Chapter 4Forces

Forces and Interaction

• Force – a “push or pull”

• Contact Force – you physically push on a wall

• Long-range Force – like magnets or gravity

• Force – a vector quantity. Has what two parts.

• SI Unit – the Newton abbreviated as “N”

4 Types Forces• Gravitational forces

• The Earth pulls and holds the moon in orbit• The moon pulls and causes tide changes

• Electromagnetic forces• due to electric charges, both static and moving.

• Strong Nuclear Forces • holds particles in the nucleus together. (Strongest

of the 4)• Weak Nuclear Forces

• Radioactive decay

• An object at rest will stay at rest unless a force acts on it.• Here, the girl is at rest until acted on by the force imposed

by the cannonball.

Newton’s 1st Law

Newton’s 1st Law• An object at rest will stay at rest unless a force

acts on it.• Here, the skateboard/log combination is at rest

until acted on by the force imposed by the rocket.

Newton’s 1st Law• An object in motion will continue in a straight line until

acted on by some “outside” force.

• Consider a baseball that is pitched to a batter. What force causes the ball to change direction?

Hit

Newton’s 1st Law

• An object in motion will continue in a straight line until acted on by some “outside” force.

• Consider the space shuttle turning while in space flight. What produces the forces?

Newton’s 1st law• An object in motion will continue in a straight line

until acted on by some “outside” force.

• Consider the space shuttle when landing. What produces the forces?

Multimedia

• The car and the wall

• The motorcyclist

• The truck and the ladder

Newton’s 1st Law

Newton’s 2nd Law

• Acceleration is directly proportional to the magnitude of the net force.

• Acceleration is inversely proportional to the mass of the object.

Newton’s 2nd Law

netFam

F ma

• Acceleration depends on both mass and the net force

Acceleration depends on net force

• A force of 10N accelerates the box

Acceleration depends on net force

• A force of 20N accelerates the box twice as fast

Acceleration depends on mass

• A force of 10N accelerates the box

Acceleration depends on mass

• A force of 10N accelerates the smaller box faster

Weight and Mass

Mass:

A measure of the amount of matter in an object has.Symbol (m) Unit (kg)

Weight:

The gravitational force exerted on a body

Symbol (W) Unit (N Newton)

W mg

Weight

A Motor cycle weights 2450N. What is its mass?

W mg /m W g

2

2450

9.8 ms

Nm

250m kg

Newton’s 2nd Law

Newton’s Second Law – Non-Equilibrium Situation

• Two 50g masses are placed 40cm on either side of a fulcrum. What is the Net force?

• If left mass slides 20cm right, what happens?

• If the left mass slides 20cm right and grows to 100g, what type of situation is this?

Newton’s 3rd Law

Newton’s 3rd Law

• When one object exerts a force on a second object, the second exerts a force on the first that is equal in magnitude but opposite in direction.

• For every action there is an equal and opposite re-action.

• Action – Reaction force pair

Newton’s 3rd LawExample

• A book rests on a table• The force from the weight

of the book pushes down.• The table provides a

supportive force up.

• Normal force (N) is perpendicular to the surface

W mg

N mg

Free-body diagrams

Draw the free-body diagram for a book is at rest on a table top.

A girl is suspended motionless from a bar which hangs from the ceiling by two ropes. A free-body diagram for this situation looks like this:

gravityF

tensionFtensionF

Free-body diagrams

An egg is free-falling from a nest in a tree. Neglect air resistance. A free-body diagram for this situation looks like this:

gravityF

Free-body diagrams WS 5b #3

A rightward force is applied to a book in order to move it across a desk at constant velocity. Consider frictional forces. Neglect air resistance. A free-body diagram for this situation looks like this:

gravityF

normalF

appFfrictionF

An egg is free-falling from a nest in a tree. Neglect air resistance. A free-body diagram for this situation looks like this:

A college student rests a backpack upon his shoulder. The pack is suspended motionless by one strap from one shoulder. A free-body diagram for this situation looks like this:

A skydiver is descending with a constant velocity. Consider air resistance. A free-body diagram for this situation looks like this:

A force is applied to the right to drag a sled across loosely-packed snow with a rightward acceleration. A free-boy diagram for this situation looks like this

A football is moving upwards towards its peak after having been booted by the punter. A free-body diagram for this situation looks like this:

A car is coasting to the right and slowing down. A free-body diagram for this situation looks like this:

Net Force

• If there is no movement–The net force is zero

• If there is no acceleration–The net force is zero

• If there is acceleration–The net force is not zero.

Free Body diagrams

• If the net forces are zero, the FBD is balanced

Examples of zero net force?

