WEBVTT
00:00:00.460 --> 00:00:02.800
A mobile phone is at rest on a table top.
00:00:03.080 --> 00:00:09.010
And the phone’s owner flicks the phone with their fingers, which applies a 0.01-newton force to the phone.
00:00:09.820 --> 00:00:14.890
The phone is accelerated at a rate of 0.1 meters per second squared in the direction of the flick.
00:00:15.690 --> 00:00:17.160
What is the mass of the phone?
00:00:18.250 --> 00:00:22.670
Alright, let’s start by underlining all of the important bits of the question so we don’t miss any information out.
00:00:23.630 --> 00:00:25.380
So we’ve got a mobile phone.
00:00:25.780 --> 00:00:28.360
Initially, it’s at rest on a table top.
00:00:29.130 --> 00:00:34.940
The phone’s owner then flicks the phone with their fingers, which applies is 0.01-newton force to the phone.
00:00:36.070 --> 00:00:43.270
The phone, due to this flick, is accelerated at a rate of 0.1 meters per second squared in the direction of the flick.
00:00:45.100 --> 00:00:47.210
We need to find the mass of the phone.
00:00:48.080 --> 00:00:51.800
So, okay, so here we’ve got a diagram of our banana phone.
00:00:52.160 --> 00:00:54.660
Quick disclaimer: other phone manufacturers are available.
00:00:55.390 --> 00:00:58.490
And we can see the owner’s hand on the left ready to flick the phone.
00:00:59.240 --> 00:01:00.330
And there it goes.
00:01:00.560 --> 00:01:03.930
The owner of the phone has flicked it, probably hurting their finger in the process.
00:01:04.390 --> 00:01:08.850
But the important thing is that the flick has applied a 0.01-newton force to the phone.
00:01:09.530 --> 00:01:12.980
We also know that the phone accelerates at 0.1 meters per second squared.
00:01:13.750 --> 00:01:15.550
And we need to find out the mass of the phone.
00:01:15.960 --> 00:01:21.600
So we need to find a relationship between the mass of the phone, the acceleration of the phone, and the force applied to the phone.
00:01:22.480 --> 00:01:26.060
The relationship we’re looking for is known as Newton’s second law of motion.
00:01:26.740 --> 00:01:33.800
Newton’s second law states that the force on an object, 𝐹, is equal to the mass of the object, 𝑚, multiplied by its acceleration, 𝑎.
00:01:34.700 --> 00:01:37.710
And these are exactly the three quantities that we’ve been working with so far.
00:01:38.430 --> 00:01:42.350
So in order to find the mass of the phone, we need to rearrange this equation first.
00:01:43.060 --> 00:01:47.050
We can do this by dividing both sides of the equation by the acceleration.
00:01:47.990 --> 00:01:50.030
The acceleration cancels on the right-hand side.
00:01:50.700 --> 00:01:53.700
And so we’re left with 𝐹 divided by 𝑎 is equal to 𝑚.
00:01:54.390 --> 00:01:57.140
The force divided by the acceleration is equal to the mass.
00:01:57.910 --> 00:01:59.940
So let’s plug in all the values we’ve been given.
00:02:00.510 --> 00:02:07.040
But before we do this, let’s quickly discuss the standard units of the quantities in the equation 𝐹 over 𝑎 is equal to 𝑚.
00:02:07.560 --> 00:02:08.800
Well we’ve got a force here.
00:02:09.200 --> 00:02:11.050
That has a standard unit of newtons.
00:02:11.770 --> 00:02:14.700
The standard unit of acceleration is meters per second squared.
00:02:15.900 --> 00:02:17.940
And for mass, it’s kilograms.
00:02:19.530 --> 00:02:28.200
That means that if our value of force is in newtons and the acceleration is in meters per second squared, then the mass that we’ll get out of this equation is gonna be in kilograms.
00:02:29.020 --> 00:02:30.310
And that is perfectly fine.
00:02:31.030 --> 00:02:32.510
So let’s get on with doing that.
00:02:33.090 --> 00:02:41.250
The mass of the phone, therefore, is equal to the force, 0.01 newtons, divided by the acceleration, 0.1 meters per second squared.
00:02:42.200 --> 00:02:46.260
Evaluating this, we find that the fraction simplifies to 0.1.
00:02:47.050 --> 00:02:48.920
And now we’ve gotta remember to put in the units.
00:02:49.180 --> 00:02:50.320
We’re working out a mass.
00:02:50.420 --> 00:02:53.500
And we’ve just said earlier that the unit is going to be kilograms.
00:02:55.130 --> 00:02:59.890
And so our final answer is that the mass of the phone is 0.1 kilograms.