Describing Motion

SpeedThe distance an object travels in a certain

amount of time.

◦Average speed – Total distance divided by total time

◦Constant speed - Speed that does not change

What is the formula for calculating speed?Speed equals distance divided by time.

Let’s Practice Calculating Speed

A football field is about 100 meters long. If it takes a person 20 seconds to run its length, how fast was the football player running?

On your paper, calculate this problem. Make sure you show all work!

Check Your WorkA football field is about 100 meters long. If it takes a person 20 seconds to run its length, how fast was the football player running?

◦Speed = Distance ÷ Time

Remember to label

the UNITS!!

Speed= 100m / 20 sec

Speed= 5m /sec

Velocity

An object’s speed and direction at a given time◦Ex: The wind is blowing 65 km/hr from the North.

Velocity gives a more accurate description of the motion of an object

On your paper, describe an example of velocity.

In a given direction! Must

include the direction it is

moving!

How do you calculate Velocity?

istance

ime

AccelerationA change in the direction or speed (velocity) of

an object over time – which may be:◦ StartingStoppingSpeeding up (positive acceleration)Slowing down (negative acceleration)

◦A change in directionAcceleration is caused by unbalanced forces.On your paper, describe an example of an

object accelerating.

How do I calculate Acceleration?

Acceleration = Force / Mass

How is this athlete preparing

for acceleration?

Rex Burkhead is taking to film sessions to try to improve his acceleration to the line of scrimmage. It's worth noting that, for a running back, the object is not to hit the line as soon as possible; it's to hit it when the hole opens up.

MotionIs a change of the position of an

object with respect to time.

Motion occurs when the position of an object changes.

Write down one example of motion.

EquilibriumWhen an object is at rest or is moving at a constant speed and direction.

InertiaIs the resistance an object has to change in its motion.

The catcher will soon feel the ball's inertia effect as it comes to a stinging stop in his mitt. Inertia is the resistance to motion changes.

A girl has a certain mass and, therefore, has inertia. She can directly feel her body's inertia as the resistance she encounters when she changes her body's momentum, such as when she:

1. comes to a skater's stop, digging her blades into the ice, and feeling the ice pushing against her feet and legs, as she slows. 2. stopped at the bottom, laboriously starts her bicycle up a steep hill. 3. catches a fast baseball, stinging her hands, as the ball's momentum decreases abruptly to zero.

Inertia Examples

ForceIs a push or pull on an object that causes a change in motion.

Balanced Force- forces that cancel each other out when acting together on an object.

Unbalanced Force-If an object has a net force acting on it, it will accelerate. The object will speed up, slow down or change direction

100N 100N100N 50N

How do we calculate Force?

Force = Mass x Acceleration or F= MxA

What unit of measurement is used to measure force?

Newtons

GravityIs the force that causes objects to fall to the ground.

Sir Isaac Newton

In 1687, English physicist and mathematician, Isaac Newton, published his three laws of motion.

1st Law of MotionAn object at rest stays at rest and an object in motion maintains its velocity unless acted upon by an unbalanced force.

2nd Law of MotionThe acceleration of an object is equal to the force exerted on the object divided by the mass of the object. F = m*a

A massive object is harder to accelerate (move) than a less massive object. It is easier to push a mosquito than to push an elephant.

3rd Law of MotionWhen an object exerts a force on a second object, the second object exerts a force on the first object that is equal in magnitude and opposite in direction.

To every action there is always an equal and

opposite reaction

Let’s go over some examples…………….

Consider a swimmer facing the wall of a pool. If she places her feet on the wall and pushes hard, what happens?

She shoots backward, away from the wall.

Clearly, the swimmer is applying a force to the wall, but her motion indicates that a force is being applied to her, too. This force comes from the wall, and it's equal in magnitude and opposite in direction.

Next, think about a book lying on a table.

What forces are acting on it?

One big force is Earth's gravity. So, if we say the book weighs 10 N, what we're really saying is that Earth is applying a force of 10 N on the book. The force is directed straight down, toward the center of the planet. Despite this force, the book remains motionless, which can only mean one thing: There must be another force, equal to 10 N, pushing upward. That force is coming from the table.

How is Newton’s 3rd Law being demonstrated in this picture?

There's no doubt the bat applies a force to the ball: It accelerates rapidly after being struck. But the ball must also be applying a force to the bat. The mass of the ball, however, is small compared to the mass of the bat, which includes the batter attached to the end of it. Still, if you've ever seen a wooden baseball bat break into pieces as it strikes a ball, then you've seen firsthand evidence of the ball's force.

WorkWork = Force x Distance

Work is measured in Joules (J)

In order for a force to qualify as having done work on an object, there must be a displacement and the force must cause the displacement.•a horse pulling a plow through the field• a father pushing a grocery cart down the aisle of a grocery store •a freshman lifting a backpack full of books upon her shoulder •a weightlifter lifting a barbell above his head,•an Olympian launching the shot-put

Read the following four statements and determine whether or not they represent examples of work. Then click on the See Answer button to view the answer.

A teacher applies a force to a wall and becomes exhausted. 

A book falls off a table and free falls to the ground. 

 

A waiter carries a tray full of meals above his head by one arm straight across the room at constant speed.

A rocket accelerates through space.

 

 This is not an example of work. The wall is not displaced. A force must cause a displacement in order for work to be done.

This is an example of work. There is a force (gravity) which acts on the book which causes it to be displaced in a downward direction (i.e., "fall").

This is not an example of work. There is a force (the waiter pushes up on the tray) and there is a displacement (the tray is moved horizontally across the room). Yet the force does not cause the displacement. To cause a displacement, there must be a component of force in the direction of the displacement.

This is an example of work. There is a force (the expelled gases push on the rocket) which causes the rocket to be displaced through space.

Potential Energystored energy of position

Kinetic Energyenergy of motion

An object that has motion - whether it is vertical or horizontal motion - has kinetic energy.

Terms I must know!1. Speed2. Velocity3. Acceleration4. Motion5. Equilibrium6. Inertia7. Force8. Balanced force9. Unbalanced force10.Gravity11.1st Law of Motion12.2nd Law of Motion13.3rd Law of Motion14.Work15.Potential Energy16.Kinetic Energy

Motion Quiz!Thursday,

November 8!Over motion and

all the terms!Study this notes

packet!