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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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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