Unit 11: Forces

Unit 11: Forces

By S Harris

A force is a push or pull.

• Measured in newtons (N)

• examples

  • catching a basketball

  • hitting a baseball with a bat

• they change the direction that the object is traveling in, but are not always noticeable.​

What's a force

Think about kicking a soccer ball. A player's foot can strike the ball with a force, causing the ball to stop, and then move in the opposite direction.

​Changing Motion

This is the sum of ALL of the forces acting on a single object.

• forces have a direction, so they can be added and subtracted like in regular math problems.

Net Force

same direction, so we add them up

opposite direction, so we subtract them

Forces are ADDED when they are going in the same direction​

• in the image the two people are ​both pushing the box forward, so we would add their newtons to get the total force.

• The net force arrow goes in the same direction as both of the forces in this case.​

Unbalanced Forces

Forces are SUBTRACTED when they are going in the opposite direction​

• In the image the two people are pushing the box. This time one is pushing it forward, while the other is pushing it backwards.

• ​The net force arrow will go in the direction of the greater force in this case

Unbalanced Forces

When forces are going in the opposite direction and are equal in size, they will CANCEL out.

• in this case the net force would be zero​

Balanced Forces

Multiple Choice

What is the net force amount and direction of the car if the guy is accelerating with a force of 10 N east, but the parachute is pulling him back (west) with a force of 7N?

• 3 N east

• 7 N west

• 10 N east

• 17 N east

Multiple Choice

What is the net force acting on the box and its direction?

• 5 N to the right

• 15 N to the left

• 15 N to the right

• 10 to the right

Do NOT cause a change in the object's motion (the object doesn't move).

​​Balanced Forces

Cause a change in an object's motion

​​Unbalanced Forces

​

Question:

• If you were riding your skateboard down a sidewalk, would you continue rolling forever or would you stop?

ANSWER:

• Eventually you will stop because of the invisible forces (such as friction) acting on you and the skateboard.

FRICTION is a force that opposes the sliding motion of two surfaces that are touching each other.

• Why?

  • surfaces, even smooth metal, have bumps and dips. this causes two surfaces to have micro-welds (sticking)

​Invisible Forces

GRAVITY is a force between any two objects.

• it depends on:

  • the masses of the objects

  • ​the distance between the objects

  ​

​Invisible Forces

• If the mass of either of the objects increases, the gravitational force between them also increases

• If the objects get closer together (distance decreases), the gravitational force between them will increase​

​Comparing Gravitational Forces

Multiple Choice

Which objects would have the greater gravitational force acting between them? (hint: each ball has the same mass)