(When blocking in football, why does a defending lineman often attempt to get his body under that of his opponent and push

upward?) What effect does this have on the friction force

between the opposing lineman’s feet and the ground?

Do Now10.21.10

http://cdn-aki.vmixcore.com/825/14811/26/531156701/448/825/5311/fbb4aa1aad4bf20dd9291a6847276a54.jpg

Do Now 10/21

What is the net force acting on a ball at each point?

(ignore air resistance)

10 m/s

10 m/s

10 m/s

10 m/s

10 m/s10 m/s

5 m/s

5 m/s

10 m/sA

C

B

Test TuesdayChapters 2, 3, and 6

◦Newton’s 1st Law (Inertia)◦Newton’s 2nd Law◦Friction◦Air Resistance

Objective

Explain freefall forcesState the two factors that affect air

resistance

The ratio of weight (F) to mass (m) is the same for the 10-kg cannonball and the 1-kg stone.

6.6 Free Fall Explained

The air resistance force an object experiences depends on the object’s speed and surface area.

6.7 Falling and Air Resistance

Terminal SpeedTerminal speed is the speed at which the acceleration of a falling object is zero because friction balances the weight.

6.7 Falling and Air Resistance

Sky divers reach terminal speed when air resistance equals weight.6.7 Falling and Air Resistance

The terminal speed for a sky diver varies from about 150 to 200 km/h, depending on the weight and orientation of the body.

• A heavier person will attain a greater terminal speed than a lighter person.

• The heavier person’s weight “pushes” through the air particles with greater force

• Body orientation also makes a difference. Dive vs. Spread out.

6.7 Falling and Air Resistance

The flying squirrel increases its area by spreading out. This increases air resistance and decreases the speed of its fall.

6.7 Falling and Air Resistance

A parachute will bring the skydiver’s speed to about 15 km/h to 25 km/h

6.7 Falling and Air Resistance

think!Which experiences a greater air resistance force, a falling piece of paper or a falling elephant?

6.7 Falling and Air Resistance

think!Which experiences a greater air resistance force, a falling piece of paper or a falling elephant?

Answer: The elephant! It has a greater frontal area and falls faster than a piece of paper—both of which mean the elephant pushes more air molecules out of the way. The effect of the air resistance force on each, however, is another story!

6.7 Falling and Air Resistance

Practice

A falling 50-kg parachutist experiences an upward acceleration of 6.2 m/s2 when she opens her parachute. What is the force of friction acting on her?

HW AnswersNewton’s 2nd Law WS1

1. 4 m/s2 and 2 m/s2

2. 3 kg3. 6 m/s2

4. 1 m/s2

5. 3 m/s2

6. 12 m/s2

7. Half as fast

8. 140,000 N9. 15 m/s2

10.7.5 m/s2

Newton’s 2nd Law WS2 Answers1. a = 0.25 m/s2

2. Fnet = 0 N3. a = 0 m/s2

4. a = 2 m/s2

5. a = 1 m/s2

6. a = 10 m/s2

7. FN = 80 N, m = 8 kg, a = 5 m/s2, Fnet = 40 N8. FN = 100 N, μ = 0.1, m = 10 kg, a = 1 m/s2, Fnet

= 10 N9. FN = 50 N, Ff = 5 N, Fg = 50 N, a = 1 m/s2, Fnet =

5 N10. μ = 0.16711. Fa = 105 N, Fa = 0 N