Newton’s Laws of Motion

Law 2 and Law 3

Newton’s 2nd Law of Motion

The acceleration of an object increases with increased force and decreases with increased mass, and is in the same direction as the force.

Force = Mass x Acceleration

       

       

Which hurts more?

Choice 1: Dropping a brick on your foot Choice 2: Gently placing a brick on your foot

Why?

Dropping the brick means higher acceleration.

The higher acceleration means a higher force.

Calculating Force

How much force must be applied to a toy car that has a mass of .25kg to achieve an acceleration of 2.4m/s2?

The unit for force is NEWTONS (kg • m/s2 = N)http://www.cchs.sbd.pvt.k12.oh.us/Assistant/MathSci/MathScienceIntegation/MathScienceIntegation-

856.htm

F = maF = .25 kg • 2.4 m/s2

F = 0.6 N

Mike's car, which weighs 1,000 kg, is out of gas. Mike is trying to push the car to a gas station, and he makes the car go 0.05 m/s/s. Using Newton's Second Law, you can compute how much force Mike is applying to the car.

Answer = 50 newtons

Newton’s 3rd Law of Motion

When one object exerts a force on another object, the second object exerts an equal and opposite force on the first object.

Equal and Opposite Forces

The rocket's action is to push down on the ground with the force of its powerful engines, and the reaction is that the ground pushes the rocket upwards with an equal force.

Let's study how a rocket works to understand Newton's Third Law.

Example 1:

What happens if you are standing on a skateboard or a slippery floor and push against a wall?

Why does that happen?

1. You are exerting a force by pushing on the wall.

2. The wall is pushing back with an equal and opposite force.

3. The force is equal because your movement depends on how hard you push against the wall.

4. The force is opposite because you accelerate away from the wall.

Example 2:

Why does it hurt so much when you stub your toe?

It hurts because…

When your toe exerts a force on a rock, the rock exerts an equal force back on your toe. The harder you hit your toe against it, the more force the rock exerts back on your toe (and the more your toe hurts).

NSF North Mississippi GK-8

Newton’s First Law:Objects in motion tend to stay in motion and

objects at rest tend to stay at rest unless acted upon by an unbalanced force.

Newton’s Second Law:Force equals mass times acceleration

(F = ma).Newton’s Third Law:

For every action there is an equal and opposite reaction.

Newton’s Laws

Sources

Steve CaseNMGK-8University of MississippiOctober 2005 www.harrisonhigh.org/weblogs/.../Discovering%20Newtons%20Laws.ppt

mohouih