Chapter 2 Forces

Acceleration; Friction; Gravity; Momentum; Newton’s 1st, 2nd,

and 3rd Laws

Section 2-1 The Nature of Force

How is a Force Described? A force is a push or pull Forces can change the

motion of objects The strength of forces is

measured in Newtons (N)

Direction and strength of a force can be represented by an arrow. The longer the arrow the

stronger the force The direction of the arrow tells

the direction of the force.

Net force: The combination of all forces

acting on an object. Determines whether an object

will move and what direction.

How are unbalanced and balanced forces related to an object’s motion?

Unbalanced forces result in a net forcecausing a change in the object’s motion.

Ex: tug of war----greater pull from one team results in a net force causing the rope to move toward them.

Balanced forces have equal forces put on them from both directions. No movement occurs.Ex: tug of war----two teams pull with equal

force and the rope does not move.

- The Nature of Force

- The Nature of ForceUnbalanced Forces

Unbalanced forces acting on an object result in a net force and cause a change in the object’s motion.

- The Nature of ForceBalanced Forces Balanced forces acting on an object

do not change the object’s motion.

Section 2-2 Friction and Gravity

What factors determine the strength of the friction force between two surfaces?

Friction: the force that two surfaces exert on each other when they rub against each other. Strength of friction depends on:

1. how hard the surfaces push together2. the type of surfaces involved

Ex: Waxing skis or snowboards causes less friction between boards and snow.

Ex: tires on a dirt track have large amounts of friction helping them to stop

Types of friction:

1. static friction: acts on objects not moving

EX: a desk on the carpet2. sliding friction: when two objects slide over each other

EX: skates on ice 3. rolling friction: when an object rolls over a surface

EX: tires on pavement 4. fluid friction: when a solid moves through a liquid. EX: Pistons and engine oil

What factors affect the gravitational force between two objects?

Gravity pulls all objects towards one another in varying amounts depending upon the size of the objects.

Law of Universal Gravitation states that the force of gravity pulls all objects in the universe towards each other.

Gravity is affected by: Mass- greater the mass the greater the

gravity Distance-greater the distance the less the

gravity

- Friction and GravityGravity Two factors affect the gravitational

attraction between objects: mass and distance.

Why do objects accelerate during free fall?

Unbalanced forces between gravity and the object cause it to free fall.

On the surface of the earth, acceleration due to gravity is a constant 9.8m/s2

Air resistance can change the acceleration rate due to fluid friction between air and object.

Projectile motion does not change the effect of gravity on the object.

Terminal velocity: Greatest velocity a falling object

reaches Occurs when the force of air resistance

= weight of the object

Section 3: Newton’s First and Second Laws of Motion

What is Newton’s first law of motion?1. An object at rest will remain at rest, and an object in motion will remain in motion unless acted upon by an unbalanced force.Ex: When a car stops quickly, you continue to move forward

2. Gravity and friction cause most unbalanced forces.Ex: When a ball is thrown it will eventually stop and fall to the ground due to gravity then fluid friction of air and rolling friction of ground stop it from rolling

3. Inertia:a. the tendency of an object to resist a change in motion.b. the greater the mass of an object the greater its inertia.

Ex: Scrambler at the fair. The larger person will squish the smaller person due to more inertia.

What is Newton’s second law of motion?1. Acceleration depends on the object’s mass and on the net force acting on the object.

Acceleration = net force / massOR

Net Force = Mass x Acceleration

2. To change acceleration you must change the force or the mass.a. Increase acceleration by increasing force or decreasing mass.b. Decrease acceleration by decreasing force or increasing mass

Calculating Force A speedboat pulls a 55-kg water-skier. The force

causes the skier to accelerate at 2.0 m/s2. Calculate the net force that causes this acceleration.

