direction of the because that is the direction of the bigger force. In fact the net bigger force. In fact the net sideways force would be 13 N to the sideways force would be 13 N to the right because we would be right because we would be subtracting the forces in opposite subtracting the forces in opposite directions.directions.

The picture also says…The picture also says…

The downward force of the weight is The downward force of the weight is equal to the upward force from the equal to the upward force from the table. By subtracting two equal table. By subtracting two equal forces, we get a net force of zero.forces, we get a net force of zero.

REMEMBER, a zero net force means REMEMBER, a zero net force means no acceleration – no CHANGE of no acceleration – no CHANGE of motion. This is why the object motion. This is why the object moves sideways, but not up and moves sideways, but not up and down.down.

But Just as Important…But Just as Important…

An object may have two equal forces An object may have two equal forces acting on it and still be moving. It acting on it and still be moving. It just isn’t CHANGING the way it is just isn’t CHANGING the way it is moving if these equal forces act from moving if these equal forces act from opposite directions.opposite directions.

For ExampleFor Example

In cruise control, your engine puts In cruise control, your engine puts out a certain force to make your car out a certain force to make your car go forward. But if your speed is go forward. But if your speed is constant, friction and air resistance constant, friction and air resistance MUST BE putting an equal force back MUST BE putting an equal force back on your car to prevent any CHANGE on your car to prevent any CHANGE of motion.of motion.

REAL LIFEREAL LIFE

Why do you keep pedaling a bike Why do you keep pedaling a bike when an object in motion wants to when an object in motion wants to stay in motion.stay in motion.

Because….friction is a force trying to Because….friction is a force trying to slow you down, so you need a force slow you down, so you need a force to keep you motion going.to keep you motion going.

Newton’s Third LawNewton’s Third Law

““If object A exerts a force on object If object A exerts a force on object B, then object B exerts an equal B, then object B exerts an equal force on object A in the opposite force on object A in the opposite direction.”direction.”

This is more commonly known as, This is more commonly known as, “for every action there is an equal “for every action there is an equal and opposite reaction”.and opposite reaction”.

What does the 3What does the 3rdrd Law Mean? Law Mean?

Does that mean that if I punch a wall Does that mean that if I punch a wall with 300 Newtons of force, the wall with 300 Newtons of force, the wall punches me back with 300 N of punches me back with 300 N of force? force?

YES – that’s why your handYES – that’s why your hand hurts. The wall is hurts. The wall is exerting a force to exerting a force to change the motion of change the motion of your hand.your hand.

Classic QuestionClassic Question

If a bee flies straight into a moving If a bee flies straight into a moving car, who experiences the greater car, who experiences the greater force? The bee or the car?force? The bee or the car?

ANSWER – they both experience theANSWER – they both experience the

same force. That’s whatsame force. That’s what

Newton’s Third Law states.Newton’s Third Law states.

How does the 3How does the 3rdrd Law Make Sense Law Make Sense

How can both the bee and the car How can both the bee and the car experience the same force? It experience the same force? It doesn’t make sense. doesn’t make sense.

Answer – Remember that forces cause Answer – Remember that forces cause CHANGES in the motion of an object. CHANGES in the motion of an object. Also remember that the more mass Also remember that the more mass an object has – the less the change an object has – the less the change will be. (Continue on next slide)will be. (Continue on next slide)

Let’s say the bee and the car each Let’s say the bee and the car each experience a force of 10 N during the experience a force of 10 N during the collision. How would a 10 N force collision. How would a 10 N force affect a bee whose mass is affect a bee whose mass is

0.002 kg?0.002 kg?

a = F/m = 10 N/.002kga = F/m = 10 N/.002kg a = 5000 m/sa = 5000 m/s22

This is a HUGE change in motion for This is a HUGE change in motion for the bee. But think about it. The bee the bee. But think about it. The bee DOES undergo a huge change in DOES undergo a huge change in motion after striking the car, so this motion after striking the car, so this makes sense.makes sense.

Now apply the same 10 Newton force to the Now apply the same 10 Newton force to the car whose mass is 2000 kilograms. car whose mass is 2000 kilograms.

a = F/m = 10 N/2000 kga = F/m = 10 N/2000 kg

a = .005 m/sa = .005 m/s22

This is a very SMALL acceleration. In other This is a very SMALL acceleration. In other words, the car’s motion CHANGES very words, the car’s motion CHANGES very little. This makes sense too. Why would a little. This makes sense too. Why would a bee striking a car really impact the way bee striking a car really impact the way the car is moving?the car is moving?

AVOID CONFUSIONAVOID CONFUSION

It is a common misconception to It is a common misconception to think that the bigger force comes think that the bigger force comes from the bigger object..- NOT TRUE!from the bigger object..- NOT TRUE!

The FORCES must be equal when two The FORCES must be equal when two objects interact. What is not equal objects interact. What is not equal are the accelerations, or the effect of are the accelerations, or the effect of the forces on motion, because of the the forces on motion, because of the difference in mass between the two difference in mass between the two objects.objects.

Common MisconceptionsCommon MisconceptionsStudents believe that objects in Students believe that objects in motion have an “impetus” force. motion have an “impetus” force. They think that if an object is They think that if an object is moving, it is retaining some of the moving, it is retaining some of the force that initially set the object in force that initially set the object in motion, and that’s why it continues motion, and that’s why it continues to move. They don’t recognize that to move. They don’t recognize that once the object is in motion, it while once the object is in motion, it while stay that way UNTIL an outside force stay that way UNTIL an outside force makes its motion change.makes its motion change.

Common MisconceptionsCommon MisconceptionsBig objects always exert the bigger Big objects always exert the bigger force when they strike a small object. force when they strike a small object.

Truth – the forces are the same. It is Truth – the forces are the same. It is the “effect” of those forces that are the “effect” of those forces that are different. The larger object has a different. The larger object has a smaller acceleration, and therefore, a smaller acceleration, and therefore, a small CHANGE in its motion after the small CHANGE in its motion after the force is applied.force is applied.

Common MisconceptionsCommon MisconceptionsForces deal with velocity.Forces deal with velocity.

Truth – forces deal with accelerations, which Truth – forces deal with accelerations, which are changes in speed or direction. are changes in speed or direction. Therefore, an object can have a constant Therefore, an object can have a constant velocity without a net force acting on it. velocity without a net force acting on it. This doesn’t mean no forces act on the This doesn’t mean no forces act on the object, it means that the forces act equally object, it means that the forces act equally from opposite directions to cancel each from opposite directions to cancel each other. So an object with no acceleration other. So an object with no acceleration can either be moving at a constant speed can either be moving at a constant speed or not moving at all.or not moving at all.

The following links go directly to the The following links go directly to the RPDP Tips Site and other Web RPDP Tips Site and other Web

Sites with Physics Animations and Sites with Physics Animations and Examples.Examples.

http://rpdp.net/sciencetips_v2/P12B1.htmhttp://rpdp.net/sciencetips_v2/P12B1.htm

http://nzip.rsnz.org/es/applets/index.htmhttp://nzip.rsnz.org/es/applets/index.htm

http://www.glenbrook.k12.il.us/gbssci/phys/mmedia/http://www.glenbrook.k12.il.us/gbssci/phys/mmedia/

http://phet.colorado.edu/new/simulations/index.php?cat=Motionhttp://phet.colorado.edu/new/simulations/index.php?cat=Motion

Please don’t hesitate to see Scott or Kris for other resources if it helps.Please don’t hesitate to see Scott or Kris for other resources if it helps.