12.1 Forces
• What is a Force?
• Combining Forces
• Friction
• Gravity
• Projectile Motion

12.1 Forces
The wind pushes against the man and his umbrella.
The push from the wind is a force.

12.1 Forces
I.
Forces
A. Force- a push or a pull that acts on an object.
1.
A force can cause a resting object to move, or it can accelerate a moving object by changing the object’s speed or direction.
2.
Measuring Force a. You may have measured forces using a spring scale.
b. The stretch of the spring in the scale depends on the amount of weight (a type of force) acting on it.

12.1 Forces
3. Classification of Forces a. Contact
1) normal forces
2) frictional forces
3) applied forces b. Non-contact
1) gravitational forces
2) electrical forces
3) magnetic force

12.1 Forces
What Is a Force?

12.1 Forces
4. Units of Force a. One newton (N) is the force that causes a 1kilogram mass to accelerate at a rate of 1 meter per second each second (1 m/s 2 ).
1 N = 1 kg•m/s 2
5. Representing Force a. Arrows can represent a force.
1) The direction of the arrow represents the direction of the force
2) The lengths of the arrows show the strength, or magnitude of the force

12.1 Forces
What Is a Force?

12.1 Forces
B. Combining Forces
1. net forcethe overall force acting on an object after all the forces are combined.
a. Forces in the same direction add together.
b. Forces in opposite directions subtract from one another.

12.1 Forces
2. Balanced Forces a. When the forces on an object are balanced, the net force is zero, and there is no change in the object’s motion b. An unlimited number of individual forces can act on an object to produce a net force of zero.

12.1 Forces
Combining Forces
The two groups pull with equal forces in opposite directions. The forces combine to make a net force of zero.

12.1 Forces
3. Unbalanced Forces a. An unbalanced force is a force that results when the net force acting on an object is not equal to zero.
b. When an unbalanced force acts on an object, the object accelerates.
1) Speeds up
2) Slows down
3) Changes direction

12.1 Forces
Combining Forces c. Forces can add together or subtract from one another.
Adding forces-
Same direction

12.1 Forces
Combining Forces
Forces can add together or subtract from one another.
Same direction
Subtracting forces-
Opposite direction

12.1 Forces
Combining Forces
Forces can add together or subtract from one another.
Same direction
Subtracting forces-
Opposite direction
Equal and opposite forces-
Results in no net force

12.1 Forces
C. Frictionforce that opposes the motion of objects that touch as they move past each other.
1. The strength of the force of friction depends on two factors.
a. How hard the surfaces push together
1) Rubbing hands together b. The types of surfaces involved
1) Sand vs snow

12.1 Forces
D. Gravity - a force that acts between any two masses.
1. Gravity is an attractive force among all objects
2. Gravity can act over large distances.
a. Moon and Earth distance b. Earth and Sun distance
3. Earth’s gravity acts downward toward the center of Earth.

12.1 Forces
Gravity
Earth exerts an attractive, downward force on this boulder. The supporting rock exerts an upward force on the boulder. The forces are balanced.

12.1 Forces
4. Both gravity and air resistance affect the motion of a falling object.
a. Gravity causes objects to accelerate downward at a rate of 9.8 m/s² b. air resistance acts in the direction opposite to the motion and reduces acceleration.
1) Terminal velocity is the constant velocity of a falling object when the force of air resistance equals the force of gravity.

12.1 Forces
Gravity
This flying squirrel takes advantage of air resistance to slow its fall and increase the distance covered in the jump.
feather and hammer

12.1 Forces
Projectile Motion
A. Their masses are different, but the blue and green balls fall at the same rate.

12.1 Forces
Projectile Motion projectile motion video
A. Their masses are different, but the blue and green balls fall at the same rate.
B. The yellow ball is a projectile, following a curved path.

12.1 Forces
E. Projectile motion - the motion of a falling object (projectile) after it is given an initial forward velocity.
1. The combination of an initial forward velocity and the downward vertical force of gravity causes the ball to follow a curved path.
2. Air resistance and gravity are the only forces acting on a projectile.

12.1 Forces
Assessment Questions
1. If an object is at rest, which of the following statements must be true? a. There are no forces acting on the object.
b. There is no friction acting on the object.
c. The forces acting on the object are unbalanced.
d. The net force acting on the object is zero.

12.1 Forces
Assessment Questions
1. If an object is at rest, which of the following statements must be true? a. There are no forces acting on the object.
b. There is no friction acting on the object.
c. The forces acting on the object are unbalanced.
d. The net force acting on the object is zero.
ANS: D

12.1 Forces
Assessment Questions
2. Which of the following is not a type of friction?
a. static friction b. sliding friction c. fluid friction d. pull friction

12.1 Forces
Assessment Questions
2. Which of the following is not a type of friction?
a. static friction b. sliding friction c. fluid friction d. pull friction
ANS: D

12.1 Forces
Assessment Questions
3.
In which direction does Earth’s gravitational force act? a. opposite the direction of motion b. downward toward the center of Earth c. upward away from the center of Earth d. in the direction of motion

12.1 Forces
Assessment Questions
3.
In which direction does Earth’s gravitational force act? a. opposite the direction of motion b. downward toward the center of Earth c. upward away from the center of Earth d. in the direction of motion
ANS: B

12.1 Forces
Assessment Questions
4. A ball thrown into the air follows a projectile course due to the initial velocity and the a. force of gravity.
b. effect of air resistance.
c. motion of Earth beneath it.
d. mass of the ball.

12.1 Forces
Assessment Questions
4. A ball thrown into the air follows a projectile course due to the initial velocity and the a. force of gravity.
b. effect of air resistance.
c. motion of Earth beneath it.
d. mass of the ball.
ANS: A

12.1 Forces
Assessment Questions
1.
The SI unit for force is 1 kg•m/s 2 , also called one kepler.
True
False

12.1 Forces
Assessment Questions
1.
The SI unit for force is 1 kg•m/s 2 , also called one kepler.
True
False
ANS: F, newton