MOTION AND FORCES

What is a force? A push or a pull. Applied Force changes the movement of

the ball when you kick a soccer ball.

Balanced force? Net force:

When two or more forces act on an object.

Inertia? The tendency of an object to resist any

change in its motion.

Newton’s Laws of Motion First Law of Motion:

An object moving at a constant velocity keeps moving at that velocity unless an unbalanced net force act on it.

Newton’s Second Law the acceleration of an object is

dependent upon two variables - the net force acting upon the object and the mass of the object. The acceleration of an object depends directly upon the net force acting upon the object, and inversely upon the mass of the object.

Relationship of Variables

If the mass of the object stays constant, as the force acting upon an object is increased, the acceleration of the object is increased.

Force mass acceleration6 N 6 kg12 N 6 kg24 N 6 kg

1 m/s22 m/s2

4 m/s2

Relationship of Variables

If the Force on the object stays constant, as the mass of an object is increased, the acceleration of the object is decreased.

Force mass acceleration12 N 2 kg12 N 3 kg12 N 6 kg

6 m/s2

4 m/s2

2 m/s2

Solving a 2nd Law Problem A tennis ball, 0.314 kg, is accelerated at a rate of

164 m/s2 when hit by a professional tennis player. What force does the player's tennis racket exert on the ball?

G:U:E:

SS

m= 0.314 kga= 164 m/s2

F = 0.314 kg x 164 m/s2 = 51.5

N

F = ?

What units do we use?Measurement Symbol

kgvolume

m/s2

velocitydistance

sForce N

m/s

time

acceleration

massL

m

Solving a 2nd Law Problem A 68 kg runner exerts a force of 59 N. What is

the acceleration of the runner?

G:U:E:

SS

m= 68 kg F= 59 Na = ?

Changing the 2nd law formula

A 68 kg runner exerts a force of 59 N. What is the acceleration of the runner?

G:U:E:

SS

m= 68 kg F= 59 Na = ?

Solving a 2nd Law Problem A 68 kg runner exerts a force of 59 N. What is

the acceleration of the runner?

G:U:E:

SS

m= 68 kg F= 59 N

.87 m/s2

a = ?

Newton’s 3rd Law For every action, there is an equal

and opposite reaction.What forces is the bird experiencing?1. Gravity = pulling down

at 9.8 m/s2

2. Force of the block = pushing in an equal and opposite reaction.

Newton’s 3rd Law If you sit on a chair, what forces are acting

upon you?According to Newton, whenever objects A and B interact with each other, they exert forces upon each other. When you sit in your chair, your body exerts a downward force on the chair and the chair exerts an upward force on your body.

Use Newton’s 3rd law to discuss what is happening in this picture?

Law of Momentum Momentum can be defined as "mass in

motion."

All objects have mass; so if an object is moving, then it has momentum - it has its mass in motion.

The amount of momentum that an object has is dependent upon two variables: mass and velocity.

Law of Momentum

Momentum = mass • velocity In physics, the symbol for the quantity

momentum is the lower case "p". Thus, the above equation can be rewritten as

p = m • v

Momentum ProblemsDetermine the momentum of a ... a. 60-kg halfback moving eastward at 9

m/s. G: ; U:E:

SS

p = m • v

m = 60-kgv = 9 m/sp = ?

p = 60 kg • 9 m/s =540 kg • m/s

Momentum ProblemsDetermine the momentum of a ... b. 1000-kg car moving northward at 20

m/s.G: ; U:E:

SS

p = m • v

m = 1000-kgv = 20 m/sp = ?

p = 1000 kg • 20 m/s =20000 kg • m/s

Momentum Problems Which has greater momentum?

A. the halfback B. the car

Relationship of Variables

If the mass of the object stays constant, as the force acting upon an object is increased, the acceleration of the object is increased.

Momentum (p)

mass velocity

2 kg3 kg6 kg

22 m/s22 m/s22 m/s

p = m • v

44 kg x m/s66 kg x m/s

132 kg x m/s