What is force?

Net force

• Net force: combination of all the forces

acting on an object

Net force

• If two forces are in opposite directions, then the net force is the difference.

Net force

• If two forces are in the same direction, they add together to get the net force.

Balanced force

Balanced Force: if two equal forces are being exerted on an object in opposite directions.

This will result in a net force of zero.

Balanced force

Unbalanced force

The effects don’t cancel each other. The objects net force is NOT zero.

The object moves.

Unbalanced force

Example

A 10-newton force and a 15-newton force are acting from a single point in opposite

directions. What additional force must be added to produce equilibrium?

Newton’s 1st law

• An object at rest tends to stay at rest,

and an object in motion tends to stay

in motion, with the same direction and

speed.

Friction

• Friction: a force that acts against the

motion between two objects; this slows

an object down

Static friction

Static friction is the attraction between

the atoms on the two surfaces that are in

contact.

This causes the surfaces to stick together.

Sliding friction

While static friction keeps an object at

rest, sliding friction slows down an

object that slides.

A result of the microscopic roughness of

two surfaces.

What do you think?

What is the difference

between static friction and

sliding friction?

Rolling friction

Needed to make a wheel or tire turn.

Occurs between the ground and the part of the tire touching the ground.

Keeps the tire from slipping on the ground.

Newton’s 2nd Law

An object acted upon by a force will

accelerate in the direction of the force.

Newton’s 2nd Law

Net Force = mass x acceleration

Force is measured in newtons (N)

Fnet

m a

Acceleration

• The second law tells how to calculate the acceleration of an object if its mass and the forces acting on it are known.

Acceleration =

Net Force Mass

example

Using what we know lets say that on the way to work my vehicle breaks down. The car weighs 1000 Kg. As I am pushing the car I am able to make it roll

at 0.05 m/s². How much force am I applying?

Example

A car with a mass of 1,500 kg has an acceleration of 3 m/s². Find the force

acting on the car.

You pull a 10 kg sled so that the net force on the sled is 5 N. What is the acceleration?

Example

example

You throw a baseball with a mass of 0.15 kg so it has an acceleration of 40 m/s². Find the force

you exerted on the baseball.

example

A human cannonball with a mass of 80 kg is fired out of a cannon with a force of 2,400 N.

Find the acceleration.

Gravity vs. Weight

Gravity: an attractive force that pulls objects toward each other.

Depends on the mass of the objects and distance between them.

Becomes weaker the farther apart the objects are.

Decreases as the mass of the objects decrease.

Gravity vs. Weight

The force of gravity causes all objects near Earth’s surface to fall with an acceleration of 9.8 m/s².

Gravitational Force:

F = m x (9.8 m/s²)

This gravitational force is also called weight.

Gravity vs. Weight

Weight: the gravitational force between you and Earth.

Newton’s 3rd law

Newton’s 3rd Law: for every force there is an equal and opposite force.

Ex. if you push on a wall, it will push back on you as hard as you are pushing on it.

Balloon Rocket lab

Paddling a Canoe

Rocket Launch

Jumping Into Pool