Motion under gravity,

In the formula for force you see m for mass.

Mass is defined as the quantity of matter in an object.

Mass is a scalar quanitity that is it has no direction with it.

The amount of matter in a body does not change (unless some part of the body is removed).

Mass is measured in grams or kilograms, (or in imperial units ounces, pounds, etc.)

Weight is often confused with mass, we get the butcher to weigh out our meat, he gives us an answer in kilograms (or pounds). This is what causes the usual confusion between mass and weight.

However in the physics realm, we see weight as a Force, looking at the equation previously, force is the product of mass and an acceleration!

What causes this acceleration ? Why do objects fall ? Gravity, g so

Weight is equal to mass times gravity

Remember Weight is a FORCE! Mass is NOT a force!

Q1) When a hot air balloon has risen 200m, a parachutist jumps out. If the mass of the parachutist is 120 kg, and the lift from the chute is 850 N, find

i) The net force acting on the parachutist as he descends

ii) The acceleration of the parachutist

iii) The time taken to reach the ground assuming he falls vertically.

When we pull or push an object across a surface we also see invisible forces at play,

Imagine there to be a a large block of granite on the ground, you may be able to push with a force of 500N, but yet it does not move.

Does this mean that it is immovable ?

What if you gathered a few friends, could you shift the block then ?

Would there come a point at which enough friends and you could move it?

What is the force that resists all the force you and you mates can muster ?

We call it Friction

Friction: a force opposing motion.

By experiment we see that it is not related to the area of the block, only the nature of the surfaces and the resultant up from the the surface cause this force. In conclusion to these experiments it can be shown that

Rules of Friction

  1. Direction of Friction is opposite to the direction of the motion of the body
  2. The Magnitude of friction, up to a point is equal to the magnitude of the force attempting to produce the motion.
  3. Only a certain amount of friction the limiting friction can be used.
  4. mu is the coefficient of friction see above.
  5. when a body is moving friction force remains and operates opposite to the motion.

Motion of a smooth surface on a smooth surface has a value for mu = 0, therefore there is no friction between the object and the surface it rests on. So there is no force against movement along the surface of a body if it is said to be smooth.

Q2) A car of mass 150kg is accelerating in a race. If the car engine provides a thrust of 200N, and the drag, or resistance is 180N, find

i) The Net force in the direction of motion

ii) The acceleration of the car

iii) The speed of the car after travelling 100m, assuming the car starts from rest.

If the driver at this stage throttles back so the engine is providing a thrust of 180N, what is the net force acting on the car?

What is the acceleration of the car?

How long does it take the car to travel the next 100 m?

To cause motion of an object that has a rough surface,

As R = W there is no movement up or down, Only if T > Fr will the body move, and only in the direction of T

Try this applet on for size .... and you will see the forces on a block that is being pulled along a table because it is connected to another block that is allowed to fall freely

again / as always with these applets,

DO put in various values,

DO observe & think

Put in more values & experiment!

An experiment to determine the friction between bodies,

  1. To determine what factors effect friction.
  2. To measure values of mu for different surfaces.

Method for static friction

  1. Get a hinged board
  2. Find the mass of the object.
  3. place object on board
  4. GENTLY raise board.
  5. if the object moves down the board with constant or increasing velocity
  6. Note position of board
  7. measure the angle or the side distances which ever is easier
  8. and determine the forces,

Method for dynamic friction

  1. Get a hinged board
  2. Find the mass of the object.
  3. place object on board
  4. GENTLY raise board.
  5. Tap the object
  6. if the object moves down the board with constant velocity
  7. Note position of board
  8. measure the angle
  9. and determine the forces,


Can you draw the forces needed to cause the object to move,

If you can do that then you can calculate them.

If you do this for a number of different materials you might notice a trend.

If you can't draw the forces ....

Use the following sections to help you out

Q4) A box of mass 40kg is pulled across a rough floor. If the box starts from rest and has a speed of 5 m/s after 10 secs, find

i) The acceleration

ii) The net force acting on the box

iii) The force of friction

iv) State two ways of reducing this friction?

Graphic showing the forces on an object on an inclined plane

Review: Nice graphic to show the way resolved vectors interact when the angle changes. A lot of the info will be advanced for a class but the graphic itself makes this a worthwhile view.

Animation showing a box sliding down a ramp. The coefficient of friction and the angle of inclination of the ramp are adjustable.

Review: Similar in style to the last website, geogebra shows this animation very nicely. It allows you to change some variables and then play the resultant piece to see what happens.


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