The concepts of forces and motion Forces & Motion The concepts of forces and motion

Motion Motion is described as when an object is changing its distance from another object. *Motion is usually very easy to see. (Ex. Walking toward the door). ** Sometimes it is difficult to see. (Ex. The Earth in motion or small or fast moving objects). The movement of an object depends on your reference point (place to compare movement from). You must assume that a reference point is not moving. To fully understand motion, you must also understand distance & distance measurements.

Speed Speed is a rate. Speed is the distance traveled over a unit of time. Speed = Distance / Time or S = D/T *Speed is usually measured in meters per second. (m/s or m/sec.) Ex. Problem If the Flash (the fastest man alive) can travel 1000 km in two seconds, what is his speed? What is his speed in meters an hour? Solution: S = D/T so S = 1000 km / 2 sec. = 500 km/sec. (T) B. Use unit multipliers to solve: 500 km/sec. X 1000 m/km X 3600 sec./hr = 1,800,000,000 m/hr

Speed (cont.) If an object moves the same direction & speed for a while it is said to be moving at a constant speed. *Most objects do not travel at a constant speed, but change speeds as they move. Because of this you must often calculate an average speed & not a specific speed. Avg. Speed = Total distance/total time

Velocity Velocity is similar to speed except velocity gives you the direction the object is moving in. Ex. Speed = 5 m/s Velocity = 5 m/s West

Acceleration Acceleration is the rate at which velocity changes. This means that acceleration can mean changing speed (speeding up or slowing down) or changing direction. *Slowing down is often called negative acceleration or deceleration. To calculate acceleration you must use this formula: Acceleration = Final Velocity – Initial Velocity / Time Ac = Fv – Iv / T * Acceleration is usually measured in meters per squared seconds (m/s2 )

Acceleration (cont.) Sample Problem A roller coaster rolling down a slope picks up speed. It begins down the slope at 4 m/s & ends up at 22 m/s at the bottom of the slope 3 seconds later. What is its acceleration? Solution: Ac = Fv – Iv / T Ac = 22 m/s – 4 m/s / 3s (Fv) (Iv) (T) Ac = 18 m/s / 3s Ac = 6 m/s2

Motion Graphs Acceleration Graphs

Motion Graphs (cont.) Speed Graph

Forces A force is simply a push or pull. Like velocity & acceleration a force is given with the direction it acts in. Force diagrams page 627 & other examples. Force diagrams

Forces (cont.) Unbalanced Forces Unbalanced forces are when 2 or more forces act in different directions & at different strengths. Unbalanced forces cause or change motion. (Fig F-2)(previous slide) When 2 different forces act in the same direction they act like a stronger force in that direction. (Ex. 2 people pushing on the same car can move it better than one) When 2 forces push in opposite directions but one force is greater than the other the object will move in the direction of the greater force but slower because some of the force is cancelled. It is like subtracting or adding a positive & negative number. When you calculate an unbalanced force the sum of all forces acting on an object is called a Net Force. Balanced Forces Balanced forces occur when the sum of all forces acting on an object = zero. *Balanced forces do not cause a change in motion.

Friction Friction is the force that opposes motion or that is caused when 2 objects are rubbed together. *Friction depends on 2 factors. Surface types How hard the objects are rubbed together. *Friction is useful. It keeps us from sliding around & is the principle force in automobile brakes. *Types of Friction: Sliding Friction – when solid surfaces slide over each other Rolling Friction – when rounded objects roll over surfaces. Less force is need to overcome rolling friction than sliding friction. This is why ball bearings are often added between rotating metal parts like in skateboards & bikes. Fluid Friction – friction that occurs when an object moves between a liquid or a gas. Usually less than sliding friction. This is why cars use brake fluid.

Newton’s Laws A review of Newton’s laws

Newton’s Laws of Motion Newton’s first 3 laws are called the laws of motion

Newton’s 1st Law Newton’s First Law is also called the Law of Inertia Inertia is defined as the tendency of an object to resist a change in its motion. It states “Every object in a state of uniform motion tends to remain in that state of motion unless an external force is applied to it.”

Newton’s 2nd Law This law defines force in terms of mass and acceleration. It says “The force of an object (in Newton’s or kgm/s2) is equal to its mass (in kg) multiplied by its acceleration (in m/s2). F=ma

Newton’s 2nd Law (cont.) Sample Problem: A 52 kg wrestler is body slammed. He is accelerated to 2 m/s2 toward the mat. What is the net force he is being slammed with? Solution: F=ma F=52 kg X 2 m/s2 F = 104 N

Newton’s 3rd Law It says: “For every action there is an equal and opposite reaction.” It explains how brakes work and why you get hurt when you fall.

Newton’s Law of Universal Gravitation Also known as the law of gravity. where: F is the magnitude of the (attractive) gravitational force between the two point masses G is the gravitational constant m1 is the mass of the first point mass m2 is the mass of the second point mass r is the distance between the two point masses Every object exerts a pull of gravity on every other object. Depends on mass of objects & the distance apart. The closer the objects the stronger the pull. The larger the object the stronger the pull.

Summary

Freefall & Projectile Motion Free Fall is the condition where gravity is the only force that is working on an object. The acceleration of gravity on an object is 9.8 m/s2. Projectile Motion Projectile Motion is motion of an object when it is thrown. *In a vacuum (a condition where there is no air resistance), a dropped ball & a projectile will hit the ground at the same time. Air Resistance A type of fluid friction caused by objects falling through the air. *It is an upward force. *It is not the same for all objects. The greater the surface area, the greater the air resistance. This is why an acorn hits the ground before a leaf. *Terminal Velocity- the greatest velocity an object can reach. **Review differences between mass & weight.

Momentum Momentum is Mass X Velocity. *Measured in kg m/s (Note difference between momentum & force!) *Has a direction.

Law of Conservation of Momentum *Law of Conservation of Momentum – the total momentum of a group of objects that interact remains the same unless an outside force acts on the objects. 2 moving objects – if 2 objects moving at different velocities collide in an elastic collision then the slower object will gain momentum & the faster object will lose momentum but total momentum will be the same. 1 moving & 1 stationary object – if a moving object collides in an elastic collision with a non moving object the non moving object will gain momentum & move but the moving object will stop. Total momentum will be the same. Connected objects – if 2 objects ( 2 moving or 1 non moving) collide & stick together ( Non elastic collision), they will move together as a single heavier unit at a slower velocity but overall momentum will be the same. *ALL the above examples assume that friction is absent. In most real situations friction will play a role.

Centripetal Force Centripetal means, “center seeking” & is any force that causes an object to move in a circle. This is the force that holds satellites in orbit around the earth. It also holds the moon & planets in orbits. Centripetal force is a product of gravity & inertia. Satellites continuously fall toward earth (gravity), but the earth is a sphere & this causes it to fall around the earth by its own inertia.