Blue Cow- Recall Fnet = ma

• A 1.2 × 103-kilogram automobile in motion strikes a 1.0 × 10-4-kilogram insect.  As a result, the insect is accelerated at a rate of 1.0 × 102 m/s2.  What is the magnitude of the force the insect exerts on the car?

a)1.0 × 10-2 Nb)1.2 × 10-2 Nc)1.0 × 101 Nd)1.2 × 103 N

Objective

• Explore what causes motion ! Apply Newton’s laws of motion to the world around us.

Homework

• Castle Learning – Newton 1

• Possible Test Next Friday.

Force

• Force is a vector quantity, therefore it has both magnitude and direction.

• The SI unit for force is the Newton.– The Newton is a derived unit where:

1N = 1kg•m/s2

– 1 Newton is the amount of force required to accelerate a 1 kg mass 1 m/s2.

The System and Environment• The object of interest is called the

System.

• The area around the object is called the Environment.

Fg

FN

System

Environment

Newton’s 2nd Law and Force vs. Acceleration

• How does acceleration relate to force?– As the force on an object increases, the rate

of acceleration will increase.

Acceleration

m1

m2

m2 > m1

Note: The slope of the line in a F vs. a graph will equal the mass of the object.The greater the slope, the greater the mass

Example: What is the rate of acceleration?

• Two people are pushing a stalled car. The mass of the car is 1850 kg. One person applies a 275 N force while the other applies a 395 N force. A third force of 560 N acts in in the opposite direction compared to the two people. What is the acceleration of the car?

Diagram the problem

Fnet = Fperson 1 + Fperson 2 – Fopposing force

State the Known and Unknowns

• What is known?– Mass (m) = 1850 kg– Force of person #1 (N) = 275 N– Force of person#2 (N) = 395 N– Opposing Force (N) = 560 N

• What is not known?– Acceleration (a) = ?

Perform Calculations

Fnet = Fperson 1 + Fperson 2 - Fopposing Where:Fnet = ma

Substitute for Fnet and solve for a:ma = Fperson 1 + Fperson 2 - Fopposing

a = (Fperson 1 + Fperson 2- Fopposing)/ma = (275 N + 395 N – 560 N)/1850 kga = 0.059 m/s2

If there was no opposing force, how would the rate of acceleration change?

What do you think is the source of the opposing force?

It would increase.

Friction.

Misconceptions about Forces

Is a force required after a ball is released while throwing to make it continue in its path? No: once the contact force between the hand

and the ball are broken, there is no longer a force propelling the ball forward.

Misconceptions about Forces

• Is inertia a force?– No: Inertia is the tendency for an object to

resist a change in velocity.– Inertia is a property of matter.– The more mass an object has, the more

inertia it has.– Forces that exist in the environment act on

objects.

What’s the difference between mass and weight?

• Mass is a property of an object that quantifies (provides a numerical value) for the amount of matter (protons and neutrons) that it contains.

• Weight is a measure of the force of attraction on a body that is directly related to the amount of mass and gravity.

• Mass is the same everywhere, while weight will change with distance from other bodies.

NOTE: MASS AND INERTIA ARE THE SAME!

Determining Weight

• If the mass of an object is known, its weight can be determined using Newton’s 2nd Law of Motion. Fg = mg

Where:• m = mass• g = acceleration due to gravity (g = 9.81 m/s2).

Example: Mass vs. Weight

• On Earth:– Mass = 1 kg – Weight = 9.8 N

• On the Moon:– Mass = 1 kg– Weight = 1.62 N (gm = 1.62 m/s2)

• Why is the weight on the moon so much less?– Because the gravitational force of attraction on the

moon is 1/6th that on Earth.