Chapter 3

• Definition of FORCE:– A push or pull on an object

• What forces cause:– An object to start moving– An object to stop moving– Speed up or slow down an object

• Measurement of force:– Newton (N) which is equal to kgm/s2

– Symbol of Force:

• EXAMPLES OF FORCES:• Anything that pushes or pulls

– Gravity– Friction– Air Resistance– Air Pressure

Unbalanced Forces Balanced Forces

Forces acting on an object that are not equal and opposite

Opposite and equal forces acting on an object

Net forces cause an object to accelerate (speed up, slow down, or change direction)

If the forces are balanced, the object will not accelerate (it will stay still OR move at a constant velocity)

Which one (unbalanced forces or balanced forces) causes a change in velocity?

Using the pink bouncy ball, perform the following tasks and answer the questions about each task.

The "Tasks"Describe what you

seeNet force or

balanced forces?

One person pushes the ball  

Two people push the ball on opposite sides at the

same time   

One person pulls the ball towards them

Two people pull the ball towards them at the

same time   

Unbalanced!

• If your velocity changes, are the forces acting on you balanced or unbalanced?

• unbalanced• If you are moving at a constant speed, are the

forces acting on you balanced or unbalanced? • balanced

• If you are at rest (not moving), are the forces acting on you balanced or unbalanced?

– Sit still in your chair. • Are the forces balanced or is there a net force?

• Imagine that different forces are acting on a hockey puck. Draw arrows to show the forces acting on each hockey puck, and then answer the questions.

• Each N = 1 cm.

ForcesCombine

d or opposed forces?

Balanced or

Unbalanced Forces?

What is the Net force on

the puck?

Will the puck

move? If so, in what direction?5 N force towards the puck from the left & opposed balanced 0 N won’t move

5 N force towards the puck from the right

7.5 N force towards the puck on the left & combined unbalanced 15 N to the right7.5 N force away from the puck on the right

4 N force towards the puck on the left & opposed unbalanced 1 N to the left5 N force towards the puck on the right

ForcesCombine

d or opposed forces?

Balanced or

Unbalanced Forces?

What is the Net force on

the puck?

Will the puck

move? If so, in what direction?

2 N force away from the puck on the left & opposed unbalanced 2.3 N to the left 4.3 N force towards the puck on the right

5 N force away from the puck on the right & opposed unbalanced 1 N to the right4 N force towards the puck on the right

3 N force pointing up at the puck & opposed balanced 0 N no movement3 N force pointing down on the puck

Newton’s Second Law:Force, Mass, and Acceleration

Acceleration and Force• Anything that accelerates is acted on by a force.

– Remember: A force is a push or pull on an object.• Acceleration is caused by applying force.• Examples:

– Hitting a bag– Steady pull of gravity

Force Causes Acceleration• When there is a net force on an object, it produces

acceleration.• Acceleration also depends on something else.

– Can you think of what that is?

The Effect of Mass• What happens when you kick a tin can?

– It accelerates (changes motion)• Now what happens when you kick the same can

full of rocks?– Ouch!– And it doesn’t accelerate as much

Inertia• The full can has more inertia than the empty can

because it has more mass.• The greater an object’s mass, the greater its

inertia.• This also means that the greater the mass, the

greater the force needed for acceleration.

What is Mass?• Mass is not volume or weight.• Mass is the amount of matter an object has• Volume is a measure space• Weight is the force due to gravity that acts on an

object’s mass– Your weight is different on the moon that it is here, but

your mass is the same.

Mass and Acceleration• Recall that the more massive an object is, the

more inertia it has.• Therefore, more massive objects are harder to

accelerate.• Acceleration is inversely proportional to mass.

– This means that when one gets bigger, the other one gets smaller.

Concept Check• Suppose you apply the same amount of force to

two carts, one cart with a mass of 4 kg, and the other with a mass of 8 kg.

• Which car will accelerate more?– The 4 kg car

• How much greater will the acceleration be?– Acceleration will be twice as much

Newton’s Second Law of Motion

• Isaac Newton was the first to realize the connection between force and mass in producing acceleration.

• His 2nd Law of Motion is one of the most important rules of nature ever proposed.

Newton’s Second Law of Motion

• It links force, mass, and acceleration.• Stated:The acceleration produced by a net force on an

object is directly proportional to the net force, is in the same direction as the net force, and is inversely proportional to the mass of the object.

Equation• Acceleration = net force

mass

a = _F_ m

• This means acceleration equals the net force divided by the mass.

Triangle and Units• Let’s make a triangle for this formula:

• Units– Acceleration = m/s2

– Mass = kg– Force = kg ● m/s2 = N

F

m a

Calculation!!• Consider a 1000-kg car pulled by a cable with

2000 N of force. What will be the acceleration of the car?

• a = _F_ m

• a = _2000 N_ 1000 kg

• a = 2 m/s2

Friction!!• What is friction?• Well, it occurs when two things rub against each

other.• IMPORTANT – Friction always works against

motion!!

MotionFriction

What affects Friction?• Type of surface

– Where is there more friction?• Between a crate and rough wooded floor• Between a crate and a polished linoleum floor

• How much they are pressed together– Where is there more friction?

• Between a crate and an even surface• Between a crate and downward ramp