1. Forces and Newton’s Laws
Video Warm Up

2. What is a force?
A force is a push or pull that causes an object to move, stop, or change direction.
In physics, a force is anything that makes an object accelerate.

3. Balanced and Unbalanced Forces
Forces occur in pairs and they can be either balanced or unbalanced.

4. Balanced Forces
Balanced forces do not cause change in motion or acceleration.
They are equal in size and opposite in direction.
I need 2 volunteers!

5. Balanced Forces
Example 1: The floor pushes upward and gravity pulls downwards on the person.
Example 2: The table pushes upward on the book and gravity pulls downward on the book.

6. Unbalanced Forces
An unbalanced force acting on an object will change the object’s motion.
An unbalanced force can cause an object to start moving, stop moving, or change direction.
The object will accelerate in the direction of the greater force.

7. Unbalanced Force Examples
Example 1: Gravity is a constant force. The parachute is working against the force of gravity.
Example 2: The person’s finger pushes the toy truck because it is using a greater force than the toy truck pushing against the finger.

8. Calculating the Net Force
Net force is the overall force acting on an object. It is a combination of the magnitude and direction.
- Magnitude = the difference between 2 forces
- Direction = direction of the largest force

9. Calculating the Net Force
If there is a net force acting on the object, the force is unbalanced.
To calculate the net force, you subtract the weaker force from the greater force.
For example, in this tug of war, the net force is 3N (Newtons) to the left, because 5N-2N = 3N.

10. What is the net force in these examples?
125N – 75N = 50N 
If the object isn’t moving or changing direction, the net force acting on it is 0 N.
10N – 7N = 3N 

11. What is the net force in this example?
To find the net force, subtract opposing sides and add same sides.
5N up and 5N down cancel each other.
10 N – 5 N = 5 N
The greater force is going left.

12. What is the net force in this example?
To find the net force, subtract opposing sides and add same sides.
10 N – 5 N = 5 N up
10 N – 5 N = 5 N right

13. Draw an arrow for the direction of movement in these examples.

14. ACCELERATION! (It accelerates as it falls.)
Gravity As a Force
We encounter the force of gravity every day.
Gravity = the force that acts to pull objects straight towards the center of the earth.
As an object falls, its velocity changes at a rate of 9.8 m/s. What is this an example of?
ACCELERATION! (It accelerates as it falls.)

15. https://PBS Video - Experiment
Make a Prediction
Does the weight of an object affect how fast it accelerates as it falls?
What will hit the ground first?
A) bowling ball
B) golf ball
C) They will hit at the same time
Video - Experiment

16. Friction As a Force
Friction is the force that one surface exerts on another when the two rub against each other.
Friction acts in a direction opposite to the object’s direction of motion.
Without friction an object would continue to move at a constant speed forever.

17. Friction As a Force
Friction is increased by the amount of surface area the object has in contact with the surface.
For example, tires with more surface area have better traction on the road.
Friction causes objects to slow down or stop.

18. Newtons (N) & Sir Isaac Newton
The unit for measuring force is called the Newton. It was named after the famous scientist, Sir Isaac Newton ( ).
Discovered the law of gravity.
Discovered the 3 Laws of Motion.

19. Newton’s Laws of Motion
= Laws that describe the motion of all objects on the scale we experience in our everyday lives.
An object in motion tends to stay in motion and an object at rest tends to stay at rest unless acted upon by an unbalanced force.
Force equals mass times acceleration (F = m*a)
For every action there is an equal and opposite reaction.

20. Newton’s First Law of Motion
An object at rest tends to stay at rest and an object in motion tends to stay in motion unless acted upon by an unbalanced force.

21. Newton’s First Law of Motion
Basically, an object will “keep doing what it was doing” unless acted on by an unbalanced force.
If the object was sitting still, it will remain stationary. If it was moving at a constant velocity, it will keep moving.
It takes force to change the motion of an object.

22. Then why don’t moving objects keep moving forever???
Things don’t keep moving forever because there’s almost always an unbalanced force acting upon it.
A book sliding across a table slows down because of the force of friction.
If you throw a ball upwards it will eventually slow down because of the force of gravity.

23. What about in space?
In outer space, away from gravity and any sources of friction, a rocket ship launched with a certain speed and direction would keep going in that same direction and at that same speed forever.