Chapter 2, Section 6Newton’s Third Law: Run and Jump

December 1, 2011HW: PTG 1-7 Due Tuesday

• Do Now:– WDYS/WDYT

• Pg. 198

• Agenda:– Do Now– LO/SC– Investigate (Demos)

• Learning Objective– Explain the forces

acting on 2 objects that are interacting

• Success Criteria– Provide evidence that

forces come in pairs, with each force acting on a different object

– Use Newton’s 3rd Law to analyze situations

– Describe how Newton’s 3rd Law explains much of the motion you see in your everyday life

Materials• Read through the lab. What

materials will you need for this investigate?

• 7 minutes

Part A: Push, Push Back and Pull, Pull Back• Investigate #1:

– When is your motion accelerated?– When is your motion a constant speed?– Thinking about F=ma, what is the source

of the force that causes the acceleration?• Identify the object that pushes on your mass

to cause acceleration– On what object do you push? In what

direction? – How do the forces compare? – 8 minutes

Part A: Push, Push Back and Pull, Pull Back• Investigate #2

– Describe the motion of student A– What force caused the motion of student

A?– Describe the motion of student B– What force caused the motion of Student

B?– 5 minutes

Part A: Push, Push Back and Pull, Pull Back• Investigate #3

– Think about the forces involved when you walk or run on a horizontal surface• Where does the forward force come from?• Compare the amount of the forward force to

the backward force exerted by your shoe with each step.

– Would it be possible to start walking or running on an extremely slippery surface? Explain why/why not in terms of force.

– 5 minutes

Part A: Push, Push Back and Pull, Pull Back• Investigate #4

– Read and complete– Record the results in your log– Draw a diagram showing the forces

exerted by you and your partner

Exit Slip• On a half sheet of paper. Answer the

following question

• How many forces act on any given object? Use your lab and provide specific evidence to support your claim.

• Do Now:– You do not have to

rewrite the LO and SC

– Explain how you can walk on the sidewalk, but it is more difficult to walk on ice

• Agenda:– Do Now– LO/SC– Physics Talk– Quiz

• Learning Objective– Explain the forces

acting on 2 objects that are interacting

• Success Criteria– Provide evidence that

forces come in pairs, with each force acting on a different object

– Use Newton’s 3rd Law to analyze situations

– Describe how Newton’s 3rd Law explains much of the motion you see in your everyday life

Part B: Observing a Meter Stick Push Back• Investigate #1:

– Draw the free body diagrams in you notebook

• #2 Set up meter stick and books• #3 Place coin on meter stick

– Record what happens using a free-body diagram

Part B: Observing a Meter Stick Push Back• Investigate #4

– Data Table– How does the deflection of the meter

stick compare to the weight it is supporting. Make a graph to show this relationship

– Is the meter stick deflecting with the coin? Explain

– Use a free body diagram to show the forces acting on 100g mass when it is at rest on the meter stick.

– 8 minutes

Exit ticket (Success Criteria)• Based on your observations from

today and yesterday, explain Newton’s 3rd Law:– Use examples to demonstrate that forces

come in pairs and act equally and opposite directions.

December 5, 2011

• Do Now:– You do not have to

rewrite the LO and SC

– Using Newton’s 3rd Law, explain why some objects don’t move while others accelerate?

• Agenda:– Do Now– LO/SC– Physics Talk– Quiz

• Learning Objective– Explain the forces

acting on 2 objects that are interacting

• Success Criteria– Provide evidence that

forces come in pairs, with each force acting on a different object

– Use Newton’s 3rd Law to analyze situations

– Describe how Newton’s 3rd Law explains much of the motion you see in your everyday life

Physics Talk 2.6• What always

accompanies acceleration?

• What was the force that pushed the person away from the wall in the chair?

• An unbalanced force

• The wall exerted a force on the person that caused her to accelerate away from the wall.

Physics Talk 2.6• What were the

forces involved when the students pushed each other on the skateboards?

• What was the forces that moves you forward when you walk across a room?

• Each student exerted an equal force against one another.

• Your foot applies a force to the ground and the ground exerts an equal force on you

Newton’s Third Law• What is Newton’s

Third Law?• For every applied

force, there is an equal and opposite force. The two forces always act on different objects.

Newton’s Third Law• What are some

examples of Newton’s Third Law?

• The person pushed (applied a force) on the wall and the wall pushes the person

• Student A on the skateboard pushed on student B and student B pushed on student A

• You push on the floor backwards and the floor pushes you forward.

• You pulled on the spring scale. The spring scale pulled on you. These forces were always equal and opposite in direction.

• You cannot touch someone without them touching you back.

Newton’s Third Law• How does the

ground provide force if it doesn’t look like it’s moving?

• Example

• The floor actually bends a little bit. If you stand in the center of a trampoline, the bend is quite noticeable; however, floors made of wood or concrete provide less of a bend.

• This is like when the meter stick bent under the force of the weight.

Newton’s Third Law• Equivalent

expressions of Newton’s Third Law

• For every force applied to object A by another object B, there is an equal and opposite for applied to object B by object A

• If you push or pull something, that something pushes or pulls back on you with an equal amount of force in the opposite direction ALWAYS

• Forces always come in pairs

Free Body Diagrams• How do you draw a

free-body diagram?• Each force is

represented by an arrow.

• The direction of the force matched the direction of the arrow.

• Each arrow labels the kind of force.

• Objects are often drawn as boxes

Free Body Diagrams• How do you draw

the force of weight?

• Center of mass

• How do you draw other forces?

• The arrow emerges from the object’s center of mass.

• The point at which all the mass of an object is considered to be concentrated.

• Arrows are drawn by arrows emerging from the point of contact

Newton’s law in the meter stick bend• What is the first

pair of forces on the meter stick?

• What is the second pair of forces acting on the mass?

• The meter stick pushes up on the mass and the mass pushes down on the meter stick.

• Earth pulls down on the mass with a force of gravity and the mass pulls up on the Earth with an equal force of gravity.

Challenging Newton’s Third Law

• Scenario: If I pull on the chair then the chair pulls on me with an equal force. Therefore, the two forces cancel and nothing should move. Newton’s law must be wrong.

• True or False?

• False! There are additional forces at work.

• The force on the ground is larger than the force on the chair, which makes the person move forward.

What do you think now?• Locate each success criteria and

complete summaries for each.