Motion: Speed, Velocity, Acceleration, and Forces

What’s the difference between distance and displacement?

• Distance: the total magnitude traveled • Displacement: the shortest distance from

the starting point to the ending point

What is speed?– Speed–The rate of change in distance with respect to

time.– Even objects that are not moving have a speed = 0

m/s– Units: m/s, cm/s, mph– Since speed is built from distance, a scalar quantity,

then speed is also a scalar quantity. – Scalar Quantities carry no direction information with

them.

What is Velocity?– The rate of change in displacement with respect to

time. – Since displacement is a vector quantity, then

velocity is also a vector quantity. – Vector Quantities have both magnitude and

direction.– v=d/t• v= velocity (m/s)• d= distance (m)• t= time (s)

D

V T

Speed & Velocity Examples

• Speed is distance divided by time– 2 cm/s

• Velocity includes the direction– 2 cm/s west

West

Acceleration“Acceleration” occurs

when your speed changes up or down (or when you change direction).

• It is the rate speed changes– Level ground- no

acceleration due to gravity

– Steep down hill- acceleration due to gravity

– Steeper hill= more acceleration

Deceleration is slowing down

Zero acceleration is at a constant speed

Acceleration

• Can use a graph to show acceleration–+ slope = +acceleration (speed up)– - slope = - acceleration (slow down)– 0 slope = no acceleration

Acceleration= change in speed (cm/sec)

time (sec)

a = vf-vi

t

Units: cm/sec*sec or cm/sec2

Forces

• Force- Ability to change motion(push or pull)–Units of lb, N=kg.m/sec2

• If forces are balanced then the object won’t move and it is said to be in equilibrium

• Unbalanced forces=move• Forces always occur in pairs!!!!!

Newton’s Laws of Motion• Newton’s First Law: – “AKA” the Law of Inertia– Inertia- property of an object to resist

change in state of motion

– an object will stay at rest until acted on by an unbalanced force

– In other words, things tend to keep on doing what they were doing in the first place unless you apply a force.

Newton’s Laws of Motion• Newton’s Second Law: – force causes an object to accelerate

while the object’s mass resists acceleration

– Fnet=ma• a=acceleration (m/sec2), F= force

(N), m=mass (kg)• Fnet= Right – Left

• Fnet= Top - Bottom

– EX. Lighter cars go faster than heavier ones pushed with equal force. Lighter cars resist the force of acceleration less allowing them to move faster than heavier ones.

Newton’s Third Law• Newton’s 3rd Law: – For every action there is a reaction force equal in

strength & opposite in direction– For every action there is an equal & opposite reaction– Can be + (move to right) or – (move to left)– Examples: rockets, stepping into a boat, throw a ball on a

a skate board

Gravity, Weight & Mass• Gravity- – Force pulling toward mass of

planet– Depends on mass– Mass- amount of matter in an object

compared to a standard

– Changes on other planets– On Earth= -9.8m/s2

• Weight- force created by gravity depends on mass– It is a force acting on object in units of

lb or N

•Fw=mg

•Fw= weight force (N)

•m=mass (kg)

•g=acceleration due to gravity (-9.8 m/sec2)

•Fw=mg

•Fw= weight force (N)

•m=mass (kg)

•g=acceleration due to gravity (-9.8 m/sec2)

• Friction: A force that…– Reduces acceleration– Works against motion– Occurs when surfaces move against on another– Causes wear on parts

• Examples of friction:– Air friction- (air resistance)- aka – “drag”– Sliding friction (rub hands together)– Viscous friction- (oil in car engines & joint fluid)– Rolling friction- (wheel on road, ball bearings)

What is friction?

Practice Problems

What is the speed of a truck that travels 450 miles in 7.5 hours?

What is the acceleration of a car that goes from 55 mph to 35 mph in 0.1 hrs?

What is the mass of an object that is accelerated at a rate of 5.3 m/s2 when a 90 N force is applied to it?

Practice Problems pg 2

How far did you ride your bike if you rode for 20 min at a speed of 3 m/min?

What is the force applied to a box with a mass of 45 kg and it causes it to accelerate at a rate of 4.5 m/s2?

What is the acceleration of an object that has a force of 350 N and a mass of 92 kg?

Practice Problems pg 3

What is the amount of force used to move a 25 kg box if it accelerates at a rate of 3 m/s2?

What is the difference between speed and velocity?

What is the acceleration of a rabbit that speeds up from 10 m/s to 45 m/s in 5 seconds?

Practice Problems pg 4

What is the velocity of a rabbit that travels 50 m in 2.5 seconds?

Which will accelerate more when dropped off the top of a building: a feather or a heavy rock?

Which has more mass: a kilogram of bricks or a kilograms of feathers?

Practice Problems pg 5

If you use cruise control while traveling down I-75, are you accelerating?

What is a common term for Newton’s 1st Law: An object at rest will stay at rest unless a force acts on it. (or An object in motion will stay in motion unless a force acts on it.)

Practice Problems pg 6

Which of Newton’s Laws is what we know as the equation F=ma.

How long does it take a turtle to go to the lake if he travels a distance of 150 m at a velocity of 0.5 m/s?

What is the mass of a rock that is accelerated at a rate of 5 m/s2 if a force of 45 N is used?

What is the acceleration of a car that is has a mass of 9 kg if a force of 63N is used?