Forces 12.1 Pg 356-362

1 / 18

# Forces 12.1 Pg 356-362 - PowerPoint PPT Presentation

Forces 12.1 Pg 356-362. 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. FORCE:. Unit of force: Newton (N)

I am the owner, or an agent authorized to act on behalf of the owner, of the copyrighted work described.

## PowerPoint Slideshow about ' Forces 12.1 Pg 356-362' - quinlan-gregory

An Image/Link below is provided (as is) to download presentation

Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author.While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server.

- - - - - - - - - - - - - - - - - - - - - - - - - - E N D - - - - - - - - - - - - - - - - - - - - - - - - - -
Presentation Transcript

### Forces12.1 Pg 356-362

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.
FORCE:
• 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
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
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
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
Unbalanced force
• When an unbalanced force acts on an object, the object accelerates
• THINK TUG OF WAR
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
Static Friction
• 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!!
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
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
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
GRAVITY:
• Force that acts between any two masses
• Acts downward toward the center of Earth
Falling objects

Gravity

• 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

Air Resistance

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

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

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
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 = √1.836734694s2 = 1.4s

9.8m/s2 9.8m/s2 9.8m/s2

• Now find the velocity (speed)

v = d v = 2.5m v = 1.8 m/s

t 1.4s

• 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