1. Forces
MYIB / Honors Physics

2. Facts about FORCE A force is by definition a push or a pull
Unit is the newton (N)
Can exist during physical contact (“contact forces”):
tension, friction, applied force, etc.
Can exist with no physical contact, called “field forces” or “non-contact forces”:
gravitational, electric, etc.

3. Balanced Forces are equal in magnitude AND opposite in direction.
When forces act on an object they are said to be balanced or unbalanced.
Balanced Forces are equal in magnitude AND opposite in direction.
5 N
5 N
10 N
10 N
10 N
10 N
5 N
5 N
When balanced forces act on an object the resultant is always
zero.
When balanced force act, the object is in a state of equilibrium.

4. Q: What is the magnitude of the unknown force that will cause equilibrium?

5. What effect will balanced forces have on the motion of an object?
NONE!
When balanced forces act on an object, there is no change in the motion of the object.

6. When unbalanced forces act on an object the resultant is always
Unbalanced Forces are not equal in magnitude OR not opposite in direction.
5 N
10 N
5 N
3 N
5 N
4 N
Q:What is the resultant force?
+15 N
-5 N
+5 N
-15 N
Q:What is the resultant force?
+5 N
0 N
-5 N
+10 N
Q:What is the magnitude of the resultant force?
5 N
7 N
+1 N
-1 N
When unbalanced forces act on an object the resultant is always
nonzero.
This resultant force is called the net force.

7. What effect will unbalanced forces have on the motion of an object?
Motion will change!
How could the motion change?
speed up
slow down
change direction
What one word describes all three of theses changes?
Acceleration!
An net force will cause a mass to accelerate.

8. Newton’s Laws of Motion

9. Suppose you are at a hockey game when you see a player strike the puck with his stick. As the puck slides across the ice, consider the following:
What force causes the puck to start moving?
(a) The force from the stick.
(b) The force from the player.
(c) The force from the ice.
(d) No force is needed.
Contact / Non-contact

10. Suppose you are at a hockey game when you see a player strike the puck with his stick. As the puck slides across the ice, consider the following:
What force keeps the puck moving as it slides across the ice?
(a) The force from the stick.
(b) The force from the player.
(c) The force from the ice.
(d) No force is needed.

11. Newton’s First Law of Motion
An object in motion will remain in motion, and an object at rest will remain at rest, unless acted on by an outside force.
Examples:
A puck sliding across the ice (constant velocity)
A spaceship traveling through space (constant velocity)
A box sitting on a table (at rest)
The tendency of an object to resist a change in its motion is called inertia.
unbalanced

12. Newton’s First Law (Inertia)
A truck is harder to set in motion than a baseball because it has more inertia.
The more massive object is said to have more inertia.

13. Newton’s Second Law 5000 kg 1 kg
When a net force acts on an object, the object will accelerate with an acceleration directly proportional to the strength of the force and inversely proportional to the object’s mass.
5000 kg
1 kg

14. A Special Case (Newton’s 2nd)
The force of gravity has a special name: “weight”.
An object’s weight is equal to the force of gravity on the object. It is equal to its mass times the acceleration due to gravity.
5000 kg
1 kg