F F S O R C E Starter (answer these in your book): What is air resistance? Explain why a parachute can slow a skydiver down. How do we measure forces? Why do you slip on ice?

Aristotle said that for an object to continue moving there must be a force acting on it. It is understandable that he thought this was the case: Snap! What couldn’t he see? In the seventeenth century Galileo argued that objects keep moving unless acted on by a force, for example friction.

It’s easy for us to imagine a situation where there is no friction. If Aristotle had been right, this would happen…

Newton later said: Objects continue to move in a state of constant velocity unless acted upon by an external net force. 1ST LAW.

Newton Astronauts need to beware!

Newton Newton also said: “Every action has an equal and opposite reaction.” - His 3rd law. Which explains why guns recoil and how rocket engines work.

Another example of the third law; this time to stop the astronaut moving.

But don’t throw the ball too hard...

The car moves at constant speed. Having EQUAL AND OPPOSITE forces acting on an object is effectively the same as having NO force acting. The car moves at constant speed. drag drive Newton’s 2nd Law: Force = Mass x Acceleration (F = ma) No net force = No Acceleration. = No change in speed.

What happens when there are unbalanced forces acting on an object? For example when a driver who is travelling at constant speed accelerates or puts on the brakes. Accelerate: Brake: drive drag Speed decreases until drag matches drive again. Speed increases until drag grows to match drive. Back arrow twice to see again…

Resultant Forces

 Situation 1: Is the car… Stationary Moving with constant speed Reaction (push back from ground)  Stationary Moving with constant speed No Drag Force No Drive Force Accelerating Decelerating Weight Resultant force = zero (how do you know?) and the car is stationary. If it was moving there would be a drag force.

 Situation 2: Is the car… Stationary Moving with constant speed Accelerating Decelerating Resultant force = zero The car remains at the same speed, in the same direction

 Situation 3: Is the car… Stationary Moving with constant speed Accelerating Decelerating Resultant force is NOT zero, so… The car accelerates in the direction of the resultant force (forwards)

 Situation 4: Is the car… Stationary Moving with constant speed Accelerating  Decelerating Resultant force is not zero, The car slows down (decelerates).

Summary Questions 1. Draw four diagrams of a car (same car in each case – nothing too elaborate). Car one is at rest. Car two is moving at constant speed forwards. Car three is accelerating. Car four is decelerating. Each diagram should show all of the labelled forces acting on the cars. 2. Explain why all cars have a top speed. A car which was moving at constant speed drives off a cliff. 3. Explain – in detail - what happens next in terms of the motion of the car and all of the forces involved. Include diagrams.

Terminal Velocity

A Long Way down… Felix Baumgartner broke several world records on Oct 14th 2012, by skydiving from an amazing height of 39,000 metres (24 miles). He was falling for a total time of 9 minutes and 3 seconds.

Faster than the speed of sound While Felix was falling he reached a maximum speed of 833.9mph. The speed of sound is 768 mph at sea level. The means that Felix managed to travel faster than the speed of sound. He is the first skydiver to ever do this.

The Previous Record Joe Kittinger previously held the record for the highest skydiver. He jumped from 19.5 miles… in 1960! Joe helped Felix in his record skydive, and was the only person that Felix was talking to throughout his dive.

Terminal velocity When an object is dropped, we can identify three stages before it hits the ground: At the start, the object accelerates downwards because of its weight. There is no air resistance. There is a resultant force acting downwards.

Terminal velocity When an object is dropped, we can identify three stages before it hits the ground: As it gains speed, the object’s weight stays the same, but the air resistance on it increases. There is a resultant force acting downwards.

Terminal velocity When an object is dropped, we can identify three stages before it hits the ground: Eventually, the object’s weight is balanced by the air resistance. There is no resultant force and the object reaches a steady speed, called the terminal velocity.

What could Felix do to change his terminal velocity? Air Resistance Gravity

Why has no one gone this fast before? Air Resistance Gravity

Terminal velocity v/t graph Less acceleration as air resistance increases

Terminal velocity extra questions Explain why a falling human reaches a higher terminal velocity than a falling cat. Explain why a falling human without a parachute reaches a higher terminal velocity than a falling human with one. Parachutes on the moon? – Comment on their usefulness. Film of parachute jumpers often makes it look as though they shoot upwards when their parachute opens. They don’t – so why does it look that way?

Terminal velocity extra questions – outline answers The object accelerates until the air resistance matches the weight. A human has a bigger weight, so has to reach a higher speed before the air resistance matches it. A falling human with a parachute has a greater air resistance at a given speed, so does not have to reach as high a speed for air resistance to match weight. Parachutes are useless on the moon, as there is no air, so no air resistance. Everything keeps on accelerating to the end of its fall. The camera operator remains in free fall while he films the parachutist opening his parachute. At this point, the parachutist suddenly slows down relative to the camera operator, who shoots past him. This makes the parachutist appear to move upwards.

Summary If forces are balanced: An object will remain stationary or moving with constant velocity. If forces are unbalanced: An object will accelerate in the direction of the force. Cars have a top speed because: As they speed up, air resistance increases until eventually it is equal to the driving force. Terminal Velocity is reached when: The forces on a moving object become equal, due to the increased air resistance. Cats can survive falls from greater heights than humans because they reach terminal velocity quicker. An objects mass and surface area affect its terminal velocity. Heavier objects have a higher terminal velocity. Objects with smaller surface areas have a higher terminal velocity.