forces 12 1 pg 356 362
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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)

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  • 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.
  • 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
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
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
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
Unbalanced force
  • When an unbalanced force acts on an object, the object accelerates
  • 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
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
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
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
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
  • Force that acts between any two masses
  • Acts downward toward the center of Earth
falling objects
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

Air Resistance

terminal velocity
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


Air Resistance

projectile motion
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
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
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.