1. 2-1 Notes – Combining Forces
Chapter 2, Lesson 1

2. Force
When more than one force acts on an object, the combined effect is caused by the sum of all applied forces.
A push or a pull on an object is a force.

3. Contact and Noncontact Forces
A contact force is exerted only when two objects are touching.
A noncontact force is exerted when two objects are not touching.
Magnets exert a noncontact force on each other.
contact force
noncontact force

4. Force is a Vector Vectors have magnitude and direction.
Force has a direction and a magnitude.
Force is measured in Newtons (N).
Arrows can be used to describe forces:
The width of the arrow indicates the magnitude
The head of each arrow points in the direction of the force.
large force
small force

5. Combining Forces
When more than one force acts on an object, the forces combine.
The combination of all the forces acting on an object is called the net force. + = + =

6. Combining Forces in the Same Direction
Forces are vectors and require a reference direction.
When forces in the same direction combine, the net force is in the same direction and the size of the force is the sum of the forces.

7. Combining Forces in the Same Direction
If two people push the dresser to the right, the forces are added together.

8. Combining Forces in Opposite Directions
When forces in opposite directions combine, the net force is in the direction of the larger force.
The size of the net force is the difference between the two forces.

9. Combining Forces in Opposite Directions
Two people exerting a force in opposite directions show how vectors combine to create net force.

10. Unbalanced and Balanced Forces
If the net force on an object is not zero, it is an unbalanced force.
If the net force on an object does equal zero, it is a balanced force.

11. Balanced Forces

12. Balanced Forces and Motion
If two people push with the same force, the dresser does not move.
The forces have the same magnitude.
The forces are in opposite directions.
The net force on the dresser is zero and the forces are balanced.
The motion of the dresser does not change.

13. Unbalanced Forces and Motion
Unbalanced forces change motion.
If two people push with different force, the dresser moves.
The forces have a different magnitude and are in opposite directions.
The net force on the dresser is not zero and the forces are unbalanced.
The dresser moves in the direction of the larger force.

14. Newton’s First Law of Motion
If the net force on an object is zero, the velocity of the object does not change.
If the net force is zero and the object is at rest, it remains at rest.
If the net force is zero and the object is moving, it continues to move in a straight line with constant speed.

15. Newton’s First Law of Motion
Also known as the “Law of Inertia”
Inertia is the tendency of an object to resist change in its motion unless acted on by a force.
The “Lazy Law”
The “Seatbelt Law”

16. Newton’s First Law of Motion
Galileo noticed what happened when you rolled a ball down a track which went down and then up again. He realized that if the nature of matter was to be stopped, it would just stop when it got to the bottom part of the track. Instead, it kept going up again, even against the force of gravity. He realized that a rolling ball, once it was moving, tended to keep moving.

17. Inertia and Mass Mass is the amount of matter in an object.
The more mass an object has, the more inertia it has and the harder it is to change its motion.

18. Inertia and Mass
A bicycle is easier to push than a car because it has less mass.

19. Inertia and Mass
Donkey Kong has more mass than Yoshi, and thus has greater inertia – harder to get him started moving from rest, but also harder to stop him.

20. 2.1 Combining Forces
Which of the following changes occur when an unbalanced force acts on an object?
A motion
B mass
C inertia
D weight

21. 2.1 Combining Forces
What is the net force acting on a door if one person pushes to the right with a force of 2000 N and a second person pushes with a force of 1500 N to the right?
A 3500 N to the right
B 3500 N to the left
C 500 N to the left
D 500 N to the right

22. B You turn at constant speed. C You coast to slow down.
2.1 Combining Forces
You are riding a bike. In which of the following situations are the forces acting on the bike balanced?
A You pedal to speed up.
B You turn at constant speed.
C You coast to slow down.
D You pedal at constant speed.

23. What causes a change in motion on an object?
A zero net force
B unbalanced force
C weight
D balanced force

24. Which of the following best describes why force is a vector?
SCI 2.a
Which of the following best describes why force is a vector?
A speed and size
B magnitude
C direction and friction
D magnitude and direction

25. SCI 2.b
Two people are pushing in opposite directions on a box. One person is pushing to the left with a force of 250 N, the other is pushing to the right with a force of 400 N. What is the resulting force vector?
A 150 N to the right
B 650 N to the right
C 150 N to the left
D 400 N to the right

26. A same because they point toward objects
A force is acting on each of the objects. What can be concluded about these forces?
A same because they point toward objects
B same because they have same magnitudes
C different because they have different magnitudes
D different because they have different directions

27. A downward and to the left B downward and to the right
Four forces are acting on a box, as shown. This box will increase in speed...
A downward and to the left
B downward and to the right
C upward and to the left
D upward and to the right

28. Two students are pushing on a cart
Two students are pushing on a cart. The cart will move as if it were acted on by a single force with a magnitude of...
A 50 N
B 150 N
C 200 N
D 350 N