Motion and Forces (p. 79-92)

Motion describes how objects travel in space and time

• The main variables in the description of motion are:

– Speed (velocity)– Travel time– Acceleration

Speed or Velocity (v)

• How fast an object is travelling

• Measured in km/h or most often in science as m/s

Travel time (t)

• The amount of time the object was moving

• Measured in seconds (s)

Acceleration (a)

• The change in speed over a given time

• Measured in m/s2

How do you calculate the speed of an object?

• Formula

Speed (velocity) = distance travelled (m)travel time (s)

or v= d/t

The units of speed will be m/s

• What is the speed of a car that travels 10 km in 12 minutes?

• Distance travelled (d) = 10km = 10 000m

• Travel time (t) = 12 min = 720 sec

v (m/s) = d (m)

t(sec)

v (m/s) = 10 000

720

v = 13.89 m/s = 50 km/h

Forces and Changes in Motion

• A force is a push or a pull on an object that can change its motion

• Forces can be represented graphically

• See Fig. 3.12 on p. 80– Line of action (dotted line)– Direction of application (arrowhead line)– Magnitude or strength (length of line)– Point of application (starting point of arrow)

Unit of Force

• Force is measured in Newtons (N)

• 1 Newton (N) is the amount of force required to accelerate a 1 kg object at a rate of 1 m/s2

What forces can do to an object

• 1. Can cause an object to start moving or increase the speed of an already moving object. This is called Acceleration.

• 2. Can cause an object to stop moving or slow down the speed of a moving object. This is called Deceleration.

• 3. Can cause a moving object to change direction.

Types of Forces

1. Gravitational

2. Electromagnetic

3. Frictional

4. Strong and Weak Nuclear

Gravitational Forces

• Force of attraction between all objects as a result of their masses and distances between them.

• The higher the masses and the shorter the distances between two objects, the greater the force of gravity.

• The smaller the masses and the larger the distances between two objects, the weaker the force of gravity.

Gravity exists between ANY two objects

Why things fall to the ground

• When we drop an object, the force of gravity is strong because the earth has a large mass and is close by. The object gets pulled to the ground.

• Near the Earth’s surface, objects accelerate to the ground at a rate of 9.8 m/s2, regardless of their masses.

• The gravitational force of the Earth decreases as you move away from the planet. (Fig. 3.14, p. 82)

• Different planets and stars will have different forces of gravity at their surfaces due to their different masses. The bigger the object the greater the force of gravity.

• Gravitational forces also explain the Earth’s tides

Mass vs. Weight

• Mass is a measure of the quantity of matter in an object. This does not depend on where the object is.

• Weight is a measure of the gravitational force acting on an object. This DOES depend on where the object is.

A useful formula

Weight = Gravitational force = Mass X gravitational field intensity

OR

w = Fg = mg

m is measured in kg

g at the earth’s surface is 9.8 m/s2

F is measured in Newtons

Example

• What is the gravitational force (weight) of a 60 kg astronaut on the earth’s surface?

F = mgF = 60 X 9.8F = 588 N

Example

• What is the gravitational force (weight) of a 60 kg astronaut on the Moon’s surface?

F=mgF= 60 X 1.67 (force of

gravity on the Moon)F= 100.2 N

Electromagnetic Force

• Force of attraction or repulsion between two charged objects or between the poles of two magnets

• We already discussed this at the beginning of the year

• Likes repel, opposites attract

Frictional Force

• The force that prevents two objects from slipping over each other

• Air resistance is also a force of friction

• Friction depends on:– The smoothness of the surfaces (the rougher

the surfaces the greater the friction)– The pressure between the surfaces (the

greater the pressure the greater the friction)

Friction

Strong and Weak Nuclear Forces

• Act within the nucleus of an atom

• Short range forces

• Holds the nucleus together

Resultant Force

• Objects are usually subjected to several forces at once.

• The RESULTANT force is equal to the combination of all forces acting on an object at the same time.

• See p. 87 Fig. 3.21

Equilibrium of Forces

• Equilibrium of forces is achieved when the resultant force is zero.

• The object will remain at rest or remain at the same velocity.

• Ex: When you ride a bike at a constant speed, the resultant force is zero….you are in equilibrium.