• A book on a desk

• A sky diver with a parachute open

• A bike moving at a constant speed.

Free Body diagrams

• If the net forces are NOT zero, the FBD is NOT balanced

Examples of non zero net force

• A book sliding across a desk

• A sky diver falling without a parachute

Net force: The vector sum of all forces

5N 10N

15N

5N

5N 5N 0N

5N 10N 5N 10N 5N

10N 5N 5N

Equilibrium or Non-equilibrium • When the helicopter is on

the ground, what state is it in?

• Why isn’t it moving?

• In order to get it moving, what would we need?

• What state is it in when it is hovering in place?

• In order to get it to move, what would we need?

• What forces are acting on the train while it is moving in a straight line at a constant speed?

• Engine Force (FE) – Force applied to propel the train along the tracks.

• Opposition Force (Fo) – friction between the tracks, wind resistance, etc. that attempts to slow the train down.

• What is the acceleration of the train? Negative, Zero, or Positive.

• Is this an equilibrium or non-equilibrium situation?

Net Force – Equilibrium in Motion

EF

OF

Net Force – Non-Equilibrium• Describe what happens in the animation below.

• What forces were acting on the train?

• Engine Force (FE) – Force applied to propel the train along the tracks.

• Opposition Force (Fo) – friction between the tracks, wind resistance, etc. that attempts to slow the train down.

• Which force was larger?

• What is the acceleration of the train? Negative, Zero, or Positive.

• Is this an equilibrium or non-equilibrium situation?

EF

OF

Net Force – Non-Equilibrium• Describe what happens in the animation below.

• What forces were acting on the train?

• Opposition Force (Fo) – friction between the tracks, wind resistance, etc. that attempts to slow the train down.

• What is the acceleration of the train? Negative, Zero, or Positive.

• Is this an equilibrium or non-equilibrium situation?

OF

Net Force

• If there is no movement–The net force is zero

• If there is no acceleration–The net force is zero

• If there is acceleration–The net force is not zero.

Newton’s Second Law – Non-Equilibrium Situation

• The result of a Non-Equilibrium situation is that the body, will begin to move.

amFFFF

...321

1F

1F

2F 1F F ma

1 2F F F ma

Newton’s laws review• Newton’s First Law - An object remains at

rest, or in uniform motion in a straight line, unless it is compelled to change by an externally imposed force.

• Newton’s first law describes an Equilibrium Situation.

• An Equilibrium Situation is one in which the acceleration of a body is equal to zero.

Newton’s laws review

• Newton’s Second Law – If there is a non-zero net force on a body, then it will accelerate.

• Newton’s Second Law describes a Non-equilibrium Situation.

• A Non-equilibrium Situation is one in which the acceleration of a body is not equal to zero.

Newton’s laws review

• Newton’s Third Law - for every action force there is an equal, but opposite, reaction force.

• Newton’s Third Law says forces must come in pairs.

• Paired force internal to a system have a Net Force of zero.

Weight and Mass

Mass:

A measure of the amount of matter in an object has.Symbol (m) Unit (kg)

Weight:

The gravitational force exerted on a body

Symbol (W) Unit (N Newton)

W mg

Weight

A Motor cycle weights 2450N. What is its mass?

W mg /m W g

2

2450

9.8 ms

Nm

250m kg

Newton’s Second Law – Non-Equilibrium Situation

• Two 50g masses are placed 40cm on either side of a fulcrum. What is the Net force?

• If left mass slides 20cm right, what happens?

• If the left mass slides 20cm right and grows to 100g, what type of situation is this?

amFFFF

...321

Friction

• Friction is a force like any other force

• Friction acts on materials that are in contact with each other

• Friction slows down motion

• Forces due to friction are always in the opposite direction of the motion.

Friction

• The force of friction depends on

1) The force pushing down (weight)

2) The surface materials in contact with each other

Friction

fF N

fF force of friction

coeffiecient of friction

N Normal force

• A horizontal force of 30N pushes a 12kg crate across a floor at a constant velocity. Find the coefficient of sliding friction.

Friction Example

W

N

aFfF

app fFnet F F

fF N fF

N

2

30

12 (9.8 )ms

N

kg

.25 app fF F

W

N

aFfF

Friction e.g.• A 4kg block has a

coefficient of friction of .22 .

• What is the force of friction for the 4kg block?

• What is the Fnet? • What is the acceleration?

Fapp=20N4kg

f NF F

2.22(4 )(9.8 )mf sF kg

8.6fF N

net app fF F F

20 8.6 11.3netF N N N netF ma

22.8 ms

a

With out friction

Galileo On Motion

With friction

With out friction

With out friction