1. Notes 2- Gravity, Friction, and Newton’s Laws

2. Forces Force: a push or pull on an object
Forces have both strength and direction.

3. 1. Gravity
Objects fall to the ground because Earth puts an attractive force on them.
Gravity: an attractive force between two objects that have mass
Strength of the force depends on the mass of each object and the distance between them

4. 1. Gravity
Due to the force of gravity, objects accelerate towards Earth at a rate of 9.8m/s2.
Practice Problem: How fast will an object be moving after 35 seconds of falling off of a tall building? Use the acceleration due to gravity.

5. 2. Friction
Friction: a resistance that one surface or object encounters when moving over another.
Ex) sliding across a smooth floor in your socks. You will eventually stop sliding.
Causes of friction:
Microscopic roughness on surfaces catch onto each other
Atomic charges (+ or -) attract each other, slowing the sliding of the object

6. 3. Newton’s Laws of Motion
Isaac Newton: English physicist and mathematician in the 1600’s
Described gravitational acceleration, and Three Laws of Motion, all driven by forces

7. 3. Newton’s Laws of Motion
Net Force: the “overall” force on an object. The combination of all forces acting on an object at one time

8. Practice: What is the net force?

9. 3. Newton’s 1st Law of Motion
1st Law: If the net force of an object is zero, the motion of the object does not change.
“An object in motion will remain in motion unless acted upon by a force”
“An object at rest will remain at rest unless acted upon by a force”
When unbalanced forces act on an object:
An object at rest will start moving
An object that was already moving will change speed, direction, or both

10. 3. Newton’s 2nd Law of Motion
2nd Law: the acceleration of an object is equal to the net force acting on the object divided by the object’s mass

11. 3. Newton’s 2nd Law of Motion
Practice: You throw a .5kg basketball with a force of 10N. What is the acceleration of the ball?

12. 3. Newton’s 3rd Law of Motion
3rd Law: when one object puts a force on a second object, the second object puts an equal force in the opposite direction on the first object
“for every action, there is an equal and opposite reaction”

13. 4. Momentum
Momentum: a measure of how difficult it is to stop a moving object
Related to the object’s mass and its velocity
Practice: What is the momentum of a 12kg bicycle moving at 5.5m/s?