Topic 2: Motion and Force Dynamics By: Mrs. Fagan.

Topic 2: Motion and Force Dynamics

By: Mrs. Fagan

A: Motion

• Motion – an object is moving if its position changes against some background that stays the same

A: Motion

• The stationary background is called a reference frame.

• Objects that you know aren’t moving; trees, buildings, lines on a road, etc.)

A: Motion

• Speed is how fast an object is moving.– To figure out speed you must measure two

quantities• The distance traveled• The time it takes to travel the distance

A: Motion

• An object which covers equal distance in equal time is going at a constant speed.

• This object has an unchanging speed

A: Motion

• One way to determine speed is to use a distance-time graph.

• Distance is measured on the vertical axis and time is measured on the horizontal axis

A: Motion

• By examining the shape of the line, you can determine what is happening with the motion of the object

• If the line is straight, the object is moving at constant speed• If the line curves upward, the object is accelerating• If the line curves downward, the object is decelerating

A: Motion

• Analyzing your results:– The slope (steepness of graph line) will show

which object was going the greatest speed • The steeper the line, the greater the slope• The greater the slope, the greater the speed

A: Motion

• Since most objects do not move at constant speed, speed is often calculated in terms of average speed.

• Average speed is the distance covered by an object divided by the time it takes to travel the distance• Speed = distance or v= d

time t

A: Motion

• If you know the speed and time, but would like to know distance traveled, use the equation

Distance = (speed)(time)or

d=vt

A: Motion

• If you know the speed and distance, you can figure out time by using

Time = distance or t = d speed v

A: Motion

1. To use the equations, always determine the given values first (what do I already know)

2. Figure out what is the unknown value (what are they asking for?)

3. Pick the equations that would solve the problem

4. Make sure your units match up!• You may have to convert units before working the

equation

A:Motion

• Example:• John ran an average of 3.5 m/s for 3 hours.

– You must first convert hours into seconds

B: Velocity

• Sometimes the words speed and velocity get confused

• Speed is how fast something is going• Velocity is how fast AND in what direction something is

going.

B: Velocity

• Example of velocity– O.J. was traveling 75 mil/h heading southeast on

Hwy 70

B: Velocity

• The velocity of an object changes if its speed, direction changes, or both.– Even when a car has a constant speed, its velocity

changes if the car turns.

B: Velocity

• Velocity is not the only important quantity when objects are in motion. Momentum is a quantity defined as the product of an object’s mass and its velocity

C: Momentum

• An object with more momentum would be more difficult to stop.

• The momentum of an object depends on both its velocity and its mass.

C: Momentum

• For an object moving in a straight line, momentum is calculated by multiplying an object’s mass by its velocity

Momentum = mass x velocityOr

p = mv

C: Momentum

• Like velocity, momentum also has direction. An object’s momentum is in the same direction as its velocity

C: Momentum

• Example: The momentum of a 6.0kg bowling ball moving at 10 m/s down the alley would be:

• 6.0kg x 10.0m/s = 60.0 kg(m/s) down the alley

• DON’T FORGET THE UNITS!!!!!! Kg(m/s)

C: Momentum

• The more mass an object has, the greater the momentum.

• The faster an object is moving, the greater the momentum

C: Momentum

• The law of Conservation of Momentum • When two objects traveling with two different masses

and velocities collide, their momentum when they collide is combines

C: Momentum

• Example: – Car A has a momentum of 7500 kg(m/s) moving

east. Car B has a momentum of 7000 kg(m/s) moving north. When they collide, their combine momentum is 14500 kg(m/s) moving north east.• The direction is a little more east than north because

the east bound car has more momentum

C: Momentum

• Policemen use this information to determine who was at fault in a collision

D: Acceleration and Force

• Any change in an object’s velocity is called acceleration.

• To find acceleration we need to measure the object’s velocity at different times.

D: Acceleration and Force

• Acceleration can be calculated by dividing the change in the object’s velocity by the time in which the change occurs.

• The SI unit for acceleration is meters per second squared (m/s2)

• Acceleration = final velocity – initial velocitytime

D: Acceleration and Force

1. Determine the initial velocity (how fast was the object moving in the beginning)

2. Determine the final velocity (how fast was the object moving at the end)

3. Determine the time (how long did it take the object to travel)

D: Acceleration and Force

4. Plug the values into the formula.5. Subtract the final – initial velocities.6. Divide (your units will be m/s2)

D: Acceleration and Force

• Positive acceleration means the object’s velocity will increase (speed up)

• Negative acceleration means the object’s velocity will decrease (slow down)

D: Acceleration and Force

• Acceleration can be shown by making a velocity-time graph.

• Velocity will be plotted on the y axis • Time will be plotted on the x axis

D: Acceleration and Force

• A straight line on a velocity-time graph means that the velocity changes the same amount each time (constant acceleration)

D: Acceleration and Force

• The slope of a line on a velocity-time graph gives you the value of the acceleration

D: Acceleration and Force

• Slope going up = acceleration• Slope going down = deceleration

D: Acceleration and Force

• The acceleration of an object is zero if its velocity is constant (the line is level)

E: Forces

• A force is a push or a pull. Forces are the cause of acceleration or change in an object’s velocity

E: Forces

• Some forces do not change an object’s motion. These forces are called balanced forces. The forces acting on an object combine to produce a net force equal to zero.

E: Forces

• These forces are opposite in direction and equal in strength.– In other words, they completely cancel each other

out.

E: Forces

• If balanced forces are acting on an object, then the object’s motion or lack of motion stays the same.

E: Forces

• If opposing forces acting on an object do not have the same strength, they are unbalanced forces.– The acceleration the object has will be determined

by the object with stronger force

E: Forces

• An object in motion is always having to fight an unbalanced force, friction.

• Friction is the force between two objects in contact that opposes the motion of the other object.– Because of friction, a constant force must be

applied to an object to keep it moving.

E: Forces

• Friction also affects objects that aren’t moving.

• Friction provides the force needed to balance the force of gravity and prevent an object from moving downhill if it is on an incline.

E: Forces

• We need friction to even have motion.• Frictional forces are relatively great when both

surfaces are rough.

E: Forces

• Another obstacle objects must overcome to stay in motion is resistance.

• Air resistance is caused by the interaction between the surface of a moving object and the air molecules.

E: Forces

• Air resistance depends primarily on three things.

1. The size of the object2. The shape of the object3. The speed the object is moving

E: Forces

• Larger objects experience greater resistance• Air resistance will also increase as an objects

speed increases.– Designers have changed the shapes of many

vehicles to reduce resistance between the vehicle and the surrounding air.

E: Forces

• Gravity – the force of attraction that exists between all objects in the universe.

E: Forces

• Every object exerts a gravitational force on every other object.

• The force of gravity acts even when the objects do not touch.

E: Forces

• The force of gravity between two objects depends on their masses and on the distance between the two objects. – The greater the mass of an object, the larger the

gravitational force it exerts on other objects.

E: Forces

• The force of gravity changes if the masses of the objects changes or the distance between the objects change.

E: Forces

• If the distance between two objects is doubled, the gravitational force between them decreases to ¼ its original value.

• Triple the distance and the force decreases to 1/9 the original value.

E: Forces

• Gravitational force is weaker than other types of forces, even though it holds planets, stars, and galaxies together.