1. forces
Ch. 12.1

2. What is a force? Force – a push or a pull that acts on an object
A force can cause a resting object to move, or it can accelerate a moving object by changing the object’s speed or direction.
Examples of a force would be wind, gravity, and friction.

3. Measuring force
Some forces are easy to measure. A scale measures weight, a type of force.
Force is measured in units called newtons (N).
1 newton = 1kg•m/s^2
You can use an arrow to represent the direction and strength of a force. The direction of the arrow shows direction and the length represents the strength of the force.

4. Combining forces
Individual forces can be combined into a larger force.
Forces in the same direction add together and forces in opposite directions subtract from one another.
Net Force – the overall force acting on an object after all the forces are combined.
Top of diagram shows forces combining into a larger force; Bottom shows opposing forces. Notice how the car still moves forward, why is that?

5. Combining forces Balanced Forces
When the net forces on an object are balanced, the net force is zero and there is no change in the object’s motion.
Unbalanced Forces
Force that results when the net force acting on an object is not equal to zero causing the object to accelerate.

6. gravity
Gravity is an attractive force that acts between any two masses.
Unlike friction, the force of gravity does not require for objects to be in contact for it to act on them. Thus gravity can act over large distances.
Earth’s gravity acts downward toward the center of the Earth.

7. gravity What forces are acting on this boulder?
If gravity is pulling it down, how come it doesn’t pass through the ground?
A supporting force is exerted upward, balancing the gravitational force. Making the net force on the boulder equal to what?
ZERO – since it is not moving.
Gravity
Supporting Force

8. Falling objects
Both gravity and air resistance affect the motion of falling objects.
Gravity causes objects to accelerate downward, whereas air resistance acts in the direction opposite to the motion and reduces acceleration.
Terminal velocity – constant velocity of a falling object when the force of air resistance equals the force of gravity.

9. friction
All moving objects are subject to friction, a force that opposes the motion of objects that touch as they move past each other.
Friction acts at the surface where objects are in contact.
“In contact” includes solid objects that are directly touching one another as well as moving through a liquid or a gas.

10. Types of friction Static Friction
Friction force that acts on objects that are not moving
Static friction always acts in the direction opposite to that of the applied force
You experience static friction every time you walk. As you push off with each step, static friction between the ground and your shoe keeps your shoes from sliding.

11. Types of friction Sliding Friction
Force that opposes the direction of motion of an object as it slides over a surface.
Because sliding friction is less than static friction, less force is required to keep an object moving than to start it moving.

12. Types of friction Rolling Friction
Rolling an object across a flat surface bends both the object and the floor slightly out of shape.
This change in shape is the cause of rolling friction – the force that acts on rolling objects.
Rolling frictions replaces sliding friction, greatly reducing the friction between surfaces; ~ times less than static or sliding.

13. Types of friction Fluid Friction
Friction also acts on objects moving through liquids and gases.
Fluid friction increases as the speed of the object moving through the fluid increases.
Air resistance – fluid friction acting on an object moving through the air.
Drag is the liquid
friction acting on
the swimmer. What
are some ways to
reduce drag?

14. Projectile motion
When you throw a ball forward, you’ll notice that it follows a curved path. This path is an example of projectile motion.
Projectile motion – the motion of a falling object after it is given an initial forward velocity.
The only forces acting on a projectile are gravity and air resistance.

15. Projectile motion
The combination of an initial forward velocity and the downward vertical force of gravity causes the object to follow a curved path.