1. Unit II Forces

2. Force = a push or pull on an object measured in Newtons (N)
4 Ways Force Effects an Object
Increase v
2. Decrease v
3. Change Direction
4. Change Shape

3. What are the forces action on box being pushed?

4. Which box is moving? A B

5. Net Force = sum of forces acting on an object
Forces are vectors
Same direction add
Opposite direction subtract

6. Balanced vs. Unbalanced Forces
Unbalanced Forces: the forces are not equal and opposite
-the object will change
-the object will accelerate
Balanced Forces: the forces on an object are equal and opposite.
-no change to the motion
-object does not accelerate

7. Friction: force that opposes motion of an object
1. Static Friction
2. Rolling Friction
3. Sliding Friction
4. Fluid Friction

8. Laws of Motion

9. 1st Law of Motion
An Object in motion will remain in motion, or an object at rest will remain at rest unless acted upon by an unbalanced force
teachertech.rice.edu/.../Newton/law1.html

10. Inertia
The tendency of an object to keep doing what it is already doing

11. 2nd Law of Motion
Force, mass and acceleration are all related

12. 2nd Law
Force = mass x acceleration
F = ma

13. 3rd Law of Motion Action and Reaction
For every action there is an equal and opposite reaction

14. How is weight different than mass?
Weight  pull of gravity acting on an object
Weight = mass x acceleration due to gravity
Mass  amount of matter in an object

15. Sample Problems
1. A boy pushes a cart of groceries forward with a total mass of 40 kg. What is the acceleration of the car if the net force on the cart is 60 N?
1.5 m/s²

16. 2. What is the upward acceleration of a helicopter with a mass of 5000 kg if a force of N act on it in an upward direction?
2 m/s²

17. 3. An automobile with a mass of 1200 kg accelerates at a rate of 3 m/s/s in the forward direction. What is the net force acting on the automobile?
3600 N

18. 4. A 25 N force accelerates a boy in a wheelchair at 0. 5 m/s/s
4. A 25 N force accelerates a boy in a wheelchair at 0.5 m/s/s. What is the mass of the boy and the wheelchair?
50 kg

19. Sample Problems
5. A girl pushes a box of books forward with a total mass of 80 kg. What is the acceleration of the box if the net force on the box is 40 N?
0.5 m/s²

20. 6. An truck with a mass of 3600 kg accelerates at a rate of 2 m/s/s in the forward direction. What is the net force acting on the automobile?
7200 N

21. 7. A 30 N force accelerates a boy on a tricycle at 0. 5 m/s/s
7. A 30 N force accelerates a boy on a tricycle at 0.5 m/s/s. What is the mass of the boy and the tricycle?
60 kg

22. 14.1 Work
Definition: the product of distance and the force in the direction an object moves
W = F x d
SI unit for work: newton-meter (N·m) or joule (J)
If there is no movement, no work is done

23. Power What is power? Power = Work SI Unit – Watt (W)
The rate of doing work
Doing work faster requires more power
Power = Work
Time
SI Unit – Watt (W)

24. Momentum Momentum = mass x velocity of an object
Momentum is large if product of mass and velocity is large
Objects with large momentum’s are hard to stop

25. Momentum
Momentum of an object at rest = 0
SI Units = kg*m s

26. Momentum Law of Conservation of Momentum
If no net force acts on the system, the total momentum of the system does not change.
In a closed system, momentum is conserved.

27. Momentum
Problem
Which has more momentum, a kg golf ball with a velocity of 60 m/s or a 7.0 bowling ball with a velocity of 6 m/s?
Remember: Momentum = mass x velocity

28. 12.4 Universal Forces What are the 4 universal forces?
Electromagnetic
Nuclear (Strong and Weak)
Gravity
How are these different from forces talked about in the ?
All act over a distance between particles

29. Describe each universal force and what makes them unique.
Electromagnetic Force
only force that can attract and repel
Gravitational force
An attractive force between 2 masses

30. Describe each universal force and what makes them unique.
Nuclear force: act within nucleus to hold it together
Strong – acts only on neutrons and protons in the nucleus (100x stronger than charges)
Weak – attractive force that acts over a short range

31. Force Can be represented by Can be Measured in units of unbalanced
Causes no change to the object
Causes the object to
Force
unbalanced

32. Force Can be represented by Can be arrows Measured in units of
unbalanced
Newtons
balanced
Causes the object to
Causes no change to the object
State of Motion
Accelerate