Read and Understand

What information have you been given? Mass of the water-skier (m) = 55 kg Acceleration of the water-skier (a) = 2.0

m/s2

Calculating ForceA speedboat pulls a 55-kg water-skier. The force causes the skier to accelerate at 2.0 m/s2. Calculate the net force that causes this acceleration.Plan and SolveWhat quantity are you trying to calculate?

The net force (Fnet) = __

What formula contains the given quantities and the unknown quantity?

a = Fnet/m or Fnet = m X a

Perform the calculation.Fnet = m X a = 55 kg X 2.0 m/s2

F = 110 kg • m/s2

F = 110 N

Calculating Force A speedboat pulls a 55-kg water-skier. The

force causes the skier to accelerate at 2.0 m/s2. Calculate the net force that causes this acceleration.

Look Back and Check

Does your answer make sense? A net force of 110 N is required to accelerate

the water-skier. This may not seem like enough force, but it does not include the force of the speedboat's pull that overcomes friction.

Calculating Force Practice Problem

What is the net force on a 1,000-kg object accelerating at 3 m/s2?

Force = mass x acceleration 3,000 N = (1,000 kg X 3 m/s2)

Calculating Force Practice Problem

What net force is needed to accelerate a 25 kg cart at 14 m/s2?

350 N (25 kg X 14 m/s2)

Section 4: Newton’s Third Law

What is Newton’s Third Law of Motion?“for every action there is an equal but opposite reaction.”

Action-Reaction Pairs:Ex: When you try to get on an inner tube in water. You push on the tube and the tube pushes back and you both go opposite directions. You fall off the tube!!!

How can you determine the momentum of an object?

1. Relates to the mass and velocity of the object.Momentum = mass x velocitymeasured in kg-m/s

2. The higher the mass or velocity the more momentum an object has.

What is the law of conservation of momentum?

1.The total amount of momentum of objects that interact does not change, unless outside forces act on the objects.

2.Conserved means the momentum before and after they interact is the same.

3. ”Outside force” could be gravity or friction

4. Momentum can be transferred to another object.

Calculating Momentum Which has more momentum: a 3.0-kg sledgehammer

swung at 1.5 m/s or a 4.0-kg sledgehammer swung at 0.9 m/s?

Read and Understand

What information have you been given? Mass of smaller sledgehammer = 3.0 kg Velocity of smaller sledgehammer = 1.5 m/s Mass of larger sledgehammer = 4.0 kg Velocity of larger sledgehammer = 0.9 m/s

Calculating MomentumWhich has more momentum: a 3.0-kg sledgehammer swung at 1.5 m/s or a 4.0-kg sledgehammer swung at 0.9 m/s?

Plan and SolveWhat quantities are you trying to calculate? The momentum of each sledgehammer = __

What formula contains the given quantities and the unknown quantity? Momentum = Mass X Velocity

Perform the calculation. Smaller sledgehammer = 3.0 kg X 1.5 m/s = 4.5 kg•m/s larger sledgehammer = 4.0 kg X 0.9 m/s = 3.6 kg•m/s

Calculating Momentum Which has more momentum: a 3.0-kg sledgehammer

swung at 1.5 m/s or a 4.0-kg sledgehammer swung at 0.9 m/s?

Look Back and Check

Does your answer make sense? The 3.0-kg hammer has more momentum than the 4.0-

kg one. This answer makes sense because the 3.0-kg hammer is swung at a greater velocity.

Calculating Momentum Practice Problem

A golf ball travels at 16 m/s, while a baseball moves at 7 m/s. The mass of the golf ball is 0.045 kg and the mass of the baseball is 0.14 kg. Which has the greater momentum?

Golf ball: 0.045 kg X 16 m/s = 0.72 kg•m/s

Baseball: 0.14 kg X 7 m/s = 0.98 kg•m/s

The baseball has greater momentum.

Calculating Momentum Practice Problem

What is the momentum of a bird with a mass of 0.018 kg flying at 15 m/s?

0.018 kg X 15 m/s = 0.27 kg•m/s