2. Force Causes Acceleration
Acceleration deals with changes in motion
There is zero acceleration at constant velocity
The cause of acceleration is force
A constant force produces motion at a constant acceleration
Acceleration ~ net force
The ~ symbol means, “directly proportional to”
The great acceleration of the racing car is due to its ability to produce large forces.

3. Mass Resists Acceleration
For a given force the acceleration is inversely proportional to the mass

4. Mass Resists Acceleration
6 times the mass
1/6 the acceleration

5. Newton’s Second Law
By using consistent units: N for force, kg for mass and m/s2 for acceleration the equation becomes exact:

6. What acceleration is produced by a force of 2000 N applied to a 1000 kg car?
2000 N/1000 kg = 2 m/s2
If the force is 4000 N, what is the acceleration?
Doubling the force on the same mass simply doubles the acceleration.

7. Newton’s Second Law
If mass and acceleration are known then the equation would look like this:

8. Newton’s Second Law
Head = 4 kg

9. Newton’s Second Law
Head = 4 kg
Acceleration of head = 25 m/s2

10. Friction Friction acts on materials that are in contact with each
other, and it always acts in a
direction to oppose motion.

11. The force of friction between the surfaces depends on the kinds of material in contact and how much the surfaces are pressed together.
Rubber against concrete produces more friction than steel against steel.

12. Why are concrete road dividers more effective than steel rails?

13. Friction also occurs in liquids and gases (fluids).
Air resistance – friction
acting on something
moving through the air

14. Applying Force--Pressure
Pressure—The amount of force per unit of area
One Newton per square meter is equal to one pascal (Pa)
Pressure
Force
Area of application

15. Applying Force--Pressure
Pressure—The amount of force per unit of area
Pressure
Force
Area of application

16. The smaller the area supporting a given force, the greater the pressure on that surface.
You exert more pressure on the ground when you stand on one foot than when you stand on both feet. (Decreased area of contact)
Stand on one toe like a ballerina and the pressure is huge.

17. Why don’t the nails puncture the skin?
The driving force
per nail is not enough
to puncture the skin.

18. Free Fall Explained
A 10 kg cannonball will hit the ground at the same time as a 1 kg stone! =

19. A 10 kg cannonball will hit the ground at the same time as a 1 kg stone!
g = acceleration of free fall
The weight at the Earth’s surface is 9.8 N per 1 kg.
1 kg stone
10 kg cannonball

20. Free Fall The net force on each object is only its weight.
The ratio of weight to mass is the same for both.
All freely falling objects undergo the same acceleration at the same place on Earth.
If you were on the moon and dropped a hammer and a feather from the same elevation at the same time, would they strike the surface of the moon at the same instant?

21. Answer
Yes
On the moon, the hammer and feather weigh only 1/6 of their Earth weights, and there is no air to provide friction.
The ratio of moon-weight to mass for each object is the same, and they both accelerate at (1/6)g.

22. Air Drag

23. Falling and Air Resistance (drag)
km/h
A heavier person will
attain a greater terminal
speed than a lighter
person.
15-25 km/h
A parachute increases
air resistance.
Terminal speed
(acceleration terminates)

24. Air drag builds up as speed increases
Air drag builds up as speed increases. The result is reduced acceleration.
More reduction can occur by increasing the surface area encountered by the air. (Diver spreads out)
If there were no air drag, like on the moon, there would be no terminal speed. (free fall and each object hits the ground at the same time).

25. Falling and Air Resistance
When air resistance equals the weight, the net force is zero and no further acceleration occurs. Acceleration terminates; the object has reached its terminal speed.
If we are concerned with direction (such as down for falling objects), the object has reached its terminal velocity.

26. a. b. c. d. e. f. His weight is 100 kg, so his weight at the Earth’s
surface is a constant 1000 N (9.8 N rounded). Air resistance (R) varies. Find his acceleration at each position.
R=0
W=1000N
A = 10 m/s2 a. b. c. d. e. f.
R=400N
W=1000N
A = 6 m/s2
R=1000N
W=1000N
A = 0 m/s2
R=1200N
W=1000N
A = -2 m/s2
Other questions in the practice book page 18.
R=2000N
W=1000N
A = -10 m/s2
R=1000N
W=1000N
A = 0 m/s2