00:01
In this example, we have a truck which has a mass of m sub t equals 2 ,000 kilograms.
00:10
It is pushing a stalled car.
00:12
That car has a mass of 650 kilograms.
00:17
So the car is put into neutral.
00:20
The engine's off.
00:22
The truck is touching the back bumper of the car, and it starts to drive, pushing them both forward.
00:30
They're accelerating at a rate of a equals 1 .2 meters per second squared.
00:38
And what we want to find is the force that the truck exerts on the car, and then the force between the truck, tires, and the road.
00:47
So first, let's look at all the forces acting on the car.
00:52
So we have the car here, m car, in a little box.
00:56
Ok, we have the normal force, fn, of the road that matches the gravitational force, mg.
01:04
They have equal magnitude opposite directions.
01:06
Acceleration in the vertical direction will be 0.
01:09
Ok, then in the horizontal direction, we have the force that the truck exerts on the car.
01:18
And that's our only force in the horizontal direction.
01:21
We have no frictional force because we have the engine in neutral.
01:26
The tires are just rolling along the roadway.
01:31
Ok, so we can see from newton's second law that the sum of the forces acting on the car in the x direction is just ft.
01:41
And this equals the car's mass times its acceleration.
01:46
So we can find the force that the truck is exerting on the car, and that equals 780 newtons.
01:59
And if this is in the direction of motion, this is the positive direction.
02:03
Ok, we'll just call this the x direction...