Newton's Laws of Motion and Forces, Slides of Science education

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Forces and Motion

Newton’s laws of motion

Terminology

• (^) Motion - change in position
• (^) Speed - rate of motion = distance / time
• (^) Direction of Motion - where an object is going. Draw a straight line to represent where object was in the past and in the present.
• (^) Force – a push or pull that one body exerts on another body
• (^) Equilibrium – a condition where all forces are balanced
• (^) Friction- force when two surfaces touch. It is always in the opposite direction of motion ( Newton’s 3 rd Law)
• (^) Velocity – the speed and direction of an object
• (^) Acceleration – a change in either the speed or the direction of an object (speed up or SLOW DOWN)
• (^) Balanced forces – equal forces acting on one object in different directions

Balanced Forces

When two forces acting on an object are equal in size, but are opposite in

direction, we say the forces are balanced.

Balanced forces do not cause a change in motion.

We can show the forces acting on an object using a force diagram.

In a force diagram, the force is shown with an arrow – the larger the arrow,

the bigger the force. The arrows in a force diagram also indicate the

direction of the force, Sometimes, the arrow will be labelled with the size of

the force in Newtons (N) or it will be depicted using a line drawn to scale

The vector sum of all forces acting on an object in mechanical equilibrium is

zero. When net force , the velocity stays the same. (same speed

and same direction). The reverse is also true; When the velocity stays the

same (constant), the net force is zero.

 F  0 

Individual Forces (^) Net Force 4 N 10 N The net force is the vector sum of all external forces acting on a single object.  F  1 2 3  F  F  F  F       Net force (^)  F  6 N  F 

Newton’s 2nd Law The acceleration (a) of an object is directly proportional to the resultant force (F) acting on it and inversely proportional to its mass (m). The direction of the acceleration is the same direction as the resulting force. F = m • a Force and acceleration are proportional to each other—the amount of force is equal to the amount of acceleration. The greater the force exerted on an object, the more it will accelerate. For example, the harder you kick a ball, the farther and faster it will travel. The opposite is true of mass. The more mass an object has, the less it will accelerate. If you kick a tennis ball and a bowling ball with the same amount of force, the heavy bowling ball is going to move slower and go a shorter distance than the tennis ball. A heavier object requires more force to set it in motion.

Newton’s Third Law of Motion For every action, there is a reaction that is equal in magnitude and opposite in direction. Every time a force, or action, occurs, it causes a reaction. We can describe the reaction in terms of its strength, or magnitude, and also its direction. The magnitude of the action is equal to the magnitude of the reaction. For example, if you toss a pebble into the water, it’s going to create a small ripple or splash. If you hurl a large boulder at the water, the splash is going to be bigger. The force of the action and reaction always match up. While an action and its reaction are equal in magnitude, they are opposite in direction. The rock plunges down into the water, but the water splashes up. When you propel something forward, the recoil of the force pushes you backward. Every time a force acts on an object, it causes a reaction force in the opposite direction.

Forces are Balanced

Objects in Motion V ≠ zero m/s Stay at Rest Stay in Motion (same speed and direction a = 0 m/s 2 a = 0 m/s 2 Object at Rest V = zero m/s Basically, objects just keep on doing whatever they are doing unless they experience some external influence.

Inertia and Mass

The natural tendency of any object is to resist changing state of motion. This is called inertia. For example, if an object is moving, it likes to keep on moving. If an object is stationary, it likes to remain stationary. It takes some measure of force to change this tendency. The more mass an object has, the greater its ability will be to resist change. For example, a big guy will be harder to push over than a small guy. A 2-kg object is moving horizontally with a speed of 4 m/s. How much net force is required to keep the object moving at this speed and in this direction? Zero, No net force is required. An object in motion will maintain its state of motion. The presence of an unbalanced force changes the velocity of the object.

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Force diagram (also known as free-body diagram)

 F  275 N  395 N  560 N  110 N

mass 1850 kg ALWAYS draw a force diagram with a force vector for each individual, external force acting on a single object. 2 +110 N m

1850 kg s F a m       Find net force and acceleration?

Definition

- A force that opposes motion When a force is applied to a body resting on a rough plane so that the body moves or tends to move, a frictional force acts on the body in opposition to the applied force. Symbol - F f (Units • Newtons – since friction is a force!) Depends on - Weight of object (normal force) - Nature of the surface between moving object and supporting surface Two types • Static friction (pushing the piano but no motion) - Kinetic (Sliding) friction (piano moves!!!)

Frictional Force Force on box by floor It’s the sum of all the forces that determines the acceleration. Every force has an equal & opposite partner. Force on box by person Force on person by box Force on floor by box