Forces and motion
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Forces and Motion. Force. FORCE is a push or a pull applied to an object that will cause it to start moving, stop moving or change its speed or direction Demonstration. Force. Force = Mass x Acceleration F = MA Force is measured in Newtons (N) which is one kilogram meter per second squared

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  • FORCE is a push or a pull applied to an object that will cause it to start moving, stop moving or change its speed or direction

  • Demonstration


  • Force = Mass x Acceleration

  • F = MA

  • Force is measured in Newtons (N) which is one kilogram meter per second squared

  • N = kg x m/s2

Newton s first law law of inertia
Newton’s First Law (law of inertia)

  • MASS is the measure of the amount of matter in an object

  • measured in grams (g)

    or kilograms (kg)

Newton s first law law of inertia1
Newton’s First Law (law of inertia)

  • WEIGHT is a measure of the force of gravity on the mass of an object

  • measured in Newtons (N)


  • But weight, what’s my mass?

  • Please do not confuse the two.

  • Weight is determined by the acceleration due to gravity.

  • If you were on another planet that had less gravity, you would weigh less.

Net force








  • In order for motion to occur, the net force must be >0

10 N


20 N

10 N


10 N

10 N

0 N


20 N

10 N

10 N

The equilibrium rule

Scales pushing up

Examples of



  • Computer setting on a table

  • Weighing yourself on a set of scales

  • Hanging from a tree

  • Car parked on an incline

Normal up

Weight down


pulling up

Weight down



Weight down

Weight down

Support force

Scales pushing up

Normal up

Weight down

Weight down


  • In the first example of mechanical equilibrium the table supplied a force upward that was called the normal force. It is a support force.

  • Consider the second example of mechanical equilibrium. The scales supply a support force on the man.

Equilibrium of moving things

  • Equilibrium is a state of no change.

  • If an object moves in a straight line with no change in speed or direction, it is in equilibrium.


Driving at constant velocity

Normal up

Air resistance



Force from road

Weight down

Terminal velocity in parachuting

Weight down

What do you weigh
What do you weigh?

  • Weight on Other Planets

Force problems
Force Problems

  • Let’s start with an easy one, your weight.

  • Remember that weight is relative – your mass isn’t changing (the amount of matter in you) but you weigh different amounts because of gravity

  • Gravity’s acceleration is 9.8m/s2

  • On earth you take your weight to be what it is

Force problems1
Force Problems

  • If you lived on another planet, such as mars for example, the acceleration due to gravity is 3.8m/s2

  • In order to find out weight, we use the following formula

  • w = m x g

Force problems2
Force Problems

  • Since gravity is a force (pulling you towards the center of the planet) this is technically a force problem

  • My weight on earth is 185lbs, or 84kg (just divide your weight by 2.2)

  • That means that if we stick my weight in and we know the acceleration due to gravity here on earth, we can find out my mass

Force problems3
Force Problems

  • w = m x g

  • 84kg x m/s2 = m x 9.8m/s2

  • M = 8.57kg

  • Kg x m/s2.. That’s also called a Newton!

  • So my mass is 8.57kg.

Force problems4
Force Problems

  • But what if we were on another planet?

  • Well, we use w = m x g

  • W = 8.57kg x 3.8m/s2

  • W = 32.57N

  • As a reminder, weight is m x g, so it equals a kg m/s2, or a N.

  • Mass is measured in kg.

Force problems5
Force Problems

  • Ok, now you try one

  • What would be your weight on Jupiter, where gravity is 22.88m/s2?

  • What would be your weight on the sun, 274.4m/s2? That’s assuming you could stand on it

Force problems6
Force Problems

  • Ok, let’s move on to earthly stuff.

  • We remember that f = ma

  • What would be the force exerted by a truck with a mass of 1818kg accelerating at 15m/s2?

  • f = ma

  • F = 1818kg x 15m/s2

  • F = 750N

Force problems7
Force Problems

  • If you accelerate a rocket with a mass of 300kg at Taber’s face at 500m/s2 with what amount of force will it hit him?

  • F = ma

  • F = 300kg x 500m/s2

  • F = 150,000

More practice
More Practice

  • Troy Polamalu, with a mass of 115kg, hits Adrian Peterson with a force of 2300N. With what acceleration does Troy hit Adrian? What force does Adrian exert on Troy?

  • F = ma

  • 2300N = 115kg x a

  • A = 2300N / 115kg

  • A = 20m/s

One more
One More

  • A 20g sparrow mistakes a pane of glass for air and slams into a window with a force of 2N. What is the bird’s acceleration?

