1. Unit 15 – Forces and Motion
Lesson 1: Forces

2. What are Forces?
A force is a push or pull which causes a change in motion.
It can cause objects to stop moving, start moving, move faster/slower, or change direction.

3. What are Forces?
Forces are measured with spring scales in units called newtons (N).
When we use a scale, we are measuring weight: the measure of the force of gravity on an object.

4. Friction Friction: a force that opposes motion.
It acts between two objects that are touching.
If it is between air and a moving object, then it is called air resistance.

5. Friction
When two objects have friction between them, we can feel heat coming off of them.
This is because the energy of motion is being changed into thermal energy.
When I say start, rub your hands together. Do not stop until I say to. What happens?

6. Unit 15 – Forces and Motion
Lesson 1: Forces

7. Magnetism
Magnetism is the force of repulsion (pushing) or attraction (pulling between poles of magnets.
Every magnet has two poles: a south pole and a north pole.

8. Magnetism
The north pole of one magnet is attracted to the south pole of another magnet.
The north pole of one magnet repels (pushes away) from the north pole of another magnet.

9. Magnetic Fields
A group of imaginary lines running from the north pole to the south pole on a magnet.

10. Gravitation
Gravity is the force that pulls all objects in the universe toward one another.

11. Gravitation
The strength of gravitation depends on the mass of the objects and the distance between them.

12. Gravitation Examples:
The mass of 2 ping pong balls is much less than the mass of a ping pong ball and the Earth.
There is a much lower gravitational force with the ping pong balls.

13. Gravitation Examples:
Two ping pong balls on the same table will have a stronger force than two ping pong balls across the room because they are closer to one another.

14. Unit 15 – Forces and Motion
Lesson1: Forces

15. Balanced Forces
forces that are equal in size and opposite in direction.
Cancel each other out.
Object’s do not move or change direction.

16. Unbalanced Forces when one force is greater than its opposite force.
Object’s motion changes – it can stop, speed up, slow down, or change direction.

17. Acceleration any change in an object’s motion.
Speed up, slow down, stop, change direction, etc.

18. Forces Always Act in Pairs
when one object acts on a second object, the second object also acts on the first object.

19. Net Force
The net force is the combined forces acting on an object at any time.

20. Net Force
If two forces are acting on an object from the same direction, you would add the forces up.
If they are acting in opposite directions, you subtract the smaller force from the larger one.

21. Example
You and a friend are helping Ms. Knapp move desks in the classroom. You stand on the same side and both push using a force of 15 N.
What is the total force?

22. Example 2
You and a friend are helping Ms. Knapp move desks in the classroom. You stand on one side, pushing with a force of 25 N, and your friend wants to go the other way. They push with a force of 20 N.
What is the total force?