Unit 2 - Forces

Types of ForcesIn this chapter we will explore;• Different types of forces and learn about

how they affect the velocity and acceleration of objects. • Newton’s Three Laws of motion to explain

how and why objects move.• Free-body Diagrams to calculate net force

and acceleration.• Problem solving related forces and motion

in one dimension.

ForcesOur previous unit of kinematics focused on how objects move without being concerned with why they move. In our next unit of study we will concentrate on why an object moves by looking at the various forces acting on it. Forces can cause objects to change their motion.Dynamics is the study of the causes of motion.

3.1 Types of ForcesA force () is simply a push or pull. Forces can cause objects to change their motion; speed up, slow down, change direction. The direction in which a force acts is very important. It is a vector quantity. SI units – Newton (N) (1 N = 1 kg·m/s2).

A system diagram is a simple sketch showing all the objects involved in a scenario. For example, two children pulling on a wagon.

A free-body diagram (FBD) is a simple drawing of an object showing all the forces that are acting on it. The forces acting on the wagon.

3.1 Everyday ForcesIn order to be able to draw useful force diagrams, we must become familiar with some of the common forces we encounter everyday.

• Applied Force () is a force that results when one object makes contact with another and pushes or pulls on it.

• Tension () is a pulling force from a rope or string that always points towards the rope or string.

• Normal Force () is a perpendicular force exerted by a surface on an object it contact with the surface; it always points away from the surface.

3.1 Everyday Forces• Friction () is a force that opposes the sliding of two surfaces across one another;

friction acts opposite to the motion or attempted motion.

• Force of Gravity () is a force of attraction between any two objects due to their mass. It is an action at a distance force; it always points down towards Earth’s centre.

• Net Force () is the sum of all the forces acting on an object.

SP #1,2 p.117-118; SP #1,2 p.119-120

3.1 Four Fundamental ForcesPhysicists have grouped all known natural forces into four categories:

• Gravitational forces are attractive and exist between any two masses in the universe. Unnoticeable between two small masses compared to Earth’s gravity.• Electromagnetic forces are caused by electric charges and magnets. This force

can both attract and repel.• Strong Nuclear forces are attractive between the protons and neutrons within

the nucleus of an atom, preventing the protons from repelling one another and flying apart.• Weak Nuclear forces are responsible for transforming protons ad neutrons into

other particles during nuclear reactions.

3.1 Homework

Practice # 2 p.119Practice # 1-3 p.120Questions # 2, 5, 7, 10, 13, 15 p.122