  • F = ma

  • 2N = .02kg x a

  • A = 100m/s2 or 10g’s!!

Oh yeah one more
Oh yeah, one more.

  • Suppose your car is parked on an incline of 10 degrees. If the parking brake lets go and your car starts rolling, with what force are you going down the hill? What is your force on the ground? Assume the car weighs 1500kg.


  • FRICTION is the force that acts in the opposite direction of the motion of the object

Types of friction
Types of Friction

  • Static Friction – Friction due to gravity when an object is at rest.

    • Demonstration

  • Sliding Friction – Friction while an object is at motion.

    • Example

  • Rolling Friction – Similar to sliding friction, but the object is on wheels or castors to reduce the sliding friction.

  • Fluid Friction – Friction through water or air

    • Terminal Velocity

Sliding friction
Sliding Friction

Ffriction = µFnormal

µ = the coefficient of sliding friction (has no units)

product of the friction b/w materials and amount of force

1. Ben is walking through the school cafeteria but does not realize that the person in front of him has just spilled his glass of chocolate milk. As Ben, who weighs 420 N, steps in the milk, the coefficient of sliding friction between Ben and the floor is suddenly reduced to 0.040. What is the sliding force of friction between Ben and the slippery floor?


  • While redecorating her apartment, Kelly slowly pushes an 82 kg china cabinet across the wooden dining room floor, which resists motion with a force of 320 N. What is the coefficient of sliding friction between the china cabinet and the floor?

    3. A rightward force is applied to a 10-kg object to move it across a rough surface at constant velocity. The coefficient of friction, µ, between the object and the surface is 0.2. Use the diagram to determine the gravitational force, normal force, applied force, frictional force, and net force.

    (Neglect air resistance.)

Projectile motion
Projectile Motion

  • What is a projectile? – Throw ball

  • Projectiles near the surface of Earth follow a curved path

  • This path is relatively simple when viewed from its horizontal and vertical component separately

  • The vertical component is like the free fall motion we already covered

  • The horizontal component is completely independent of the vertical component (roll ball)

  • These two independent variables combined make a curved path!

Projectile motion2
Projectile Motion

No Gravity

With Gravity

Horizonta l ly launched project i le i n i tial speed vx 25 m s

Time Horizontal Displacement (x)

0s 0m

1s 25m

2s 50m

3s 75m

4s 100m

5s 125m

Ts vxt

Vertical Displacement (y)







½ gt2

Horizontally Launched Projectile(initial speed (vx) = 25 m//s)

Horizontally launched projectiles
Horizontally Launched Projectiles

  • What will hit the ground first, a projectile launched horizontally, a projectile dropped straight down, or a project fired up?

Projectile motion4
Projectile Motion

  • Remember that nothing is accelerating the projectile after it leaves

  • The only thing accelerating the projectile after launch is gravity

  • The two vectors can be separated into the velocity at launch and the acceleration of gravity

Truck and ball
Truck and Ball

  • Imagine a pickup truck moving with a constant speed along a city street. In the course of its motion, a ball is projected straight upwards by a launcher located in the bed of the truck. Imagine as well that the ball does not encounter a significant amount of air resistance. What will be the path of the ball and where will it be located with respect to the pickup truck?

Fast moving projectiles satellites
Fast-Moving Projectiles—Satellites

  • What if a ball were thrown so fast that the curvature of Earth came into play?

  • If the ball was thrown fast enough to exactly match the curvature of Earth, it would go into orbit

  • Satellite – a projectile moving fast enough to fall around Earth rather than into it (v = 8 km/s, or 18,000 mi/h)

  • Due to air resistance, we launch our satellites into higher orbits so they will not burn up

Aristotle on motion

  • Aristotle attempted to understand motion by classification

  • Two Classes:

    • Natural and Violent

Natural motion
Natural Motion

  • Natural motion depended on nature of the object.

  • Examples:

  • A rocks falls because it is heavy, a cloud floats because it’s light

  • The falling speed of an object was supposed to be proportional to its weight.

Natural motion1
Natural Motion

  • Natural motion could be circular (perfect objects in perfect motion with no end).

Violent motion
Violent Motion

  • Pushing or pulling forces imposed motion.

  • Some motions were difficult to understand.

  • Example: the flight of an arrow

  • There was a normal state of rest except for celestial bodies.


  • Aristotle was unquestioned for 2000 years.

  • Most thought that the Earth was the center of everything for it was in its normal state.

  • No one could imagine a force that could move it.

Galileo and the leaning tower

  • 17th Century scientist who supported Copernicus.

  • He refuted many of Aristotle's ideas.

