Measuring Motion Chapter 5

Observing Motion by Using a Reference Point How do you know an object is moving? When you watch the motion of an object, you are actually watching the object in relation to another object that appears to stay in place. The object that appears to stay in place is a reference point.

Observing Motion by Using a Reference Point When an object changes position over time relative to a reference point, the object is in motion. Common Reference Points: tree building door objects in motion – a bird flying while in a hot air balloon.

Speed Depends on Distance and Time Speed-Distance traveled by an object divided by the time taken to travel that distance. Ex. A balloon traveled 50 m in 10 s. What is the balloons speed?   The SI unit for speed is meters per second (m/s).

Determining Average Speed Most of the time, objects do not travel at a constant speed – you probably do not walk at a constant speed from one class to the next. Average speed= total distance Total time

Average Speed Examples: An athlete swims a distance from one end of a 50m pool to the other end in a time of 25s. What is the athletes average speed? Jake jogs to a store 72 m away in a time of 36s. What is Jake’s average speed?

Recognizing Speed on a Graph Distance vs. Time What is the average speed? Distance (m) Time (s)

Velocity: Direction Matters Imagine that 2 students leave the same classroom at the same time. They both walk at 10km/hr for 5 min, 12 km/hr for 8 min, and 5 km/hr for 10 min. Why don’t they end up at the same place?

Velocity Velocity- Speed of an object in a particular direction You must always include a direction in your answer Constant velocity only occurs if neither speed nor direction changes. Velocities can be added or subtracted together to give a resultant velocity. Add velocities that are in the same direction. Ex: Walking on a bus as the bus is moving forward. Subtract velocities that are in opposite directions. Ex. Walking to the back of the bus while it is moving forward

Finding Resultant Velocities Same direction Resultant Velocity = 2 km/min east 15 km/min east

Finding Resultant Velocities Opposite directions Resultant Velocity= 2 km/min west 15 km/min east

Acceleration Acceleration is the rate at which velocity changes. Velocity changes if: speed changes direction changes both change Increase in velocity = positive acceleration Decrease in velocity = negative acceleration (deceleration)

Acceleration Calculating Average Acceleration Avg Accl = final velocity-starting velocity time it takes for velocity to change A = vf – vi = A = Δv t t Velocity is expressed in m/s and time is expressed in s. Therefore, acceleration is expressed in meters per second per second (m/s/s) or m/s2

Calculating Acceleration 0:02 0:03 0:04 0:05 0:01 1 m/s 2 m/s 3 m/s 4 m/s 5 m/s Equation: Calculation:

Acceleration Problems A skater goes from a standstill to a speed of 6.7 m/s in 12 seconds.  What is the acceleration of the skater? A plane passes point A at a velocity of 240 m/s north. Forty seconds later, it passes point B at a velocity of 260 m/s north. What is the planes average acceleration?

Centripetal Acceleration Centripetal Acceleration- the acceleration that occurs in a circular motion. Ex: ferris wheel, the moon’s orbit

5.2 Forces A force is a push or pull that causes a resting object to move, or it can accelerate a moving object by changing its speed or direction. Unit = Newton (N) 1 kg m/s2 Combination of all forces acting on an object is called the net force.

Types of Forces Balanced Force When the forces on an object are balanced, the net force on an object is zero and there is no change in the object’s motion. Unbalanced force When the forces on an object are unbalanced, there is a net force and the object accelerates.

Calculating Net Force Forces in the same direction Add forces to determine net force

Calculating Net Force Forces in opposite directions Subtract forces to determine net force

Forces Affect of force on moving object Change in speed or direction Ex. When soccer ball is passed to another player and is kicked Affect on force on stationary object Cause a nonmoving object to start moving. Ex. Stationary soccer ball kicked

5.3 Friction Friction - A force that opposes motion between two surfaces that are in contact. Friction causes moving objects to slow down and eventually stop. What causes friction? When the hills and valleys of one surface come in contact with the hills and valleys of the other surface

Friction Rough surfaces have more hills and valleys than smooth surfaces do. The rougher the surface, the greater the friction. Ex. A soccer ball rolling on grass, verses a hockey puck on ice. The greater the weight of the object, the greater the friction will be

Types of Friction Static Friction is the friction force that acts on a stationary object. It opposes the applied force. Kinetic Friction is the force that opposes the direction of motion of an object as it slides over a surface. Solid over solid

Types of Kinetic Friction Rolling Friction is the force of friction felt on rolling objects Example: wheels and balls. Fluid friction opposes the motion of a moving object in a fluid (gas or liquid) Example: Air resistance

Is friction helpful or harmful? Movement of tires Walking Eraser Harmful Machine wear Erosion Burns/blisters

Decreasing Friction Lubricants- Substances that are applied to surfaces to reduce the friction between the surfaces. Ex: motor oil, wax, grease Lubricants are usually liquids, but can be solids or gases. (Air in air-hockey)

Decreasing Friction Switching from sliding friction to rolling friction. EX. Ball bearings are placed between the wheels and axles of skates Make surfaces that rub against each other smoother.

Gravity Gravity is a force of attraction that acts between objects with mass. Objects close to Earth accelerate at 9.8m/s2. All objects fall at the same rate.

Gravity If gravity affects all objects with mass – why aren’t all objects stuck together? Masses of most objects are too small, you can’t detect this force Earth has a huge mass, so the gravitational force of Earth is large. It pulls everything toward the center of the Earth. – Dropped objects fall to the floor.

Law of Universal Gravitation All objects in the universe attract each other through gravitational force. The size of the force depends on the masses of the objects and the distance between the objects. Greater the mass the greater the force. Smaller the distance, the greater the force.

Mass vs. Weight Mass is the amount of matter in an object. Unit = kilogram Measuring device is a balance. Weight is a measure of gravitational force on an object. Unit – Newton Measuring device is a spring scale. Weight = mass x gravity N = (kg) x (m/s2)