Chapter 4 Free-Body Diagram

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# Chapter 4 Free-Body Diagram - PowerPoint PPT Presentation

Chapter 4 Free-Body Diagram. Swinging on a monkey bar By Dawn Peterson With help from Darby Peterson. Free-Body Diagram.

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### Chapter 4 Free-Body Diagram

Swinging on a monkey bar

By Dawn Peterson

With help from Darby Peterson

Free-Body Diagram

Darby has a tension force in his arms from the bar, and air resistance. These are both contact forces. The force of gravity is a field force that is pulling him toward the ground.

Ftension

Fgravity

Fair resistance

These forces are unbalanced.

I will show you why with vector addition.

Fair resistance

Ftension

Ftension

Fgravity

Fresultant

Fresultant

Fgravity

Fair resistance

After adding all the forces head to tail, I get a resultant force that pulls Darby back and down.

The Acceleration and Force …

…are both in the same direction. I can see that Darby is accelerating back and down each time he is in front of the post.

When he is moving toward the center pole his velocity is increasing as he accelerates back and down.

When he is moving forward his acceleration is still back and down but this decreases his velocity and he is slowing down.

The Inertia…

…is constant due to his mass but it helps him keep moving when the forces are balanced. His momentum = mass *velocity so it varies with his velocity.

His highest velocity is at the bottom so his inertia keeps him moving even though the forces are nearly balanced.

He has a velocity of zero at the furthest point in his swing, but here the forces are most unbalanced so he accelerates down and back.

How do I stop him?

When he is at rest, all the force vectors need to add to zero!

Fair resistance

Ftension

Ftension

Fgravity

Fresultant

Fresultant

Fgravity

-Fresultant

Fair resistance

The forces are balanced and he stays at rest.

How do I stop him?

Watch as I grab his feet. I balance the forces so his acceleration is zero and he is at rest!

( A little extra force is needed to overcome his inertia if I don’t catch him when his velocity is zero.)

• Swimmer doing a turn or stroke
• Soccer player planting for a kick
• Football player making a tackle
• Basketball player making a foul or being fouled.
• Wrestler being pinned or pinning someone