1. What is a Force?
Unit 7
Section 2

2. I. What is a Force? A force is a push or pull on an object.
b. Scientists express force using newtons (N).
I. For example-the boy pushed the desk with 20 N of force.
c. A force changes the speed and direction or an object.

3. II. Unseen Forces
a. Sitting on a chair is an example of an unseen force.
i. You are exerting a force by sitting on the chair, but the force does not cause the chair to move.
b. For a force to occur something must receive a push or pull.
typing on the keyboard: keys are receiving a push
opening a textbook: pages are receiving a pull
c. Forces can change the acceleration of an object.
Acceleration = change in speed and Direction (velocity)

4. III. Net Force Usually more than one force is acting on an object.
Net Force is the combination of all forces acting on an object.
If the forces applied are moving in the same direction – add the forces.
If forces applied are moving in opposite directions– subtract the forces.
The net force is always in the direction of the greater force.
For example, determine the net force below.
The forces are going in opposite directions so you should subtract the forces.
360 N -270 N = 90 N to the right

5. Try this one:
You and a friend move a box across the room. Your friend PUSHES with 20 N of force to the right, and you PUSH with 25 N of force to the right.
Same direction so you add the forces.
20 N + 25 N = 45 N to the right

6. What if you were playing tug of war and one side was PULLING to the left with 50N of force, and the other side was PULLING to the right with 20N of force?
In this case, you subtract the smaller number from the bigger number. The net force is 30N to the left.

7. IV. Balanced Forces
When the force on an object produce a net force of 0 N, the forces are balanced, and there will be NO MOVEMENT.
Balanced forces do not cause a nonmoving object to start moving.
i. An example would be a desk sitting on the classroom floor. If the desk is not moving, the forces between the desk and the floor are BALANCED. The desk is pushing down on the floor with the same force that the floor is pushing up on the desk.
10N
10N

8. V. Unbalanced Forces
When the net force on an object is NOT 0 N, the forces are unbalanced. There IS movement.
Unbalanced forces produce a change in motion
i. Speed
ii. Direction
Unbalanced forces are necessary to cause a nonmoving object to start moving and to change direction.
An example would be if you got pushed in the hallway and you fell down. The forces were unbalanced because there was movement as you fell.
The soccer ball is already moving when it is passed from one player to another. When the ball reaches another player, that player exerts an unbalanced force – a kick – on the ball. After the kick, the ball moves in a new direction and has a new speed.