Sem 2 2014 EDUH 1017 Sports Mechanics L8 1

EDUH 1017Sports Mechanics

Lecture 8Force

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Newton’s First Law….

• What makes a person or object accelerate?• It is perhaps contrary to usual experience, but the normal

state of motion is to keep doing what you have been doing• Moving objects tend to keep moving• Stationary objects tend to stay stationary

• This is Newton’s First Law – the “Law of Inertia”“Every body continues in a state of rest or motion in a straight line unless it is compelled to change its motion by external forces exerted upon it.”

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Force is a vector

• The unit of force is the Newton (N) = kg.m/s2

• Several forces acting at one point on an object act add up and as a single force – force is a vector

• There are 5 horizontal forces in the tug-of-war – the man on the left experiences one large force although there are 4 individual forces

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Weight – Gravitational Force

• Weight is the downward gravitational force experienced by an object. The weight force is related to the acceleration due to gravity (g = 9.8 m/s2)

• We might say a person’s “weight” is 50 kg. We should say her mass is 50 kg and her weight is 50 kg x 9.8 m/s2 = 490 N

Fgravitation = mg

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Momentum

• Velocity alone fails to give us some aspect of the motion of a body

• Why would you rather be hit by a fly travelling at 60 km/h than a fire engine at the same speed?

• The “drive” that a moving body has cannot be attributed to the velocity alone. The more mass a body has, the more difficult it is to change its motion

• This concept is embodied in a new vector quantity momentum (p)

p = mv

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Newton’s Second Law

• The first law tells us that the motion of an object changes with the application of a net force – but by how much?

• This is expressed in Newton’s Second Law which we will write as“The force exerted on a body equals the resulting change

in the body’s momentum divided by the time elapsed in the process. The change takes place in the direction in which the force acts.”

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F = ma

• i.e.

• At any instant the net force applied to an object causes it to accelerate at a rate that is inversely proportional to its mass – i.e.

F =

mvf −mvit

F = m

v f −vit

⎛ ⎝ ⎜

⎞ ⎠ ⎟ = ma

a = F /m

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F = ma

• F is the net force (the sum of all the forces added as vectors) acting on the object

• If there is a net force then there is an acceleration• If there is an acceleration then there is a net force

• Force and acceleration are vectors in the same direction, thus F = ma

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Examples

• The blocks accelerate in the direction of F for as long as the force is applied.

Sem 2 2014 EDUH 1017 Sports Mechanics L8

No (net) force = no acceleration

10

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Examples Hecht 4.3

• A ball of 0.142 kg left a player’s hand at a speed of 20 m/s. If the straight throw lasted 0.020 s, determine the magnitude of the force exerted on the ball, assuming it is constant.

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Illustrating gravity

Dropping keys or throwing them upwards on a moving train

• What does the woman on the train see?

• What do we see (on the platform)?

• Why?

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Newton’s Third Law

• Yet another law!• “To every action there is always an equal an opposite

reaction”• More precisely:• The interaction of two objects always occurs by way of

two forces• A force of the first object acting on the second, and• An equal-magnitude and oppositely directed force of

the second object acting on the first

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Standing still

• Weight is an external force acting on a person. The Earth pulls on the person and the person pulls on the Earth – an action-reaction pair.

• The person isn’t accelerating up or down, so there must be a force on her to balance the weight force.

• Her weight causes her feet to push on the ground and so the ground pushes back on her – another action-reaction pair.

• The forces on her are the weight and the reaction of the floor – in balance (if the floor is strong enough!)

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Example

Try bouncing a spinning ball on the ground. What happens? Watch the motion.

Force on a spinning ball

coming in from the right (A) and bouncing off to the left (B).

The resulting direction is shown by the velocity arrow (V).

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Summary

• If there is a net force then there is an acceleration• If there is an acceleration then there is a net force

• Forces in balance (no net force) implies no acceleration i.e. the object remains stationary or continues moving with constant velocity

• NEXT: Examples of Forces

F = ma