Newton’s Second Law of Motion

Newton’s Second Law – the Law of Acceleration

The acceleration (a) of an object depends on the objects mass (m) and the amount of force (F) applied to the object.

Remember, acceleration occurs when an object…

1. Speeds up2. Slows Down

3. Changes Direction

What does this look like?

Force

accelerationmass X

Most familiar version:F = m•a

Actual definition of Newton’s 2nd Law:

a = F/m

OtherManipulation:

m = F/a

Practice!

1. What is the acceleration of a 15 kg mass if a force of 45 Newton’s is used to move it? - Which formula will you use?

F = m X A A = F/m

A = F/m

A = 45 N / 15 kg

A = 3 m/s2

With a partner!

What force is necessary to accelerate 200 kg object at a rate of 40 m/s2?

F = m X A A = F / m

On your own…

What is the force necessary to move an object with a mass of 25 kg at an

acceleration of 5 m/s2?

F = m X A A = F / m

Net Forces

• Total of forces acting on an object

• the refrigerator has many forces acting on it, but when added together they cancel out.• therefore there is no

change in it’s motion

Forces in the Same Direction

– In this example, there are two forces acting on the object in the same direction.

– These two forces are added together.

– The objects resulting motion is to the right.

15 N

20 N

35 NResulting Net Force=

Net Force of Zero

–Because the forces from Man A and Man B are equal but opposite in direction, there is no change in the objects motion.

A B

Net Force Other Than Zero

If there is a difference between two opposing forces, the resulting change in motion is in the direction of the larger force.– The force of the car

is 10 N to the right while the parachute provides 7 N of force in the opposite direction.

Net Force = 10 N – 7 N F = 3 N to the right

Practice

What would the resulting motion be from this group of forces?

More Practice

The mass of the car and the driver equals 240 kg. How fast will the driver and his car accelerate?

How much force would the man on the left need to provide in order to stay in the same position?

10 N 15 N

7 N3 N

?

If the man on the right was able to provide 30 N of force, what would the resulting acceleration be?

85 kg75 kg 90 kg 55 kg 50 kg

Always remember…

If Force remains the same and mass increases, then acceleration

will decrease. Acceleration increases, then the

mass will have to decrease.2nd Law VideoNewton’s 2nd Law and trains