4.10 Acceleration Due To Gravity

Question 1

What is freefall?

Answer 1

The motion of an object undergoing an acceleration of ‘g’

Question 2

What type of quantity is acceleration?

Answer 2

Vector

Question 3

Which direction does ‘g’ act?

Answer 3

Vertically downwards

Question 4

What forces are acting on an object in freefall?

Answer 4

The only force acting on the object is weight

Question 5

Can object in freefall have an initial velocity?

Answer 5

They can have an initial velocity in any direction, but the force providing the initial velocity must not be acting for them to undergo freefall

Question 6

At what rate do different objects in freefall accelerate?

Answer 6

All objects in freefall accelerate to the ground at the same rate

Question 7

Why do all objects fall at the same rate?

Answer 7

• f = ma and f = w = mg
• mg = ma
• so m cancels to give g = a
• g is the same for all objects

Question 8

What are the signs when calculating freefall?

Answer 8

• g is always downwards, so is usually negative
• t is always positive
• u, v and s can be positive or negative (depends if they’re going up or down)

Question 9

How do you determine g using freefall?

Answer 9

• set up an electromagnet with a ball bearing attached. Set up a switch with a timer and attach a trapdoor.
• measure the height, h, from the bottom of the ball bearing to the trapdoor
• flick the switch to start the timer and disconnect the electromagnetic, releasing the ball bearing
• the ball bearing falls, knocking the trapdoor down and breaking the circuit, stopping the timer.
• record the time, t, shown on the timer
• repeat 3 times and average
• change the height and repeat

Question 10

How do you reduced error when experimentally determining g using freefall?

Answer 10

• the most significant source of random error will be the measurement of h. Using a ruler you’ll have an uncertainty of about 1mm
• using a small and heavy ball bearing means you can assume air resistance is small
• having a computer automatically release and time the ball bearings fall can measure times with smaller uncertainty than doing it manually

Question 11

How would you calculate g from your experimental results from ‘determining g using freefall’?

Answer 11

• plot a graph of height, h against the time it takes for the ball to fall squared, t^2
• draw a line of best fit
• calculate the gradient, which is equally to 1/2g
• double it to find g