1.1.1 Newton’s first law

Any body will remain in it’s state of rest or uniform motion in a straight line unless caused by some external NET FORCE to act otherwise. It essentially means that a body will do one of two things: · accelerate if you apply a force to it · not accelerate if you don’t

Explain the motion of the following, refer to all the forces acting on the object: a car going “flat out” at 120 kmh-1 No acceleration as balanced forces of drag and weight reach equilibrium a parachutist hitting the ground at 200 kmh-1 without a parachute if he jumps from 2000m or 5000m Has reached this equilibrium point - terminal velocity - before he has fallen 2000m the rate of acceleration of car decreasing as it gets faster. Engine produces a constant force - accelerates and so drag increases. This decreases the net accelerating force

Using the air track and light gates make a decision about how friction free the motion of the glider actually is and whether it can be used to demonstrate motion according to Newton’s first law. Does it remain stationary when no forces are acting? Does the velocity remain constant as it passes from one light gate to the next? What happens to the velocity if the glider is allowed to bounce off the ends?

Using your knowledge of balanced forces draw on the size of forces and direction to simplified diagrams of these people travelling at constant velocity. Remember the size of the arrow indicates the size of the force.

Objects in Equilibrium Decide for yourself what would happen to these objects! In all cases there is no resultant force so the balls stay in place!

In order to decide whether forces or velocity vectors do cause a resultant in any given direction we need to “add” them, taking into account their direction. We have done this since Year 7 with Forces acting along the same line of action, even in 2 or 3D. However the previous slide showed 3 forces at totally different angles. How, apart from the general feeling, can we prove they cancel each other out? How can we calculate the resultant? In all cases the Resultant is the “vector sum” of the components…………WHAT!!!!!

For example, if you were swimming in a moving river, what direction would you end up moving in and how fast? Again you can get a feel for this from your everyday experience – USE THIS “FEEL” IN THE EXAM TO GAUGE IF YOU ARE RIGHT! There are two methods to solve this 1) Using Trigonometry and Pythagoras 2) Drawing a scale diagram and measuring the size and direction of the resultant.

Trigonometry and Pythagoras If a person can swim at 1.5 ms-1 in still water but the current of the stream flows at 2 ms-1 at 90o to the swimmer. What is their speed and direction? A person swims at 1.5 ms-1 in still water The vectors should always be drawn “nose to tail” as shown on the left. The RESULTANT is the vector that joins the start of one vector with the end of the last one after joining them as described! The current flows at 2 ms-1

1.5ms-1 Tan = 2 / 1.5  = 53.10  2 ms-1 So the RESULTANT is the person travelling at 2.5ms-1 in a direction 53o from the direction that the person was swimming in. Using Pythagoras’ theorem 1.52 + 22 = 6.25 so R = 6.25 = 2.5ms-1

How do you add vectors if they are not in the same line of action or at 90o ? This is best achieved at A level by drawing 2) A scale diagram and measure the angle between the vectors accurately. Make sure you specify the direction of the angle eg from the horizontal, vertical or one of the vectors This method is known as the parallelogram law as the two forces make up 2 sides of the parallelogram allowing us to measure the size of the resultant! RESULTANT R = A + B B A

Example A body of mass 0.6kg falls vertically. A wind blows horizontally with a force of 8N. What is the magnitude and direction of the resultant force on the mass? (g=10 Nkg-1) 8N 6N Tan() = 6/8   = 37o R2 = 62 + 82 R2 = 36 + 64 R2 = 100 R = 10N Which angle on the diagram is measured though?!

These forces can be investigated using a force board where the forces are in equilibrium and the angles indicated by the string and magnitude by the weights suspended at the 3 points.

Resolve them vertically and horizontally Resolving Vectors If we want to know what the effect of a force is in a certain direction or if we are to add vectors, we need to know what they are doing in specific directions. The easiest to use are vertical and horizontal directions ie. Resolve them vertically and horizontally Tension in the rope Vertical component Horizontal component

Wind direction Force produced by keel Direction of boat As you can see from the example above we can make a triangle of forces from just about any situation. If a force is not acting vertically or horizontally we can consider it being made up of a vertical and horizontal force, just as you can walk somewhere by going forwards, backwards, left and right without moving diagonally!

Sideways pull of rope on barge Forward pull of horse Actual Pull of horse So how can we calculate the components of a force at 90o to each other? If the Horse pulls with a force of 750 N at an angle of 45o What would be the forwards force? Show your working and annotate the actions you are taking in these questions!!

If the force needed to pull this sled is 100 N and you pull at an angle of 25o what are the vertical and horizontal components of this force? Diagonal force upwards eg dragging a sledge  If a husky pulls a sled at an angle of 10o to it’s line of travel, what is the vertical component of the force and the horizontal component if the dog pulls with a force of 700N. If the sled and load weighs 100Kg what acceleration will the dog cause the sled to have?

Arrange the apparatus as shown Masses Newton meter  Arrange the apparatus as shown Vary the masses on both stacks recording the values Record the value from the Newton meter Check to see if the horizontal force and vertical force should theoretically combine to produce the force on the Newton meter.