Ch3+Forces+and+Pressure

You can make use of the slides to see the lesson material = = =Forces Learning Outcome= Candidates should be able to: (a) describe the effect of balanced and unbalanced forces on a body (b) describe the ways in which a force may change the motion of a body (c) identify forces acting on an object and draw free body diagram(s) representing the forces acting on the object (for cases involving forces acting in at most 2 dimensions) (d) recall and apply the relationship resultant force = mass x acceleration to new situations or to solve related problems (e) explain the effects of friction on the motion of a body

**A force can do one of four things to an object:**
If something is doing one of these four things there must be a **net force** acting upon it. (**Note:** Other names for a **net force** are an **unbalanced force** or a **resultant force**.)
 * 1) Make it **speed up** - accelerate.
 * 2) Make it **slow down** - decelerate.
 * 3) **Change its direction**.
 * 4) **Change its shape**.

Well, forces are vectors - you must take their **direction** into account as well. The unit we measure force in is the **Newton**, named after Sir Isaac Newton, which leads us onto **Newton's first law.**
 * What do we mean by a 'net' force?**
 * It is easy to add up forces, just look at these three examples:**

**Newton's First Law**
Something without a **net** force acting on it will either **stay still** or **move at a constant speed** in a straight line until you apply a force to it.
 * states that:** **'Every body continues in a state of rest or uniform motion unless acted upon by an external force.'**

Also known as **Newton's Second Law** - you will have seen this equation during your course. This shows that if you keep the mass constant and double the applied force the acceleration will double.
 * F is the force in Newtons, N.
 * m is the mass in kilograms, kg.
 * a is the acceleration in m/s2.
 * If you plot a graph of force against acceleration it will look like this:**

You can see here that force is proportional to acceleration. As you double the force the acceleration doubles, as you triple the force the acceleration triples.
 * If you plot a graph of acceleration against mass it will look like this:**

You can see here that if you keep the force constant and increase the mass the acceleration will fall. Acceleration is inversely proportional to mass. If you double the mass the acceleration will halve. It is helpful if you can rearrange this equation. **The triangle for this is as follows:**

//**Some examples:**// //**Answer:**// //**Note:**// Don't forget the units! //**Answer:**// //**Important point:**// The equation works in exactly the same way for deceleration as it does for acceleration!
 * 1. A 500kg car accelerates at 3 m/s2.**
 * How much force is exerted by the wheels to accelerate the car?**
 * **Write down the formula:** F = ma
 * **Plug in the numbers:** F = 500 x 3
 * **Write down the answer:** F=1500N
 * 2. A 500kg car is accelerated by a force of 2000N. What is its acceleration?**