# 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.

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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) 1 Newton is the force that causes a 1 kg mass to accelerate at a rate of 1 m/s 2 1 N = 1 kgm/s 2 kg = mass and m/s 2 = 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 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

Terminal Velocity 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 Terminal velocity of a skydiver is approximately 195 km/h (122 mph or 54 m/s)

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 2m away)? Given info: Height of cliff = 9 m Distance forward = 2.5 m Gravity = 9.8 m/s 2 Find Velocity (v) and Time (t) First find time t = √2(h)t = √2(9m) t = √18m = √1.836734694s 2 = 1.4s 9.8m/s 2 9.8m/s 2 9.8m/s 2 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

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