3.1.2 Linear motion

Okay so we need to learn 4/5 of the suvat equations (no. 1, 2, 3, and 4) :

Tbh, number 5 is useful if you want to learn it, but you can use the others to deduce the answer so not the end of the world (you do need to know no.5 if you're doing maths though).


Okay so one experiment we need to know about is about free fall and determining g (the gravitational field strength), acceleration due to gravity. As we know, objects with mass exert a gravitational force on each other. When an object is accelerating under gravity (with  no other force acting on it) it is said to be in free fall. As I mentioned just a few moments ago, we need to be able to determine g (in a laboratory):

• Hold a small steel ball over a trapdoor using an electromagnet
• Turn the current off, a timer will trigger and the electromagnet will demagnetise (hence the ball will fall)
• the heavy steel ball will drop over a known distance (measure with a meter rule)
• it's decent will be timed using the timer (use a stopwatch for backup)
• Calculate the value for g using a = v/t = g

We can also use light gates:

• Two light beams are placed one above the other with detectors connected to a timer
• When the ball falls through the first beam it interrupts the light and starts the timer
• When the ball falls through the second beam a known distance further down the timer will stop
• use a = v/t = g

We can also use pictures:

• Drop a small metal ball from rest next to a metre rule
• Record its fall with a camera in rapid0-fire repeating mode
• Hold the camera shutter open to produce a photograph with multiple images of the falling ball/put a long exposure setting on
• Examine the photograph and determine the position of the ball at regular time intervals
• do a = v/t

Okay so we also need to know a little bit about things such as reaction time, thinking distance breaking distance, and stopping distance for a vehicle. Basically:

• The stopping distance is the total distance travelled from when the driver first sees the reason to stop to when the vehicle stops. It is thinking distance + breaking distance
• Thinking distance is the distance travelled between the moment when you first see a reason to stop to the moment you use the brake. For a vehicle moving at constant speed thinking distance = speed x reaction time. The greater the speed or reaction time the further the vehicle will travel before its driver applies the brakes. 
• Breaking distance is the distance travelled between the time the brake is applied until the vehicle stops.

NOTE: One point in this section of the spec states that we need to know '(ii) techniques and procedures used to investigate the motion and collisions of objects'. You should have done this as a PAG experiment (usually in your first year of covering the course).