Newton’s first and second laws

Galileo said that it doesn’t require a force to keep an object moving… In fact, there must be a force to cause

an object to stop moving!

Newton’s First Law: Law of Inertia— An object at rest, stays at rest— An object in motion, stays in motion— Unless acted on by a force

If an object is at rest:

There are no forces acting on it.

There are forces acting on it.

We need more information to know.

Determine the Net Force

A100 N

100 N 200 N

100 N

0 N

- 200 N

This object must be:

A100 N

100 N Moving

At rest

Slowing down

We need more information

Determine the Net Force

B

20 N

20 N30 N

10 N

70 N

0 N 40 N

Determine the Net Force

D

2 N 2 N

7 N 7 N

??? NFnet = 0 N

2 N

7 N

0 N

4 N

Acceleration

• Acceleration is how speed changes over time – the rate of change of speed

• A car speeds up by 10 m/sec every second

• The car accelerates at 10 meters per second per second– 10 m/sec/sec– 10 m/sec2

1 sec10 m/sec

2 sec20 m/sec

3 sec30 m/sec

Example

While traveling along a highway a driver speeds up from 15 m/sec to 24 m/sec in 12 seconds. What is the car’s acceleration?

What is the acceleration if the same car then comes to a stop in eight seconds?

Why does the speed change?

Remember that Galileo said that changing motion requires a force. Newton went further: He said that the acceleration of an object is proportional to the force.

Galileo

Correctly written, the law should say NET force.

• A net force causes acceleration• No acceleration = no net force

• Acceleration = net force/mass

the SI unit of force: the Newton (N)

• If a = F/m, then F = ma• The units of force (a Newton) are the units of

mass * acceleration• Therefore, the units of a Newton are

What is the rate of acceleration of a 2,000-kilogram truck if a net force of 4,200 N is used to make it start moving forward?

If the engine provides 5,000 N of forward force, how much friction is pushing the truck back?

How much net force is needed to accelerate an 500 gram baseball at a rate of 10 m/sec2?

What is the mass of an object that requires a net force of 30 N to accelerate at a rate of 5 m/sec2?

1. Dani and Gina are pushing on a 50 kg box. Dani pushes with 250 N of force and Gina pushes with 100 N of force.

a. What is the net force if they both push in the same direction?

b. What is the acceleration of the box? (Assume no other forces)

b. What is the acceleration of the box if they push in opposite directions?

Mass, Weight, Gravity, and falling• Weight is the pull of the Earth on an object. We call that the force of

gravity (heaviness)• The force of gravity causes unsupported objects to accelerate downward.• When air resistance is ignored, all objects accelerate at the same rate: 9.8

m/s2. This constant is abbreviated: g = 9.8 m/s2

• If you drop a ball off a high bridge, how much does it speed up in one second?

• One kilogram on Earth weighs 9.8 Newtons. Two kilograms weighs 19.6 N.• What is the acceleration caused by this force (when unsupported)?• This is why objects of different mass have the same acceleration when they fall!

Weight and mass

1 kg 2 kg

9.8 N19.6 N

Air resistance and terminal speed

Here’s a video about skydiving

HomeworkSkydiving worksheet and:

Page 55-56 “Solving Problems” # 1, 2, 3, 4, 6, 7, 9, 10