  • Worked on falling object problem - used experiment.

Galileo s inclined planes

  • Knocked down Aristotle's push or pull ideas.

  • Rest was not a natural state.

  • The concept of inertia was introduced.

  • Galileo is sometimes referred to as the

  • “Father of Experimentation.”

Newton s first law of motion

  • Newton finished the overthrow of Aristotelian ideas.

  • Law 1 (Law of Inertia)

  • An object at rest will stay at rest and an object in motion will stay in motion unless acted upon by an outside force.

Newton s first law law of inertia2
Newton’s First Law (law of inertia)

  • INERTIA is a property of an object that describes how hard it is to change the motion of the object

  • More mass = more inertia

  • F=MA

Newton s second law force causes acceleration
Newton’s Second LawForce Causes Acceleration

  • In order to make an object at rest move, it must accelerate

    • Suppose you hit a hockey puck

      • as it is struck it experiences acceleration, but as it travels off at constant velocity (assuming no friction) the puck is not accelerating

      • if the puck is struck again, then it accelerates again; the force the puck is hit with causes the acceleration

  • Acceleration depends on net force

    • to increase acceleration—increase net force

    • double acceleration—double the force

  • Force ~ Acceleration

    • directly proportional

  • Newton s second law
    Newton’s Second Law

    • Acceleration depends on mass

      • to decrease acceleration—increase mass

      • to increase acceleration—decrease mass

      • double the mass = ½ the acceleration

    • Acceleration ~ 1/mass

      • inversely proportional

    Newton s second law1
    Newton’s Second Law

    • Newton was the first to realize that acceleration produced when something is moved is determined by two things

      • how hard or fast the object is pushed

      • the mass of the object

  • Newton’s 2nd Law

    • The acceleration of an object is directly proportional to the net force acting on the object and is inversely proportional to the object’s mass

  • Newton s second law2
    Newton’s Second Law

    • Second Law Video

    Newton s second law3
    Newton’s Second Law

    • a = F/m or F = ma

      • Robert and Laura are studying across from each other at a wide table. Laura slides a 2.2 kg book toward Robert. If the net force acting on the book is 1.6 N to the right, what is the book’s acceleration?

      • A = F/m

        = 1.6N / 2.2kg

        = .73m/s2

    Newton s second law4
    Newton’s Second Law

    2. An applied force of 50 N is used to accelerate an object to the right across a frictional surface. The object encounters 10 N of friction. Use the diagram to determine the normal force, the net force, the mass, and the acceleration of the object. (Neglect air resistance.)

    3. Rose is sledding down an ice-covered hill inclined at an angle of 15.0° with the horizontal. If Rose and the sled have a combined mass of 54.0 kg, what is the force pulling them down the hill?

    Newton s 2 nd law kinematics
    Newton’s 2nd Law & Kinematics

    • A 4.60 kg sled is pulled across a smooth ice surface. The force acting on the sled is of magnitude 6.20 N and points in a direction 35.0° above the horizontal. If the sled starts at rest, what is its velocity after being pulled for 1.15 s?

      • V = 1.265 m/s

    • The fire alarm goes off, and a 97 kg fireman slides 3.0 m down a pole to the ground floor. Suppose the fireman starts from rest, slides with a constant acceleration, and reaches the ground floor in 1.2 s. What was the force exerted by the pole on the fireman?

      • F = 201.76N

    Sample problems
    Sample Problems

    • Which exerts a greater force on a table: a 1.7kg physics book lying flat or a 1.7kg physics book standing on end? Which applies a greater pressure? If each book measures .26m x .210m x .04m, calculate the pressure for both.

    • Same

    • Standing

    • Flat = 311N/m2; Standing = 2.0x103 N/m2

    Sample problems1
    Sample Problems

    • A 1250kg slippery hippo slides down a mud-covered hill inclined at an angle of 18 degrees to the horizontal. If the coefficient of sliding friction between the hippo and the mud is .09 what force of friction impedes the hippo? If the hill were steeper, how would this affect the coefficient of friction?

    • 1068.75N

    Sample problems2
    Sample Problems

    • Mr. Micek loves to ride his motorcycle. Mr. Micek and his motorcycle have a combined mass of 518kg. With what force must each tire push down on the ground to hold the bike up? If the contact pattern of the tire is 15cm x 35cm (for each wheel), what pressure do they exert? If Mr. Micek accelerates at 8.8m/s2 what force will he exert? If he parks his bike on a hill at an angle of 25 degrees what must the force due to friction be to keep it there?

    • 2590N; 49.3kPa; 4558.4N; 2175.6N