Unit 2 PhysicsArea of Study 1

Motion

Area of Study 1Ch 4 Aspects of Motion

Chapter 5Newton’s

Laws

On completion of this chapter, You will have covered material from the study

of movement including: vector techniques in two dimensions forces in two dimensions Newton's laws of motion problems in mechanics including weight and

friction.

Area of Study 1Ch 5 Newton’s Laws

Force as a vector In this chapter we will consider the forces

that cause motion to occur. This branch of Physics is called mechanics

and in particular – this section is called dynamics.

In simple terms; a force can be thought of as a push or a pull.

Area of Study 1Ch 5.1 Force as a vector

Force as a vector Forces are fundamental to the nature of

matter and the structure of the universe. Consider each of the photographs in Figure

5.1 and identify each force - push or pull - that is acting.

Area of Study 1Ch 5.1 Force as a vector

Force as a vector Figure 5.1 push or pull

5.1 Force as a vector

Force as a vector Forces that act directly on a body are called contact

forces They require one object or material touching another

because the body will only experience the force while contact is maintained.

Forces that act on a body at a distance are non-contact forces.

Gravitational, magnetic and electric forces are non-contact forces.

5.1 Force as a vector

Force as a vector The action of a force is usually recognised through

its effect on an object or body. A force may do one or more of a number of things to

the object. It may change its shape, change its speed or change only the direction of its motion.

The tennis racquet in Figure 5.1a has applied a force to the tennis ball, and, as a consequence, the speed of the ball changes along with its direction. The ball also changes shape while the force acts!

5.1 Force as a vector

Force as a vector The amount of force acting can be measured using

the SI unit called the newton. One newton (1 N) is defined as the force required to

make a mass of 1 kg accelerate at 1 m s- 2. The unit, honours Sir Isaac Newton (1642-1727), who

is still considered to be one of the most significant physicists to have lived.

A force of one newton, 1 N, is approximately the force you have to exert when holding a 100 g mass (a small apple) against the downward pull of gravity.

5.1 Force as a vector

Force as a vector FORCE is a vector quantity. It requires a magnitude and a direction to describe it

fully. In a diagram, a force is

usually shown as an arrowwhose length representsthe magnitude of the forceand whose direction is indicated by the arrow.

Consider the case of a soccer player who kicks the ball horizontally with a force of 95 N towards the east.

5.1 Force as a vector

Net Force If more than one force acts on a body at the same

time, the body behaves as if only one force—the vector sum of all the forces—is acting.

The vector sum of the forces is called the resultant or net force, ΣF (shown as a double-headed arrow).

The NET FORCE acting on a body experiencing a number of forces acting simultaneously is given by the vector sum of all the individual forces acting:

ΣF = F1 + F2 + … + Fn

5.1 Force as a vector

Net Force If the forces are acting at an angle to each other, the

resultant force must still be found by performing a vector addition.

Consider the example of a shopping trolley that is being simultaneously pushed from behind by one person and pushed from the side by another.

This situation is illustrated in Figure 5.6.

5.1 Force as a vector

Net Force If the forces are acting at an angle to each other, the

resultant force must still be found by performing a vector addition.

Consider the example of a shopping trolley that is being simultaneously pushed from behind by one person and pushed from the side by another.

http://phet.colorado.edu/en/simulation/vector-addition

5.1 Force as a vector

Net Force Consider the example of a shopping trolley that is

being simultaneously pushed from behind by one person and pushed from the side by another.

http://phet.colorado.edu/en/simulation/vector-addition

5.1 Force as a vector

Vector Components It is often helpful to divide a force acting in a two-

dimensional plane into two vectors. These two vectors are called the components of the

force. Consider, for example, the pulling force of 45 N

acting on the cart shown in Figure 5.9. This pulling force is acting through the rope and is

known as tension or a tensile force. It is usual to construct a right-angled triangle around

the force vector.

5.1 Force as a vector

Vector Componentshttp://phet.colorado.edu/en/simulation/vector-addition

5.1 Force as a vector