1. Forces 12.1 Pg

2. A push or a pull that acts on an object
FORCE:
A push or a pull that acts on an object
Can cause a resting object to move, or it can accelerate a moving object by changing the object’s speed or direction.

3. Unit of force: Newton (N)
1 Newton is the force that causes a 1 kg mass to accelerate at a rate of 1 m/s2
1 N = 1 kgm/s2
kg = mass and m/s2 = acceleration so…
Force = mass x acceleration

4. Representing Force
Example: Measuring force with a spring scale at the grocery store
When the vegetables are in the basket the force pushes the basket down
To represent force use an arrow in the direction of the force.
Vectors can be used to represent force because force has magnitude and direction

5. Combining forces Forces CAN be combined (vector addition)
3 people pushing a car out of gas
  
Forces in the SAME direction you ADD
Forces in the OPPOSITE direction you SUBTRACT
NET FORCE: overall force acting on an object after all the forces are combined

6. Balanced forces:
When the forces on an object are balanced, the net force is ZERO and there is no change in the object’s motion   0
The forces are balanced when they are equal in size and opposite in direction
THINK equal push and pull

7. Unbalanced force
When an unbalanced force acts on an object, the object accelerates
THINK TUG OF WAR

8. FRICTION:
A force that opposes the motion of objects that touch as they move past each other
Friction makes
Walking possible
Food stay on your fork
Tires on your car grip the road so the car moves forward
A feather slow down as it falls
There are four main types of friction: static friction, sliding friction, rolling friction, and fluid friction

9. Static Friction It is not the same as static electricity!!
The friction force that acts on objects that are not moving
Always acts in the direction opposite to the applied force
Every time you take a step and push off the ground you are experiencing static friction
It is not the same as static electricity!!

10. Sliding Friction
A force that opposes the direction of motion of an object as it slides over a surface
Sliding friction is less than static friction, therefore once you get something moving it is easier to keep it moving

11. Rolling Friction
The friction force that acts on rolling objects, occurs when the floor and object are bent slightly out of shape
Rolling friction is is about 100 to 1000 times less than the force of static or sliding friction
Ball bearings are often used to reduce friction in machines
Friction is greatly reduced because it changes sliding friction to rolling friction
Think in-line skates and skate boards

12. Fluid Friction
The force that opposes motion of an object as it moves through a fluid
Fluids are liquids (like water) or mixtures of gases (like air)
Fluid friction increases as as the speed of the object increases
Air resistance is fluid friction acting on an object moving through air

13. GRAVITY:
Force that acts between any two masses
Acts downward toward the center of Earth

14. Falling objects
Gravity causes objects to accelerate downward, whereas air resistance acts in the direction opposite to the motion and reduces acceleration
Air resistance acts in the direction OPPOSITE to the motion
Gravity
Air Resistance

15. Terminal Velocity
Terminal velocity of a skydiver is approximately 195 km/h (122 mph or 54 m/s)
As speed increases air resistance increases
If an object falls for a long time the upward force of air resistance becomes equal to the downward force of gravity
Then the object continues to fall at a constant velocity (or constant speed)
Terminal velocity is the constant velocity of a falling object when the force of air resistance equals the force of gravity
Gravity
Air Resistance

16. Projectile Motion
The motion of a falling object after it is given an initial forward velocity
The path of a projectile is always a curve
Air resistance and gravity are the only forces acting on a projectile
The combination of an initial forward velocity and the downward force of gravity causes the ball to follow a curved path
Both marbles fall at the same rate even if one is also moving forward

17. Projectiles and Speed
In the diagram the marbles roll off the table and fall to the floor
The marbles continue to move forward while they fall
Therefore, as the marble’s velocity increases the distance away it travels will increase
All marbles hit the ground at the same time no matter how fast they travel forward

18. Projectile Motion Problem
How fast would the person have to run in order to land in the water safely (land 2.5m away)?
Given info:
Height of cliff = 9 m
Distance forward = 2.5 m
Gravity = 9.8 m/s2
Find Velocity (v) and Time (t)
First find time
t = √2(h) t = √2(9m) t = √18m = √ s2 = 1.4s
9.8m/s m/s m/s2
Now find the velocity (speed)
v = d v = 2.5m v = 1.8 m/s
t s
So the person would have to run at a velocty of 1.8 m/s in order to land at least 2.5 m away from the cliff. If not he could be seriously injured on the rocks below.
2.5m