# Forces and Motion

Not all that you need but the basics.

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• Created by: mel
• Created on: 21-05-10 12:29

## Representing motion

The slope on a velocity-time graph represents the acceleration of an object. The distance travelled is equal to the area under a velocity-time graph.When an object is moving with a constant velocity, the line on the graph is horizontal. When an object is moving with a constant acceleration, the line on the graph is straight, but sloped.

the steeper the line on a distance time graph, the greater the speed of the object.

• the gradient of a velocity-time graph represents the acceleration
• the area under a velocity-time graph represents the distance covered
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## Force, mass and acceleration 1

An object may have several different forces acting on it, which can have different strengths and directions. But they can be added together to give the resultant force. When all the forces are balanced, the resultant force is zero. In this case:

• a stationary object remains stationary
• a moving object keeps on moving at the same speed in the same direction
• In this diagram of the weightlifter, the resultant force on the bar is not zero. The upwards force is bigger than the downwards force. The resultant force acts in the upwards direction, so the bar moves upwards.

In this diagram of the weightlifter, the resultant force on the bar is not zero. The upwards force is smaller than the downwards force. The resultant force acts in the downwards direction, so the bar moves downwards.

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## Force, mass and acceleration 2

The bigger the force, the greater the acceleration. Doubling the size of the (resultant) force doubles the acceleration.

resultant force (newton, N) = mass (kg) × acceleration (m/s2)

• For example, the force needed to accelerate a 10kg mass by 5m/s2 is: 10 x 5 = 50N
• A truck has a mass of 2,000kg.
• The driving force created by the engine is 3,000 newtons.
• Calculate the acceleration caused by this force.
• Air resistance - drag :When an object moves through the air, the force of air resistance acts in the opposite direction to the motion. Air resistance depends on the shape of the object and its speed.
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## Force, mass and acceleration 3

• Contact force :This happens when two objects are pushed together. They exert equal and opposite forces on each other. The contact force from the ground pushes up on your feet even as you stand still. This is the force you feel in your feet. You feel the ground pushing back against your weight pushing down.
• Friction :This is the force that resists movement between two surfaces which are in contact.
• Gravity :This is the force that pulls objects towards the Earth. We call the force of gravity on an object its weight. The Earth pulls with a force of about 10 newtons on every kilogram of mass.

what forces at on a parachutist falling at a steady speed?

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