1. Measuring Forces & the First Law of Motion
Chapter 2: Forces
Measuring Forces
& the First Law of Motion

2. What is a Force? A force is a push or a pull.
There are three main types of force that we are going to focus on.

3. Contact Forces
A contact force is when one object pushes or pulls another object by touching it.

4. Gravity Gravity is the force of attraction between two masses.
the strength of the gravitational force between two objects depends on their masses.
The greater their masses, the greater the force.

5. Friction
Friction is a force that resists motion between two surfaces that are pressed together.
For example, carpet has greater friction that wood floors.

6. Size and Direction of Forces
Force is a vector because it has both size and direction.
The overall force acting on an object when all of the forces are combined is called the net force.

7. Net Force
If the net force on an object is zero, then it will not move. The forces are said to be balanced forces.
When the net force is not zero, then the object will move. The forces in this case are unbalanced forces.

8. Net Force
If two forces are pushing on an object from the same side, then you add the forces together.
If forces are acting from opposing directions, you would subtract the lower force from the higher force.
Which direction will the object go?

9. Sir Isaac Newton Came up with three laws of motion.
Worked closely with other scientists, including Galileo.
Influenced several other physicists and mathematicians.

10. First Law of Motion
Objects at rest will remain at rest, and objects in motion will remain in motion with the same velocity, unless acted upon by an unbalanced force.
A soccer ball will stay at rest until you kick it. Once you kick it, it will stay in motion until an outside force stops it (friction, air resistance, another player, etc.).

11. Inertia
Inertia is the resistance of an object to a change in velocity (speed or direction).
Closely related to mass – the more mass an object has, the higher resistance to change it will have.

12. Chapter 2: Forces
Second Law of Motion

13. Newton’s Second Law of Motion
The acceleration of an object increases with increased force and decreases with increased mass.
An object moves in the direction the force is acting on it.

14. Newton’s Second Law of Motion
Example:
If you are pushing a full grocery cart and a friend is pushing an empty grocery cart, you would have to apply more force to your cart to have the same acceleration.
Both of your carts would move in the direction of the force.

15. Newton’s Second Law of Motion
Formula:
Force = mass x acceleration
If mass OR acceleration increases, so will the force needed to act on the object. If either of them decreases, so will the force.

16. Newton’s Second Law of Motion
Changing the formula:
Force = mass x acceleration
Mass = force / acceleration
Acceleration = force / mass

17. Newton’s Second Law of Motion
Measuring Force: newtons (N)
Mass = kg; acceleration = m/s2
So 1 N = (1 kg)(1 m/s2)

18. Practice
If a 5 kg ball is accelerating 1.2 m/s2, what is the force acting on it?
A person on a scooter is acceleration 2 m/s2. If the person has a mass of 50 kg, how much force is acting on that person?

19. Practice
A girl pulls a wheeled backpack with a force of 3 N. If the backpack has a mass of 6 kg, what is its acceleration?
A boy pushes a shopping cart with a force of 10 N, and the cart accelerates 1 m/s2. What is the mass of the cart?

20. Centripetal Force
Centripetal force: any force that keeps an object moving in a circular path.
Force points to the center of the circle; if it is “let go” from its path, it will fly off in a straight line.
You can use F = (m)(a) with centripetal force.

21. Third Law of Motion & Momentum
Chapter 2: Forces
Third Law of Motion
& Momentum

22. Newton’s Third Law of Motion
Every time one object exerts a force on another object, the second object exerts a force that is equal in size and opposite in direction back on the first object.
A jellyfish moving through water; rowing a boat

23. Action and Reaction Pairs
The first force in the pair is the action force, and the second force is the reaction force.
See examples on page 58 in your textbook.

24. Action and Reaction Pairs
Not the same as balanced forces; balanced forces act on the same object but action/reaction pairs act on different objects.

25. Momentum Momentum is a measure of mass and its velocity.
Similar to inertia because they both depend on mass.

26. Momentum Momentum = mass x acceleration Or, p = (m)(v) Unit: (kg)(m/s)
*kilogram-meter/second

27. Momentum
A 3 kg ball is moving with a velocity of 1 m/s. What is the ball’s momentum?
What is the momentum of 0.5 kg ball moving 0.5 m/s.