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Acceleration is the change in motion over time
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Newton’s 2nd LawThe Law of Acceleration
Newton’s 1st LawTells us how an object behaves when there is no net force acting on it.
Newton’s 2nd LawTells us how an object behaves when there is a net force acting on it.
Acceleration is the
change in motion over time
Q. A car changes speed from 100km/hr to 35 km/hr.
Yes
Q. A car speeds up by 20 km/hr.
Yes
Q. A car traveling at 60 km/hr turns to the left.
Yes
Q. A skydiver jumps out of a plane and falls towards the ground faster and faster.
Yes
Q. A skydiver reaches a terminal velocity and falls towards the ground at a constant speed.
No
Q. A car driving on a straight road at a constant speed.
No
Q. A person sitting on the grass.
No
Newton’s 2nd Law
A net force on an object causes the object to accelerate in the same direction as the net force.
F=ma
Q. A car drives at 50 km/hr east along a flat highway. The car feels a net force to the east. How does the motion of the car change?
A. The car starts to speed up but continues to travel east.
Q. A car drives at 50 km/hr east along a flat highway. The car feels a net force to the west. How does the motion of the car change?
A. The car starts to slow down but continues to travel east.
Q. A car drives at 50 km/hr east along a flat highway. The car feels a net force to the north. How does the motion of the car change?
A. The car starts to turn towards the north.
Q. A car drives at 50 km/hr east along a flat highway. The car feels a net force to the south. How does the motion of the car change?
A. The car starts to turn towards the south.
Q. A skydiver falls straight towards the earth at a terminal velocity of 120 km/hr. She feels a net force downward.
a. How does her motion change?
b. What situation could make this happen?a. She starts speeding up until terminal
velocity is reached, but continues to fall straight downwards.
b. She could be falling through an area of less wind resistance.
Q. A skydiver falls straight towards the earth at a terminal velocity of 120 km/hr. She feels a net force upward.
a. How does her motion change?
b. What situation could make this happen?
a. She starts slowing down, but continues to fall straight downwards.
b. She could be falling through an area of more wind resistance.
If the mass is twice as big, then the acceleration
must be half as big for the same force.
Q. A skydiver falls straight towards the earth at a terminal velocity of 120 km/hr. She feels a net force to the left.
a. How does her motion change?
b. What situation could make this happen?
a. She starts to fall diagonally – mostly downward, but partially to the left.
b. She could be falling through an area where the wind is blowing to the left.
Q. A skydiver falls straight towards the earth at a terminal velocity of 120 km/hr. She feels a net force to the right.
a. How does her motion change?
b. What situation could make this happen?
a. She starts to fall diagonally – mostly downward, but partially to the right.
b. She could be falling through an area where the wind is blowing to the right.
Newton’s 2nd Law tells us that
F = ma
The units of mass are ________________. Kilograms (kg)
The units of acceleration are ____________________.Meters per second²(m/s²)
The units of force are ________________.Newtons (N)
So 1N = 1kg x 1m/s²
1. A 1000 kg car accelerates at 2.0m/s². What force is being made on the car?
F=ma
F=(1000kg)(2.0m/s²)
F=2000N
2. A 2000 kg car accelerates at 1.0m/s². What force is being made on the car?
F=(2000kg)(1.0m/s²)
F=2000N
3. A 500 kg car accelerates at 4.0m/s². What force is being made on the car?
F=(500kg)(4.0m/s²)
F=2000N
4. What do you notice about the answers to questions 1, 2, and 3?
They are the same.
We see that there needs to be two things that control how big a force is:
Acceleration
Mass
So a big acceleration on a small mass
F = maand a small acceleration on a big mass
could both be caused by the same force.
F = ma
1. A 5000 kg car accelerates at 2.4 m/s². If an equal force is made on a 10 000 kg truck, what will the truck’s acceleration be?
Fcar=(5000kg)(2.4m/s²)
FTruck=(10 000kg)(?)
We know…
Fcar=FTruck
(5000kg)(2.4m/s²)=(10 000kg)(aTruck)
a=1.2m/s²
2. A 1000 kg car accelerates at 4.0m/s². If an equal force is made on a 4000 kg car, what will the acceleration be?
FcarA=FcarB
(1000kg)(4.0m/s²)=(4000kg)(a)
a=1.0m/s²
3. A 1000 kg car accelerates at 4.0m/s². If an equal force is made on a 500 kg car, what will the acceleration be?
FcarA=FcarB
mcarAacarA=mcarBacarB
(1000kg)(4.0m/s²)=(500kg)(a)
a=8.0m/s²
4. A 900 kg car accelerates at 2.5m/s². If an equal force causes a second car to accelerate at 7.5 m/s², what is the mass of the second car?
FcarA=FcarB
(900kg)(2.5m/s²)=(m)(7.5m/s²)
m=300kg
5. A 1000 kg car accelerates at 2.0 m/s². If an equal force causes a second car to accelerate at 8.0 m/s², what is the mass of the second car?
FcarA=FcarB
(1000kg)(2.0m/s²)=(m)(8.0m/s²)
m=250kg
6. A 1000 kg car accelerates at 2.0 m/s². If an equal force causes a second car to accelerate at 0.5 m/s², what is the mass of the second car?
FcarA=FcarB
(1000kg)(2.0m/s²)=(m)(0.5m/s²)
m=4000kg
Since the force equation has three quantities, (force, mass, and acceleration), we can really make three equations:
Fm =a
Fa =m
F = ma
