1. Newton’s Laws of Motion
Grade 8 Science

2. Force Pop Quiz What is a force?
What is the difference between a balanced and unbalanced force?
What are the four types of friction?
What is the law of universal gravitation?
What two factors affect the gravitational attraction between object?

3. The First Law of Motion
Newton’s first law of motion states that an object at rest will remain at rest, and an object moving at a constant velocity will continue moving at a constant velocity, unless it is acted upon by an unbalanced force
Example: Clothes on the floor of your room will stay there unless you pick them up
Example: A tennis ball flies through the air once you hit it with a racket. If your friend doesn’t hit the ball back, the forces of gravity and friction will eventually stop the ball
On Earth, gravity and friction are unbalanced forces that often change an object’s motion

4. Inertia An object’s resistance to a change in motion
The amount of inertia an object has depends on its mass
Objects with greater mass have more inertia, and require a greater force to cause a change in motion

5. The Second Law of Motion
States that acceleration depends on the object’s mass and the net force acting on the object
Written as:
Acceleration = Net Force / Mass
Force = Mass X Acceleration (F=mXa)
Force is measured in kilograms x meters per second per second or in a unit called newton (N)
1 N = 1 kg x 1 m/s/s

6. The Second Law of Motion (Cont.)
The acceleration of an object will increase if the force increases
According to the equation, acceleration and force change in the same way – they both get larger
The equation also shows that the acceleration will increase if the mass decrease

7. The Second Law of Motion - Examples
What is the net force on a 1,000 kg object accelerating at 3 m/s/s?
Force = Mass X Acceleration
Force =

8. The Second Law of Motion - Examples
What net force is needed to accelerate a 25 kg cart at 14 m/s/s?
Force = Mass X Acceleration
Force =

9. The Third Law of Motion
Newton’s third law of motion describes the relationship between two forces
States that if one object exerts a force on another object, then the second object exerts a force of equal strength in the opposite direction on the first object
“For every action, there is an opposite and equal reaction”

10. The Third Law of Motion (Cont.)
Newton’s third law refers to forces on two different objects
The action and reaction forces described by this law cannot be added together because they are each acting on difference objects
Forces can be added together only if they are acting on the same object

11. Momentum
Momentum is a characteristic of a moving object that is related to the mass and the velocity of the object
Momentum = Mass X Velocity
Unit for Momentum is kilogram-meters per second (kg-m/s)
The momentum of an object is in the same direction as the velocity of the object
The more momentum a moving object has, the harder it is to stop

12. Momentum Practice Problems
A golf ball travels at 16 m/s, while a baseball moves at 7m/s. The mass of the golf ball is 0.045kg and the bass of the baseball is 0.14kg. Which has greater momentum?
Momentum = Mass X Velocity

13. Momentum Practice Problems
What is the momentum of a bird with a mass of kg flying at 15 m/s?
Momentum = Mass X Velocity

14. Law of Conservation of Momentum
States that, in the absence of outside forces, the total momentum of the objects that interact does not change. It is the same before and after the interaction
The total momentum of any group of objects remains the same, or is conserved, unless outside forces act on the objects
Friction would be an example of an outside force that might act on the objects