Free fall

Philip's car has fallen off the cliff. Now gravity takes over. It pulls the car downwards. If we ignore air resistance, the only force acting on the car is gravity. We say that the car is in free fall.

Watch how the car falls. This is the car's position recorded every second. See how the spacing of the points increases? The car is falling with an increasing velocity. It's accelerating, downwards.

Let's rewind a bit, and pause. The car is about to fall over the edge. Next to the car we'll drop a bowling ball at the same height. The car weighs two thousand kilos, and the bowling ball two kilos. The car weighs a thousand times more than the ball.

Which object will hit the ground first? Think about that for a few seconds. Pause the film if you want to discuss it with a classmate. Aha. They hit the ground at the same time.

The smaller and lighter ball falls with the same velocity as the car? So the acceleration is equal for the two objects, even though they have different masses. From Newton's Second Law we know that the force acting on an object equals the object's mass times the acceleration. The car's mass is larger than the ball's. A thousand times larger.

We see that the force of gravity - that's the same as the weight of an object - is larger for the car than for the ball. A thousand times larger. The force of gravity acting on an object depends on the mass of the object. The larger the mass, the larger the force of gravity. The acceleration due to gravity is called the gravitational acceleration.

We write it with a lower case g. The gravitational acceleration, g, on the Earth is about nine point eight meters per second squared. This means that all objects in free fall accelerate - increase their velocity - approximately nine-point-eight meters per second, every second. The first second, the object increases its velocity by nine-point-eight meters per second. After two seconds, the object has increased velocity by nine-point-eight times two meters per second.

After three seconds, nine-point-eight times three meters per second. From this, we can create a formula. Because the velocity, v, is apparently equal to the gravitational acceleration, g, times the time, t. V equals g times t. This formula applies for all objects in free fall. -- How fast was the car falling as it hit the ground?

We can use the formula to calculate that. It took six seconds for the car to reach the ground. So the time t is six. Then we multiply the time with the gravitational acceleration, g, nine-point-eight meters per second squared. So the car's velocity when it hits the ground is 58.8 meters per second, directed downwards. -- If the car initially is moving horizontally, what happens then?

Like this car driving at high speed over the cliff's edge... The moment the car shoots off the cliff, gravity pulls it down. The car keeps moving horizontally at the same velocity it had when it left the cliff's edge. The horizontal movement with constant speed and the vertical movement that accelerates cause the car to move in an arc. The car will hit the ground at the same time as if it had fallen from rest.

The gravitational acceleration makes sure of that, and this is always the same. Of course, it won't land in the same spot. Whenever something is blasted or thrown through the air, we describe the movement as projectile motion. In the case of Philip's car there is air resistance, which resists the car's downward movement. So it's not actually in free fall, meaning that it accelerates slightly slower than nine-point-eight meters per second squared.

But... it falls... And the car... explodes. But don't worry, Philip.

It's a cartoon. It's hard to explain... Anyway, everything is back to normal in the next film.