1. Chapter 8: Forces & Motion
Week 4
Chapter 8: Forces & Motion

2. Forces
April 12 & 13

3. Today’s Objectives Students will be able to:
Identify different types of forces.
Explain how mass and distance affect gravity.
Differentiate mass and weight.
Distinguish between contact forces and noncontact forces.

4. Bellringer #3
Which friend do you agree with most?
Explain your thinking. Describe what you think a force is.

5. Bellringer #4
Think back to activity you participated in during the previous period.
What caused the book and sheet of paper to move?
Did anything make contact with the items to make them move?
Did anything push or pull the objects without making contact with them?

6. Forces A force is a push or a pull on an object.
Force has both size and direction.
You can use arrows to show the size and direction of a force.
The unit for force is the newton (N).
How do forces relate to energy?
A force imparts energy to an object and can cause a change in motion

7. Forces
A contact force is a push or a pull on one object by another object that is touching it.
What types of contact forces exist?
A force that one object can apply to another object without touching it is a noncontact force.
What type of noncontact forces exist?
1. Applied force, friction, tension force, spring force
2. Electrostatic, magnetic, gravity

8. Gravity
Gravity is an attractive force that exists between all objects that have mass.
Although gravitational forces always exist between objects, they only become observable when the masses are as large as those of planets, moons, or stars.
The size of a gravitational force depends on the masses of the objects and the distance between them.
If the mass of an object increases, the gravitational force increases between it and another object.

9. Gravity
If the mass of an object increases, the gravitational force increases between it and another object.
This fact may lead you to believe that falling objects accelerate at different rates.
By applying the formula F = ma, you can see that if the force due to gravity increases as mass increases, the acceleration will remain constant.
Near the Earth’s surface, the acceleration due to gravity is 9.8 m/s^2
Mass does not affect the acceleration due to gravity in any measurable way. The two quantities are independent of one another.
Or 9.8 m/s/s, meaning the speed increases by 9.8m/s every second.

10. Gravity How do we measure the force of gravity?
Weight is a measure of the gravitational force acting on an object’s mass.
What is the difference between mass and weight?
Weight=mass x g (w= Newtons, m=kg, g= 9.8 m/s/s)
The weights of objects on the Moon are smaller than objects on Earth because the mass of the Moon is smaller.
g = acceleration due to gravity

11. Gravity
The size of a gravitational force also depends on the distance between two objects.
Gravitational force between an apple and Earth is about 2N on Earth’s surface.
It would be about .001 N if the apple was 380,000 km away.
If the size of the force of gravity did not affect the object’s acceleration, what caused the objects to fall differently?
Air resistance (next slide)

12. Friction & Air Resistance
Objects can have their motion opposed by friction with air.
Air resistance is the frictional force between air and objects moving through it.
Friction is a contact force that resists the sliding motion of two surfaces that are touching.
The force of friction acts in the opposite direction of an object’s motion.
Air resistance opposes the downward motion of falling objects.
As objects fall air resistance gradually becomes equal to the pull of gravity.
When this happens, there is no acceleration.
What does this imply about the object’s motion?
The object continues to fall, but at a constant velocity.
Fun fact: sky divers cannot accelerate any further once they reach a terminal velocity (air resistance= force of gravity) of about 190 km/hr. At this point, the sky divers continue their descent, although there is no longer any sensation of falling!

13. Wrap Up How do contact and noncontact forces affect objects’ motion?
How did our activity demonstrate this?

14. Acceleration
Acceleration is a measure of how quickly the velocity of an object changes.
When the velocity of an object changes, it accelerates.
The action of slowing down is called negative acceleration.
When an object’s acceleration is in the same direction as its motion, this is called positive acceleration.
When the roller-coaster car increases speed, decreases speed, or changes direction, it accelerates.
Acceleration takes place whenever velocity changes.
*optional review

15. Combining Forces
April 15

16. Bellringer # 5 What happens when two forces act on an object at once?
Think of a real-life example.
Remember, a force is a push or pull on an object.

17. Today’s Objectives Students will be able to:
Explain what happens when forces combine.
Relate balanced and unbalanced forces to motion.

18. Combining Forces
When more than one force acts on an object, the forces combine and act as one force.
The sum of all the forces acting on an object is called the net force.
When two forces act on the same object in opposite directions, you must include the direction of the forces when you add them to calculate net force.
How can you include direction when calculating net force?
If the net force on an object is 0 N, the forces acting on the object are called balanced forces.
When the net force on an object is not 0 N, the forces acting on the object are unbalanced.

19. Unbalanced Forces
Forces that are unbalanced are unequal in size and act in the same or opposite in direction.
When unbalanced forces act on an object, the object’s velocity changes.
Recall: What is velocity?
Unbalanced forces can change either the speed or the direction of motion.
The train is able to pull away from the station because the force of the engine is greater than the force of friction.
How does a curve in the track change the train’s velocity?
Does the train accelerate as it goes around the curve? How do you know?
1. The speed and direction of a moving object.
2. The track exerts a sideways force on the train’s wheels, causing it to change direction, and therefore, velocity.
3. Yes, the train changes direction so the velocity changes. Remember, acceleration is any change in velocity.

20. Balanced Forces
How are objects at rest and objects moving at constant motion similar?
Forces that are balanced are equal in size, but opposite in direction
When balanced forces act on an object, the motion is constant.
The object is either at rest or moving at a constant velocity.
What forces keep the train resting on the track?
Because the forces are balanced, motion or the lack of motion stays constant.
Gravity pulls down on the train and the tracks push up with an equal force.

21. Wrap Up How do balanced and unbalanced forces affect motion?
Apply the concept of combining forces to Tuesday’s activity with the book and sheet of paper.
Forces may work together or they may be opposite forces. Two or more opposite forces are balanced forces if their effects cancel each other out and they do not cause a change in an object's motion. If two forces of equal strength act on an object in opposite directions, the forces will cancel out, resulting in a net force of zero and no change in speed or direction.
For the paper, air resistance became equal to the force of gravity, causing its velocity to remain constant.
For the book, its greater mass caused the forces to be unbalanced, allowing it to continue accelerating.

22. Exit Ticket: Concept Mapping
With the person next to you, use ten or more of the terms below to create a concept map.
Forces Balanced Unbalanced Contact Noncontact
Time
Position
Motion
Speed
Velocity
Acceleration
Friction
Air resistance
Gravity